Calculating correct longitude when it's over |180|?











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I'm trying to develop a "formula" to correct the lat-lng values.



I'm using vue-leaflet but when you pan outside the "first" world you get big numbers. Over +180 or under -180.



For example: when I pan to America to the right (east direction), I get as lng 215.
In my mind, I would just correct it with 215-360=-145



The same is for when I pan to east russia to the left (west direction) and I get for example -222. Now I need to calculate -222+360=138



However, since the world is indefinite the user could pan to the 8th world and I had to adjust the values.



Is it possible to calculate the right longitude? (and another requirement is when the user is in the first world, 24 lng should still be 24 lng.










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    up vote
    9
    down vote

    favorite












    I'm trying to develop a "formula" to correct the lat-lng values.



    I'm using vue-leaflet but when you pan outside the "first" world you get big numbers. Over +180 or under -180.



    For example: when I pan to America to the right (east direction), I get as lng 215.
    In my mind, I would just correct it with 215-360=-145



    The same is for when I pan to east russia to the left (west direction) and I get for example -222. Now I need to calculate -222+360=138



    However, since the world is indefinite the user could pan to the 8th world and I had to adjust the values.



    Is it possible to calculate the right longitude? (and another requirement is when the user is in the first world, 24 lng should still be 24 lng.










    share|improve this question









    New contributor




    Shadrix is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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      up vote
      9
      down vote

      favorite









      up vote
      9
      down vote

      favorite











      I'm trying to develop a "formula" to correct the lat-lng values.



      I'm using vue-leaflet but when you pan outside the "first" world you get big numbers. Over +180 or under -180.



      For example: when I pan to America to the right (east direction), I get as lng 215.
      In my mind, I would just correct it with 215-360=-145



      The same is for when I pan to east russia to the left (west direction) and I get for example -222. Now I need to calculate -222+360=138



      However, since the world is indefinite the user could pan to the 8th world and I had to adjust the values.



      Is it possible to calculate the right longitude? (and another requirement is when the user is in the first world, 24 lng should still be 24 lng.










      share|improve this question









      New contributor




      Shadrix is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.











      I'm trying to develop a "formula" to correct the lat-lng values.



      I'm using vue-leaflet but when you pan outside the "first" world you get big numbers. Over +180 or under -180.



      For example: when I pan to America to the right (east direction), I get as lng 215.
      In my mind, I would just correct it with 215-360=-145



      The same is for when I pan to east russia to the left (west direction) and I get for example -222. Now I need to calculate -222+360=138



      However, since the world is indefinite the user could pan to the 8th world and I had to adjust the values.



      Is it possible to calculate the right longitude? (and another requirement is when the user is in the first world, 24 lng should still be 24 lng.







      leaflet javascript latitude-longitude






      share|improve this question









      New contributor




      Shadrix is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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      share|improve this question









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      share|improve this question








      edited 2 days ago









      PolyGeo

      52.7k1779236




      52.7k1779236






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      asked 2 days ago









      Shadrix

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          5 Answers
          5






          active

          oldest

          votes

















          up vote
          11
          down vote



          accepted










          You need to repeatedly add (or subtract) 360 to your value until it lies in the range of -180 - 180. So usually a pair of loops like:



          lon = -187;
          while(lon < -180){
          lon +=360;
          }
          while (lon > 180){
          lon -= 360;
          }





          share|improve this answer























          • signs wrong way round? Should be lon +=360 in first case.
            – JimT
            2 days ago






          • 2




            bleeugh, coding before coffee. Thanks
            – Ian Turton
            2 days ago






          • 3




            you can do it with only one loop while (Math.abs(lon) > 180) { lon -= Math.sign(lon) * 360 } I am not providing it as an answer though because your version actually matches the explanation, while my version is just an optimization that likely doesn't make any difference. I keep it as a comment only as a reminder that things can be done in multiple ways, some more optimized than others.
            – Andrei
            2 days ago






          • 2




            I don't think I would ever use that one as it uses 2 function calls per loop and only one of my loops would ever execute. Probably makes no difference in this example but that's my prejudice
            – Ian Turton
            2 days ago






          • 2




            You can't do lon %= 180?
            – Nic Hartley
            2 days ago


















          up vote
          10
          down vote













          An answer that avoids conditionals and function calls:



          longitude = (longitude % 360 + 540) % 360 - 180


          I wrote a quick microbenchmark at https://jsperf.com/longitude-normalisation and the conditional code seems to be faster (in Chrome on my machine) for 'reasonable' ranges of input values. In general you probably shouldn't be worrying in advance about performance in small calculations like this, giving more weight to readability and consistency with the rest of your codebase.



          Probably more important in this case is the question of whether your code could ever come across extreme input values (1e10, Infinity etc.). If so, the looping implementation could end up running really slowly or silently hanging your program. This might occur with calculations performed near the poles, e.g. trying to pan east or west by some distance (rather than angle) from a pole could easily result in an infinite longitude.






          share|improve this answer










          New contributor




          Joe Lee-Moyet is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.














