[Proj] Convergence
Charles Karney
ckarney at sarnoff.com
Wed Sep 17 08:49:19 EDT 2008
I believe the convention is:
meridian convergence = bearing of grid north (measured clockwise
from true north)
This is the sign convention suggested by those little figures at the
bottom of large scale maps (USGS and UKOS for example) which indicate
the bearing of magnetic and grid north relative to true north. (And the
universal convention is, I believe, that "bearings" are measured
clockwise.)
This is consistent with
http://www.lantmateriet.se/templates/LMV_Meridiankonvergens.aspx?id=11496&lang=EN
The UK OS page
http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidetonationalgrid/page19.html
seems to have it backward. However, the previous page
http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidetonationalgrid/page18.html
says
true bearing = grid bearing + convergence
which is consistent with the convention (assuming bearing are always
measured clockwise from the reference direction).
--
Charles Karney <ckarney at sarnoff.com>
Sarnoff Corporation, Princeton, NJ 08543-5300
URL: http://charles.karney.info
Tel: +1 609 734 2312
Fax: +1 609 734 2662
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