[Proj] Convergence

[Charles Karney]

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
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