          • 1




            Interesting. Let's race a conditional jmp against a FP divide. Hmmm I wonder.
            – Joshua
            2 days ago






          • 1




            @Joshua You can't use a conditional jump. You have to use multiple conditional jumps, aka a loop. (Plus the loop contains floating point additional, which isn't free.) How many iterations the loop needs depends on the input. So you have to know something about the data to look at performance. If the vast majority are near the desired range and require few iterations, sure, the addition loop might be faster, but it isn't as obvious as your sarcasm suggests.
            – jpmc26
            2 days ago








          • 1




            @jpmc26: In this case, expecting to go around the loop more than once is silly.
            – Joshua
            2 days ago






          • 1




            There was no sarcasm. I actually don't know which way it would fall.
            – Joshua
            2 days ago






          • 1




            @Joshua yep, I wasn't sure either :). I added more to the answer on performance (and a potential failure case of the loop code)
            – Joe Lee-Moyet
            2 days ago


















          up vote
          4
          down vote













          One-liner:



          normalized = remainder(longitude, 360);


          Explanation: You want to know what remains after you disregard full rotations (360°).



          This process is called normalizing.



          Example (cpp.sh)






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          • 1




            Wouldn't this result in a [0, 360) value, not [-180, 180] as Shadrix requested?
            – The Guy with The Hat
            2 days ago










          • @TheGuywithTheHat Check this example: cpp.sh/7uy2v
            – Peter Paff
            2 days ago










          • Ah, didn't know this was C++. In Shadrix's context of JavaScript, I interpreted remainder as modulus. Modulus in JS would result in [0, 360).
            – The Guy with The Hat
            2 days ago








          • 1




            I don't think that would work. You would need to subtract 360 iff result > 180. Another problem I just realized with JavaScript is that modulo is symmetrical across 0, e.g. -1 % 3 is -1, not 2 as would be necessary for it to work here. remainder is a great C++ solution, but unfortunately there's just no function/operator in JS that's similar enough to be useful.
            – The Guy with The Hat
            2 days ago


















          up vote
          0
          down vote













          Another option: longitude = atan2(cos(long), sin(long))






          share|improve this answer

















          • 1




            This doesn't seem like a good idea. It's very hard to understand, computationally expensive and potentially subject to rounding errors.
            – David Richerby
            2 days ago


















          up vote
          0
          down vote













          If the programming language you're using supports the % (mod) operator on floating point numbers (like Python and Ruby), I'd recommend using that. Otherwise, some other languages (like C and C++) allow you to use fmod().



          (Whichever mod operator you use, make sure ahead of time that it will do mod operations on floating-point numbers, and that it'll always give you non-negative answers. Otherwise you'll get a nasty surprise later when many of your lat/lon points are not correct.)



          Use it like this:



          # Put the longitude in the range of [0,360):
          longitude %= 360

          # Put the longitude in the range of [-180,180):
          if longitude >= 180:
          longitude -= 360


          If you'd prefer to do it all in one line:



          # Put the longitude in the range of [-180,180):
          longitude = (longitude + 180) % 360 - 180


          These approaches have no loops, so they'll normalize longitude values without needing to repeatedly add or subtract, no matter how many times your observation has circled around the earth.



          Edit:



          Hmmm... I just noticed that Javascript doesn't seem to handle % with negative values like I thought it would.



          In that case, try this one-liner:



          longitude = (longitude + 36180) % 360 - 180


          The 36180 we're adding is 36,000 + 180. The 36,000 is to move a negative value into the positive domain, and the 180 is to shift it over so that when it is modded by 360, it'll be in the range of [0,360). The - 180 part shifts it back to the range of [-180,180).



          Here's another one-liner, one that doesn't rely on 36,000 being big enough:



          longitude = (longitude % 360 + 360 + 180) % 360 - 180


          The longitude % 360 + 360 part will ensure the value stays in the positive domain when it's later modded by 360. The + 180 part shifts it over so that when it later gets 180 subtracted from it (with - 180), it'll be in the desired range of [-180,180).






          share|improve this answer










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          • 1




            Note: C,C++ fmod(longitude, 360) --> (-360.0 ... +360.0) and ilongitude % 360 --> [-359 ... +359].
            – chux
            yesterday












          • @chux - I didn't know about that, so I just tested it, and it appears that you are correct. Thank you for pointing that out.
            – J-L
            yesterday











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          5 Answers
          5






          active

          oldest

          votes








          5 Answers
          5






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes








          up vote
          11
          down vote



          accepted










          You need to repeatedly add (or subtract) 360 to your value until it lies in the range of -180 - 180. So usually a pair of loops like:



          lon = -187;
          while(lon < -180){
          lon +=360;
          }
          while (lon > 180){
          lon -= 360;
          }





          share|improve this answer























          • signs wrong way round? Should be lon +=360 in first case.
            – JimT
            2 days ago






          • 2




            bleeugh, coding before coffee. Thanks
            – Ian Turton
            2 days ago






          • 3




            you can do it with only one loop while (Math.abs(lon) > 180) { lon -= Math.sign(lon) * 360 } I am not providing it as an answer though because your version actually matches the explanation, while my version is just an optimization that likely doesn't make any difference. I keep it as a comment only as a reminder that things can be done in multiple ways, some more optimized than others.
            – Andrei
            2 days ago






          • 2




            I don't think I would ever use that one as it uses 2 function calls per loop and only one of my loops would ever execute. Probably makes no difference in this example but that's my prejudice
            – Ian Turton
            2 days ago






