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The Geodesy & Geophysics web page at the NGA.mil site has the Isogonic (magnetism) model that can be downloaded.<br>
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<div id="AppleMailSignature">Sent from Cliff Mugnier's iPhone</div>
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On Jan 30, 2018, at 6:37 AM, Thomas Knudsen <<a href="mailto:knudsen.thomas@gmail.com">knudsen.thomas@gmail.com</a>> wrote:<br>
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<div>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:<br>
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<span style="font-family:monospace,monospace">$ echo 12 55 | proj -V +proj=utm +zone=32 +ellps=GRS80</span><br>
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<span style="font-family:monospace,monospace">#Universal Transverse Mercator (UTM)<br>
# Cyl, Sph<br>
# zone= south<br>
# +proj=utm +zone=32 +ellps=GRS80<br>
#--- following specified but NOT used<br>
# +ellps=WGS84<br>
#Final Earth figure: ellipsoid<br>
# Major axis (a): 6378137.000<br>
# 1/flattening: 298.257222<br>
# squared eccentricity: 0.006694380023<br>
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Longitude: 12dE [ 12 ]<br>
Latitude: 55dN [ 55 ]<br>
Easting (x): 691875.63<br>
Northing (y): 6098907.83<br>
Meridian scale (h) : 1.00005168 ( 0.005168 % error )<br>
Parallel scale (k) : 1.00005168 ( 0.005168 % error )<br>
Areal scale (s): 1.00010336 ( 0.01034 % error )<br>
Angular distortion (w): 0.000<br>
Meridian/Parallel angle: 90.00000<br>
Convergence : 2d27'29.52" [ 2.45819987 ]<br>
Max-min (Tissot axis a-b) scale error: 1.00005 1.00005<br>
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<span style="font-family:monospace,monospace"><font face="arial,helvetica,sans-serif">Where the convergence (here 2.45819987)</font></span><span style="font-family:monospace,monospace"><font face="arial,helvetica,sans-serif"><span style="font-family:monospace,monospace"><font face="arial,helvetica,sans-serif">
is what you need</font></span>, if I understand your question correctly.<br>
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See the reply by Martin F over at <a href="https://gis.stackexchange.com/questions/61743/applying-proper-conversion-from-true-to-grid-bearing">
https://gis.stackexchange.com/questions/61743/applying-proper-conversion-from-true-to-grid-bearing</a> for instructions of how to apply this.</font><br>
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<div class="gmail_quote">2018-01-30 12:42 GMT+01:00 Chen Barnoy <span dir="ltr"><<a href="mailto:jicrbuh1@gmail.com" target="_blank">jicrbuh1@gmail.com</a>></span>:<br>
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<div dir="ltr">Hi,</div>
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<div dir="ltr">Is there a way to convert between grid north, true north and magnetic north in Proj4 or another library?</div>
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<div dir="ltr">Cheers,</div>
<div dir="ltr">Chen</div>
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