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

Clifford J Mugnier cjmce at lsu.edu
Tue Jan 30 08:06:07 EST 2018

The Geodesy & Geophysics web page at the NGA.mil site has the Isogonic (magnetism) model that can be downloaded.

Sent from Cliff Mugnier's iPhone

On Jan 30, 2018, at 6:37 AM, Thomas Knudsen <knudsen.thomas at gmail.com<mailto:knudsen.thomas at gmail.com>> wrote:

The magnetic north varies from location to location is not easy to handle, but you can get deviation from grid north using the -V option to proj:

\$  echo 12 55 | proj -V +proj=utm +zone=32 +ellps=GRS80

#Universal Transverse Mercator (UTM)
#    Cyl, Sph
#    zone= south
# +proj=utm +zone=32 +ellps=GRS80
#--- following specified but NOT used
# +ellps=WGS84
#Final Earth figure: ellipsoid
#  Major axis (a): 6378137.000
#  1/flattening: 298.257222
#  squared eccentricity: 0.006694380023

Longitude: 12dE [ 12 ]
Latitude:  55dN [ 55 ]
Easting (x):   691875.63
Northing (y):  6098907.83
Meridian scale (h) : 1.00005168  ( 0.005168 % error )
Parallel scale (k) : 1.00005168  ( 0.005168 % error )
Areal scale (s):     1.00010336  ( 0.01034 % error )
Angular distortion (w): 0.000
Meridian/Parallel angle: 90.00000
Convergence : 2d27'29.52" [ 2.45819987 ]
Max-min (Tissot axis a-b) scale error: 1.00005 1.00005

Where the convergence (here 2.45819987) is what you need, if I understand your question correctly.

See the reply by Martin F over at https://gis.stackexchange.com/questions/61743/applying-proper-conversion-from-true-to-grid-bearing for instructions of how to apply this.

2018-01-30 12:42 GMT+01:00 Chen Barnoy <jicrbuh1 at gmail.com<mailto:jicrbuh1 at gmail.com>>:
Hi,

Is there a way to convert between grid north, true north and magnetic north in Proj4 or another library?

Cheers,
Chen

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