          • 2




            You can't do lon %= 180?
            – Nic Hartley
            2 days ago















          up vote
          11
          down vote



          accepted










          You need to repeatedly add (or subtract) 360 to your value until it lies in the range of -180 - 180. So usually a pair of loops like:



          lon = -187;
          while(lon < -180){
          lon +=360;
          }
          while (lon > 180){
          lon -= 360;
          }





          share|improve this answer























          • signs wrong way round? Should be lon +=360 in first case.
            – JimT
            2 days ago






          • 2




            bleeugh, coding before coffee. Thanks
            – Ian Turton
            2 days ago






          • 3




            you can do it with only one loop while (Math.abs(lon) > 180) { lon -= Math.sign(lon) * 360 } I am not providing it as an answer though because your version actually matches the explanation, while my version is just an optimization that likely doesn't make any difference. I keep it as a comment only as a reminder that things can be done in multiple ways, some more optimized than others.
            – Andrei
            2 days ago






          • 2




            I don't think I would ever use that one as it uses 2 function calls per loop and only one of my loops would ever execute. Probably makes no difference in this example but that's my prejudice
            – Ian Turton
            2 days ago






          • 2




            You can't do lon %= 180?
            – Nic Hartley
            2 days ago













          up vote
          11
          down vote



          accepted







          up vote
          11
          down vote



          accepted






          You need to repeatedly add (or subtract) 360 to your value until it lies in the range of -180 - 180. So usually a pair of loops like:



          lon = -187;
          while(lon < -180){
          lon +=360;
          }
          while (lon > 180){
          lon -= 360;
          }





          share|improve this answer














          You need to repeatedly add (or subtract) 360 to your value until it lies in the range of -180 - 180. So usually a pair of loops like:



          lon = -187;
          while(lon < -180){
          lon +=360;
          }
          while (lon > 180){
          lon -= 360;
          }






          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited 2 days ago

























          answered 2 days ago









          Ian Turton

          46.8k546109




          46.8k546109












          • signs wrong way round? Should be lon +=360 in first case.
            – JimT
            2 days ago






          • 2




            bleeugh, coding before coffee. Thanks
            – Ian Turton
            2 days ago






          • 3




            you can do it with only one loop while (Math.abs(lon) > 180) { lon -= Math.sign(lon) * 360 } I am not providing it as an answer though because your version actually matches the explanation, while my version is just an optimization that likely doesn't make any difference. I keep it as a comment only as a reminder that things can be done in multiple ways, some more optimized than others.
            – Andrei
            2 days ago






          • 2




            I don't think I would ever use that one as it uses 2 function calls per loop and only one of my loops would ever execute. Probably makes no difference in this example but that's my prejudice
            – Ian Turton
            2 days ago






          • 2




            You can't do lon %= 180?
            – Nic Hartley
            2 days ago


















          • signs wrong way round? Should be lon +=360 in first case.
            – JimT
            2 days ago






          • 2




            bleeugh, coding before coffee. Thanks
            – Ian Turton
            2 days ago






          • 3




            you can do it with only one loop while (Math.abs(lon) > 180) { lon -= Math.sign(lon) * 360 } I am not providing it as an answer though because your version actually matches the explanation, while my version is just an optimization that likely doesn't make any difference. I keep it as a comment only as a reminder that things can be done in multiple ways, some more optimized than others.
            – Andrei
            2 days ago






          • 2




            I don't think I would ever use that one as it uses 2 function calls per loop and only one of my loops would ever execute. Probably makes no difference in this example but that's my prejudice
            – Ian Turton
            2 days ago






          • 2




            You can't do lon %= 180?
            – Nic Hartley
            2 days ago
















          signs wrong way round? Should be lon +=360 in first case.
          – JimT
          2 days ago




          signs wrong way round? Should be lon +=360 in first case.
          – JimT
          2 days ago




          2




          2




          bleeugh, coding before coffee. Thanks
          – Ian Turton
          2 days ago




          bleeugh, coding before coffee. Thanks
          – Ian Turton
          2 days ago




          3




          3




          you can do it with only one loop while (Math.abs(lon) > 180) { lon -= Math.sign(lon) * 360 } I am not providing it as an answer though because your version actually matches the explanation, while my version is just an optimization that likely doesn't make any difference. I keep it as a comment only as a reminder that things can be done in multiple ways, some more optimized than others.
          – Andrei
          2 days ago




          you can do it with only one loop while (Math.abs(lon) > 180) { lon -= Math.sign(lon) * 360 } I am not providing it as an answer though because your version actually matches the explanation, while my version is just an optimization that likely doesn't make any difference. I keep it as a comment only as a reminder that things can be done in multiple ways, some more optimized than others.
          – Andrei
          2 days ago




          2




          2




          I don't think I would ever use that one as it uses 2 function calls per loop and only one of my loops would ever execute. Probably makes no difference in this example but that's my prejudice
          – Ian Turton
          2 days ago




          I don't think I would ever use that one as it uses 2 function calls per loop and only one of my loops would ever execute. Probably makes no difference in this example but that's my prejudice
          – Ian Turton
          2 days ago




          2




          2




          You can't do lon %= 180?
          – Nic Hartley
          2 days ago




          You can't do lon %= 180?
          – Nic Hartley
          2 days ago












          up vote
          10
          down vote













          An answer that avoids conditionals and function calls:



          longitude = (longitude % 360 + 540) % 360 - 180


          I wrote a quick microbenchmark at https://jsperf.com/longitude-normalisation and the conditional code seems to be faster (in Chrome on my machine) for 'reasonable' ranges of input values. In general you probably shouldn't be worrying in advance about performance in small calculations like this, giving more weight to readability and consistency with the rest of your codebase.



