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I have a netCDF file that contains climate data in a Gaussian grid. The longitudes are given from 0 to 360. I need to convert this to -180 to 180.

Is this the correct formula -

If longitude is greater than 180 subtract 360

I am looking to do this in Fortran.

PolyGeo
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2 Answers2

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Here is an example for fortran:

Variables in explanation:

long1 is the longitude varying from -180 to 180 or 180W-180E
long3 is the longitude variable from 0 to 360 (all positive)

To convert longitude from (0-360) to (-180 to 180)

 Matlab and fortran long1=mod((long3+180),360)-180

 Matlab long1=rem((long3+180),360)-180

 Fortran long1=modulo((long3+180),360)-180

To convert longitude from (-180 to 180) to (0-360) 

 Matlab lon3=mod(lon1,360)

 Fortran lon3=modulo(lon1,360)

The “mod” function is Fortran is equivalent to “rem” function in Matlab.

The “modulo” function in Fortran is equivalent to “mod” function in Matlab.

http://vikas-ke-funde.blogspot.com/2010/06/convert-longitude-0-360-to-180-to-180.html

kttii
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If you know for certain that your input longitudes range between 0 and 360, then yes: just subtract 360 from values that are greater than (or equal to) 180.

If there is any possibility of "crazy" values (less than -180 or greater than 540) you might consider using the IEEE_REM function:

fixed_longitude = IEEE_REM(longitude, 360.0)

This will gracefully handle any longitude value with no other logic required.

csd
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