[Proj] Converting between true north, grid north, and magnetic north

[Charles Karney]

You can also use GeographicLib to compute the meridian convergence (the
bearing of grid north measured clockwise from true north) for the UTM
system.  Continuing with the Boston Logan Airport example:

     echo 42.363 -71.006 | GeoConvert -u => 19n 334812 4692030

     echo 42.363 -71.006 | GeoConvert -c => -1.35200 0.9999357

The first command gives you the UTM coordinates and the second the
meridian convergence and scale.

On 02/04/2018 07:08 AM, Chen Barnoy wrote:
> Thanks!
> 
> And is there a similar library to convert between grid\UTM north to true 
> north?
> 
> 2018-01-30 23:59 GMT+02:00 Kurt Schwehr <schwehr at gmail.com 
> <mailto:schwehr at gmail.com>>:
> 
>     Be aware that the magnetic field is time varying.  It can change
>     pretty quickly near the magnetic poles.
> 
>     Yet another source... igrf.py
> 
>     https://earthref.org/PmagPy/cookbook/#x1-1190005.2.54
>     <https://earthref.org/PmagPy/cookbook/#x1-1190005.2.54>
> 
> 
> On Tue, Jan 30, 2018 at 7:03 AM, Charles Karney <charles.karney at sri.com 
> <mailto:charles.karney at sri.com>> wrote:
> 
>     GeographicLib https://geographiclib.sourceforge.io
>     <https://geographiclib.sourceforge.io> provides access to
>     several magnetic models via the MagneticField utility.  E.g.,
> 
>         echo 42.363 -71.006 <tel:42.363%20-71.006> | MagneticField -t now
> 
>     =>
> 
>         -14.67 67.08 20228.0 19569.1 -5121.1 47840.0 51940.7
> 
>     The first number is the current magnetic declination at Boston Logan
>     Airport using the wmm2015 magnetic model.
> 
>     The magnetic models need to be downloaded separately from the
>     GeographicLib library using
> 
>         geographiclib-get-magnetic
> 
>     On 01/30/18 06:42, Chen Barnoy wrote:
>      > Hi,
>      >
>      > Is there a way to convert between grid north, true north and magnetic
>      > north in Proj4 or another library?
>      >
>      > Cheers,
>      > Chen