          Probably more important in this case is the question of whether your code could ever come across extreme input values (1e10, Infinity etc.). If so, the looping implementation could end up running really slowly or silently hanging your program. This might occur with calculations performed near the poles, e.g. trying to pan east or west by some distance (rather than angle) from a pole could easily result in an infinite longitude.






          share|improve this answer










          New contributor




          Joe Lee-Moyet is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.














          • 1




            Interesting. Let's race a conditional jmp against a FP divide. Hmmm I wonder.
            – Joshua
            2 days ago






          • 1




            @Joshua You can't use a conditional jump. You have to use multiple conditional jumps, aka a loop. (Plus the loop contains floating point additional, which isn't free.) How many iterations the loop needs depends on the input. So you have to know something about the data to look at performance. If the vast majority are near the desired range and require few iterations, sure, the addition loop might be faster, but it isn't as obvious as your sarcasm suggests.
            – jpmc26
            2 days ago








          • 1




            @jpmc26: In this case, expecting to go around the loop more than once is silly.
            – Joshua
            2 days ago






          • 1




            There was no sarcasm. I actually don't know which way it would fall.
            – Joshua
            2 days ago






          • 1




            @Joshua yep, I wasn't sure either :). I added more to the answer on performance (and a potential failure case of the loop code)
            – Joe Lee-Moyet
            2 days ago















          up vote
          10
          down vote













          An answer that avoids conditionals and function calls:



          longitude = (longitude % 360 + 540) % 360 - 180


          I wrote a quick microbenchmark at https://jsperf.com/longitude-normalisation and the conditional code seems to be faster (in Chrome on my machine) for 'reasonable' ranges of input values. In general you probably shouldn't be worrying in advance about performance in small calculations like this, giving more weight to readability and consistency with the rest of your codebase.



          Probably more important in this case is the question of whether your code could ever come across extreme input values (1e10, Infinity etc.). If so, the looping implementation could end up running really slowly or silently hanging your program. This might occur with calculations performed near the poles, e.g. trying to pan east or west by some distance (rather than angle) from a pole could easily result in an infinite longitude.






          share|improve this answer










          New contributor




          Joe Lee-Moyet is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.














          • 1




            Interesting. Let's race a conditional jmp against a FP divide. Hmmm I wonder.
            – Joshua
            2 days ago






          • 1




            @Joshua You can't use a conditional jump. You have to use multiple conditional jumps, aka a loop. (Plus the loop contains floating point additional, which isn't free.) How many iterations the loop needs depends on the input. So you have to know something about the data to look at performance. If the vast majority are near the desired range and require few iterations, sure, the addition loop might be faster, but it isn't as obvious as your sarcasm suggests.
            – jpmc26
            2 days ago








          • 1




            @jpmc26: In this case, expecting to go around the loop more than once is silly.
            – Joshua
            2 days ago






          • 1




            There was no sarcasm. I actually don't know which way it would fall.
            – Joshua
            2 days ago






          • 1




            @Joshua yep, I wasn't sure either :). I added more to the answer on performance (and a potential failure case of the loop code)
            – Joe Lee-Moyet
            2 days ago













          up vote
          10
          down vote










          up vote
          10
          down vote









          An answer that avoids conditionals and function calls:



          longitude = (longitude % 360 + 540) % 360 - 180


          I wrote a quick microbenchmark at https://jsperf.com/longitude-normalisation and the conditional code seems to be faster (in Chrome on my machine) for 'reasonable' ranges of input values. In general you probably shouldn't be worrying in advance about performance in small calculations like this, giving more weight to readability and consistency with the rest of your codebase.



          Probably more important in this case is the question of whether your code could ever come across extreme input values (1e10, Infinity etc.). If so, the looping implementation could end up running really slowly or silently hanging your program. This might occur with calculations performed near the poles, e.g. trying to pan east or west by some distance (rather than angle) from a pole could easily result in an infinite longitude.






          share|improve this answer










          New contributor




          Joe Lee-Moyet is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          An answer that avoids conditionals and function calls:



          longitude = (longitude % 360 + 540) % 360 - 180


          I wrote a quick microbenchmark at https://jsperf.com/longitude-normalisation and the conditional code seems to be faster (in Chrome on my machine) for 'reasonable' ranges of input values. In general you probably shouldn't be worrying in advance about performance in small calculations like this, giving more weight to readability and consistency with the rest of your codebase.



          Probably more important in this case is the question of whether your code could ever come across extreme input values (1e10, Infinity etc.). If so, the looping implementation could end up running really slowly or silently hanging your program. This might occur with calculations performed near the poles, e.g. trying to pan east or west by some distance (rather than angle) from a pole could easily result in an infinite longitude.







          share|improve this answer










          New contributor




          Joe Lee-Moyet is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          share|improve this answer



          share|improve this answer








          edited 2 days ago





















          New contributor




          Joe Lee-Moyet is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          answered 2 days ago









          Joe Lee-Moyet

          2014




          2014




          New contributor




          Joe Lee-Moyet is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.





          New contributor





          Joe Lee-Moyet is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.






          Joe Lee-Moyet is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.








          • 1




            Interesting. Let's race a conditional jmp against a FP divide. Hmmm I wonder.
            – Joshua
            2 days ago






          • 1




            @Joshua You can't use a conditional jump. You have to use multiple conditional jumps, aka a loop. (Plus the loop contains floating point additional, which isn't free.) How many iterations the loop needs depends on the input. So you have to know something about the data to look at performance. If the vast majority are near the desired range and require few iterations, sure, the addition loop might be faster, but it isn't as obvious as your sarcasm suggests.
            – jpmc26
            2 days ago








          • 1




            @jpmc26: In this case, expecting to go around the loop more than once is silly.
            – Joshua
            2 days ago






          • 1




            There was no sarcasm. I actually don't know which way it would fall.
            – Joshua
            2 days ago






          • 1




            @Joshua yep, I wasn't sure either :). I added more to the answer on performance (and a potential failure case of the loop code)
            – Joe Lee-Moyet
            2 days ago














          • 1




            Interesting. Let's race a conditional jmp against a FP divide. Hmmm I wonder.
            – Joshua
            2 days ago






          • 1




            @Joshua You can't use a conditional jump. You have to use multiple conditional jumps, aka a loop. (Plus the loop contains floating point additional, which isn't free.) How many iterations the loop needs depends on the input. So you have to know something about the data to look at performance. If the vast majority are near the desired range and require few iterations, sure, the addition loop might be faster, but it isn't as obvious as your sarcasm suggests.
            – jpmc26
            2 days ago








          • 1




            @jpmc26: In this case, expecting to go around the loop more than once is silly.
            – Joshua
            2 days ago






          • 1




            There was no sarcasm. I actually don't know which way it would fall.
            – Joshua
            2 days ago






          • 1




            @Joshua yep, I wasn't sure either :). I added more to the answer on performance (and a potential failure case of the loop code)
            – Joe Lee-Moyet
            2 days ago








          1




          1




          Interesting. Let's race a conditional jmp against a FP divide. Hmmm I wonder.
          – Joshua
          2 days ago




          Interesting. Let's race a conditional jmp against a FP divide. Hmmm I wonder.
          – Joshua
          2 days ago




          1




          1




          @Joshua You can't use a conditional jump. You have to use multiple conditional jumps, aka a loop. (Plus the loop contains floating point additional, which isn't free.) How many iterations the loop needs depends on the input. So you have to know something about the data to look at performance. If the vast majority are near the desired range and require few iterations, sure, the addition loop might be faster, but it isn't as obvious as your sarcasm suggests.
          – jpmc26
          2 days ago






          @Joshua You can't use a conditional jump. You have to use multiple conditional jumps, aka a loop. (Plus the loop contains floating point additional, which isn't free.) How many iterations the loop needs depends on the input. So you have to know something about the data to look at performance. If the vast majority are near the desired range and require few iterations, sure, the addition loop might be faster, but it isn't as obvious as your sarcasm suggests.
          – jpmc26
          2 days ago






          1




          1




          @jpmc26: In this case, expecting to go around the loop more than once is silly.
          – Joshua
          2 days ago




          @jpmc26: In this case, expecting to go around the loop more than once is silly.
          – Joshua
          2 days ago




          1




          1




          There was no sarcasm. I actually don't know which way it would fall.
          – Joshua
          2 days ago




          There was no sarcasm. I actually don't know which way it would fall.
          – Joshua
          2 days ago




          1




          1




          @Joshua yep, I wasn't sure either :). I added more to the answer on performance (and a potential failure case of the loop code)
          – Joe Lee-Moyet
          2 days ago




          @Joshua yep, I wasn't sure either :). I added more to the answer on performance (and a potential failure case of the loop code)
          – Joe Lee-Moyet
          2 days ago










          up vote
          4
          down vote













          One-liner:



          normalized = remainder(longitude, 360);


          Explanation: You want to know what remains after you disregard full rotations (360°).



          This process is called normalizing.



          Example (cpp.sh)






          share|improve this answer










          New contributor




          Peter Paff is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.














          • 1




            Wouldn't this result in a [0, 360) value, not [-180, 180] as Shadrix requested?
            – The Guy with The Hat
            2 days ago










          • @TheGuywithTheHat Check this example: cpp.sh/7uy2v
            – Peter Paff
            2 days ago










          • Ah, didn't know this was C++. In Shadrix's context of JavaScript, I interpreted remainder as modulus. Modulus in JS would result in [0, 360).
            – The Guy with The Hat
            2 days ago








          • 1




            I don't think that would work. You would need to subtract 360 iff result > 180. Another problem I just realized with JavaScript is that modulo is symmetrical across 0, e.g. -1 % 3 is -1, not 2 as would be necessary for it to work here. remainder is a great C++ solution, but unfortunately there's just no function/operator in JS that's similar enough to be useful.
            – The Guy with The Hat
            2 days ago















          up vote
          4
          down vote













          One-liner:



          normalized = remainder(longitude, 360);


          Explanation: You want to know what remains after you disregard full rotations (360°).



          This process is called normalizing.



          Example (cpp.sh)






          share|improve this answer










          New contributor




          Peter Paff is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.














          • 1




            Wouldn't this result in a [0, 360) value, not [-180, 180] as Shadrix requested?
            – The Guy with The Hat
            2 days ago










          • @TheGuywithTheHat Check this example: cpp.sh/7uy2v
            – Peter Paff
            2 days ago










          • Ah, didn't know this was C++. In Shadrix's context of JavaScript, I interpreted remainder as modulus. Modulus in JS would result in [0, 360).
            – The Guy with The Hat
            2 days ago








          • 1




            I don't think that would work. You would need to subtract 360 iff result > 180. Another problem I just realized with JavaScript is that modulo is symmetrical across 0, e.g. -1 % 3 is -1, not 2 as would be necessary for it to work here. remainder is a great C++ solution, but unfortunately there's just no function/operator in JS that's similar enough to be useful.
            – The Guy with The Hat
            2 days ago













          up vote
          4
          down vote










          up vote
          4
          down vote









          One-liner:



          normalized = remainder(longitude, 360);


          Explanation: You want to know what remains after you disregard full rotations (360°).



          This process is called normalizing.



          Example (cpp.sh)






          share|improve this answer










          New contributor




          Peter Paff is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          One-liner:



          normalized = remainder(longitude, 360);


          Explanation: You want to know what remains after you disregard full rotations (360°).



          This process is called normalizing.



          Example (cpp.sh)







          share|improve this answer










          New contributor




          Peter Paff is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          share|improve this answer



          share|improve this answer








          edited 2 days ago





















          New contributor




          Peter Paff is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          answered 2 days ago









          Peter Paff

          412




          412




          New contributor




          Peter Paff is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.





          New contributor





          Peter Paff is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.






          Peter Paff is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.








          • 1




            Wouldn't this result in a [0, 360) value, not [-180, 180] as Shadrix requested?
            – The Guy with The Hat
            2 days ago










          • @TheGuywithTheHat Check this example: cpp.sh/7uy2v
            – Peter Paff
            2 days ago










          • Ah, didn't know this was C++. In Shadrix's context of JavaScript, I interpreted remainder as modulus. Modulus in JS would result in [0, 360).
            – The Guy with The Hat
            2 days ago








          • 1




            I don't think that would work. You would need to subtract 360 iff result > 180. Another problem I just realized with JavaScript is that modulo is symmetrical across 0, e.g. -1 % 3 is -1, not 2 as would be necessary for it to work here. remainder is a great C++ solution, but unfortunately there's just no function/operator in JS that's similar enough to be useful.
            – The Guy with The Hat
            2 days ago














          • 1




            Wouldn't this result in a [0, 360) value, not [-180, 180] as Shadrix requested?
            – The Guy with The Hat
            2 days ago










          • @TheGuywithTheHat Check this example: cpp.sh/7uy2v
            – Peter Paff
            2 days ago










          • Ah, didn't know this was C++. In Shadrix's context of JavaScript, I interpreted remainder as modulus. Modulus in JS would result in [0, 360).
            – The Guy with The Hat
            2 days ago








          • 1




            I don't think that would work. You would need to subtract 360 iff result > 180. Another problem I just realized with JavaScript is that modulo is symmetrical across 0, e.g. -1 % 3 is -1, not 2 as would be necessary for it to work here. remainder is a great C++ solution, but unfortunately there's just no function/operator in JS that's similar enough to be useful.
            – The Guy with The Hat
            2 days ago








          1




          1




          Wouldn't this result in a [0, 360) value, not [-180, 180] as Shadrix requested?
          – The Guy with The Hat
          2 days ago




          Wouldn't this result in a [0, 360) value, not [-180, 180] as Shadrix requested?
          – The Guy with The Hat
          2 days ago












          @TheGuywithTheHat Check this example: cpp.sh/7uy2v
          – Peter Paff
          2 days ago




          @TheGuywithTheHat Check this example: cpp.sh/7uy2v
          – Peter Paff
          2 days ago












          Ah, didn't know this was C++. In Shadrix's context of JavaScript, I interpreted remainder as modulus. Modulus in JS would result in [0, 360).
          – The Guy with The Hat
          2 days ago






          Ah, didn't know this was C++. In Shadrix's context of JavaScript, I interpreted remainder as modulus. Modulus in JS would result in [0, 360).
          – The Guy with The Hat
          2 days ago






          1




          1




          I don't think that would work. You would need to subtract 360 iff result > 180. Another problem I just realized with JavaScript is that modulo is symmetrical across 0, e.g. -1 % 3 is -1, not 2 as would be necessary for it to work here. remainder is a great C++ solution, but unfortunately there's just no function/operator in JS that's similar enough to be useful.
          – The Guy with The Hat
          2 days ago




          I don't think that would work. You would need to subtract 360 iff result > 180. Another problem I just realized with JavaScript is that modulo is symmetrical across 0, e.g. -1 % 3 is -1, not 2 as would be necessary for it to work here. remainder is a great C++ solution, but unfortunately there's just no function/operator in JS that's similar enough to be useful.
          – The Guy with The Hat
          2 days ago










          up vote
          0
          down vote













          Another option: longitude = atan2(cos(long), sin(long))






          share|improve this answer

















          • 1




            This doesn't seem like a good idea. It's very hard to understand, computationally expensive and potentially subject to rounding errors.
            – David Richerby
            2 days ago















          up vote
          0
          down vote













          Another option: longitude = atan2(cos(long), sin(long))






          share|improve this answer

















          • 1




            This doesn't seem like a good idea. It's very hard to understand, computationally expensive and potentially subject to rounding errors.
            – David Richerby
            2 days ago













          up vote
          0
          down vote










          up vote
          0
          down vote









          Another option: longitude = atan2(cos(long), sin(long))






          share|improve this answer












          Another option: longitude = atan2(cos(long), sin(long))







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 2 days ago









          Andres

          163211




          163211








          • 1




            This doesn't seem like a good idea. It's very hard to understand, computationally expensive and potentially subject to rounding errors.
            – David Richerby
            2 days ago














          • 1




            This doesn't seem like a good idea. It's very hard to understand, computationally expensive and potentially subject to rounding errors.
            – David Richerby
            2 days ago








          1




          1




          This doesn't seem like a good idea. It's very hard to understand, computationally expensive and potentially subject to rounding errors.
          – David Richerby
          2 days ago




          This doesn't seem like a good idea. It's very hard to understand, computationally expensive and potentially subject to rounding errors.
          – David Richerby
          2 days ago










          up vote
          0
          down vote













          If the programming language you're using supports the % (mod) operator on floating point numbers (like Python and Ruby), I'd recommend using that. Otherwise, some other languages (like C and C++) allow you to use fmod().



          (Whichever mod operator you use, make sure ahead of time that it will do mod operations on floating-point numbers, and that it'll always give you non-negative answers. Otherwise you'll get a nasty surprise later when many of your lat/lon points are not correct.)



          Use it like this:



          # Put the longitude in the range of [0,360):
          longitude %= 360

          # Put the longitude in the range of [-180,180):
          if longitude >= 180:
          longitude -= 360


          If you'd prefer to do it all in one line:



          # Put the longitude in the range of [-180,180):
          longitude = (longitude + 180) % 360 - 180


          These approaches have no loops, so they'll normalize longitude values without needing to repeatedly add or subtract, no matter how many times your observation has circled around the earth.



          Edit:



          Hmmm... I just noticed that Javascript doesn't seem to handle % with negative values like I thought it would.



          In that case, try this one-liner:



          longitude = (longitude + 36180) % 360 - 180


          The 36180 we're adding is 36,000 + 180. The 36,000 is to move a negative value into the positive domain, and the 180 is to shift it over so that when it is modded by 360, it'll be in the range of [0,360). The - 180 part shifts it back to the range of [-180,180).



          Here's another one-liner, one that doesn't rely on 36,000 being big enough:



          longitude = (longitude % 360 + 360 + 180) % 360 - 180


          The longitude % 360 + 360 part will ensure the value stays in the positive domain when it's later modded by 360. The + 180 part shifts it over so that when it later gets 180 subtracted from it (with - 180), it'll be in the desired range of [-180,180).






          share|improve this answer










          New contributor




          J-L is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.














          • 1




            Note: C,C++ fmod(longitude, 360) --> (-360.0 ... +360.0) and ilongitude % 360 --> [-359 ... +359].
            – chux
            yesterday












          • @chux - I didn't know about that, so I just tested it, and it appears that you are correct. Thank you for pointing that out.
            – J-L
            yesterday















          up vote
          0
          down vote













          If the programming language you're using supports the % (mod) operator on floating point numbers (like Python and Ruby), I'd recommend using that. Otherwise, some other languages (like C and C++) allow you to use fmod().



          (Whichever mod operator you use, make sure ahead of time that it will do mod operations on floating-point numbers, and that it'll always give you non-negative answers. Otherwise you'll get a nasty surprise later when many of your lat/lon points are not correct.)



          Use it like this:



          # Put the longitude in the range of [0,360):
          longitude %= 360

          # Put the longitude in the range of [-180,180):
          if longitude >= 180:
          longitude -= 360


          If you'd prefer to do it all in one line:



          # Put the longitude in the range of [-180,180):
          longitude = (longitude + 180) % 360 - 180


          These approaches have no loops, so they'll normalize longitude values without needing to repeatedly add or subtract, no matter how many times your observation has circled around the earth.



          Edit:



          Hmmm... I just noticed that Javascript doesn't seem to handle % with negative values like I thought it would.



          In that case, try this one-liner:



          longitude = (longitude + 36180) % 360 - 180


          The 36180 we're adding is 36,000 + 180. The 36,000 is to move a negative value into the positive domain, and the 180 is to shift it over so that when it is modded by 360, it'll be in the range of [0,360). The - 180 part shifts it back to the range of [-180,180).



          Here's another one-liner, one that doesn't rely on 36,000 being big enough:



          longitude = (longitude % 360 + 360 + 180) % 360 - 180


          The longitude % 360 + 360 part will ensure the value stays in the positive domain when it's later modded by 360. The + 180 part shifts it over so that when it later gets 180 subtracted from it (with - 180), it'll be in the desired range of [-180,180).






          share|improve this answer










          New contributor




          J-L is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.














          • 1




            Note: C,C++ fmod(longitude, 360) --> (-360.0 ... +360.0) and ilongitude % 360 --> [-359 ... +359].
            – chux
            yesterday












          • @chux - I didn't know about that, so I just tested it, and it appears that you are correct. Thank you for pointing that out.
            – J-L
            yesterday













          up vote
          0
          down vote










          up vote
          0
          down vote









          If the programming language you're using supports the % (mod) operator on floating point numbers (like Python and Ruby), I'd recommend using that. Otherwise, some other languages (like C and C++) allow you to use fmod().



          (Whichever mod operator you use, make sure ahead of time that it will do mod operations on floating-point numbers, and that it'll always give you non-negative answers. Otherwise you'll get a nasty surprise later when many of your lat/lon points are not correct.)



          Use it like this:



          # Put the longitude in the range of [0,360):
          longitude %= 360

          # Put the longitude in the range of [-180,180):
          if longitude >= 180:
          longitude -= 360


          If you'd prefer to do it all in one line:



          # Put the longitude in the range of [-180,180):
          longitude = (longitude + 180) % 360 - 180


          These approaches have no loops, so they'll normalize longitude values without needing to repeatedly add or subtract, no matter how many times your observation has circled around the earth.



          Edit:



          Hmmm... I just noticed that Javascript doesn't seem to handle % with negative values like I thought it would.



          In that case, try this one-liner:



          longitude = (longitude + 36180) % 360 - 180


          The 36180 we're adding is 36,000 + 180. The 36,000 is to move a negative value into the positive domain, and the 180 is to shift it over so that when it is modded by 360, it'll be in the range of [0,360). The - 180 part shifts it back to the range of [-180,180).



          Here's another one-liner, one that doesn't rely on 36,000 being big enough:



          longitude = (longitude % 360 + 360 + 180) % 360 - 180


          The longitude % 360 + 360 part will ensure the value stays in the positive domain when it's later modded by 360. The + 180 part shifts it over so that when it later gets 180 subtracted from it (with - 180), it'll be in the desired range of [-180,180).






          share|improve this answer










          New contributor




          J-L is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          If the programming language you're using supports the % (mod) operator on floating point numbers (like Python and Ruby), I'd recommend using that. Otherwise, some other languages (like C and C++) allow you to use fmod().



          (Whichever mod operator you use, make sure ahead of time that it will do mod operations on floating-point numbers, and that it'll always give you non-negative answers. Otherwise you'll get a nasty surprise later when many of your lat/lon points are not correct.)



          Use it like this:



          # Put the longitude in the range of [0,360):
          longitude %= 360

          # Put the longitude in the range of [-180,180):
          if longitude >= 180:
          longitude -= 360


          If you'd prefer to do it all in one line:



          # Put the longitude in the range of [-180,180):
          longitude = (longitude + 180) % 360 - 180


          These approaches have no loops, so they'll normalize longitude values without needing to repeatedly add or subtract, no matter how many times your observation has circled around the earth.



          Edit:



          Hmmm... I just noticed that Javascript doesn't seem to handle % with negative values like I thought it would.



          In that case, try this one-liner:



          longitude = (longitude + 36180) % 360 - 180


          The 36180 we're adding is 36,000 + 180. The 36,000 is to move a negative value into the positive domain, and the 180 is to shift it over so that when it is modded by 360, it'll be in the range of [0,360). The - 180 part shifts it back to the range of [-180,180).



          Here's another one-liner, one that doesn't rely on 36,000 being big enough:



          longitude = (longitude % 360 + 360 + 180) % 360 - 180


          The longitude % 360 + 360 part will ensure the value stays in the positive domain when it's later modded by 360. The + 180 part shifts it over so that when it later gets 180 subtracted from it (with - 180), it'll be in the desired range of [-180,180).







          share|improve this answer










          New contributor




          J-L is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          share|improve this answer



          share|improve this answer








          edited yesterday





















          New contributor




          J-L is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.









          answered 2 days ago









          J-L

          1011




          1011




          New contributor




          J-L is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.





          New contributor





          J-L is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.






          J-L is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.








          • 1




            Note: C,C++ fmod(longitude, 360) --> (-360.0 ... +360.0) and ilongitude % 360 --> [-359 ... +359].
            – chux
            yesterday












          • @chux - I didn't know about that, so I just tested it, and it appears that you are correct. Thank you for pointing that out.
            – J-L
            yesterday














          • 1




            Note: C,C++ fmod(longitude, 360) --> (-360.0 ... +360.0) and ilongitude % 360 --> [-359 ... +359].
            – chux
            yesterday












          • @chux - I didn't know about that, so I just tested it, and it appears that you are correct. Thank you for pointing that out.
            – J-L
            yesterday








          1




          1




          Note: C,C++ fmod(longitude, 360) --> (-360.0 ... +360.0) and ilongitude % 360 --> [-359 ... +359].
          – chux
          yesterday






          Note: C,C++ fmod(longitude, 360) --> (-360.0 ... +360.0) and ilongitude % 360 --> [-359 ... +359].
          – chux
          yesterday














          @chux - I didn't know about that, so I just tested it, and it appears that you are correct. Thank you for pointing that out.
          – J-L
          yesterday




          @chux - I didn't know about that, so I just tested it, and it appears that you are correct. Thank you for pointing that out.
          – J-L
          yesterday










          Shadrix is a new contributor. Be nice, and check out our Code of Conduct.










           

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