Mikael.Rittri at carmenta.com
Wed Sep 17 07:41:51 EDT 2008
I think you are right that PROJ.4 is inconsistent here.
Before I read your post, I thought there was a standard sign convention
for meridian convergence, although I could never remember which way it went.
But I have done some googling now, and the situation seems as bad as for
the rotation angles in a 7-parameter transform.
If I use a slash and a backslash to describe how a piece of a meridian
looks in a projected grid, then:
The National Land Survey of Sweden agrees with +proj=utm, and says:
\ has positive meridian convergence,
/ has negative.
But the Ordnance Survey of Great Britain agrees with +proj=lcc, and says:
\ has negative meridian convergence,
/ has positive.
I didn't search further. Possibly, there might be a clear majority
viewpoint among all national mapping agencies, but I got tired.
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mikael.rittri at carmenta.com
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Kees Krikke
Sent: den 15 september 2008 17:28
To: PROJ.4 and general Projections Discussions
Subject: [Proj] Convergence
I've made a piece of s/w that uses proj's convergence calculation to find the true north for a given grid north.
When testing with a utm and lcc projection i found some confusing results regarding the sign (direction) of the convergence.
To demonstrate this i modified proj.exe so it prints the convergence (in
deg) at the end of the line when using the -S flag:
C:\proj-4.6.1\src>proj -vS +proj=utm +datum=WGS84 +lon_0=3
#Universal Transverse Mercator (UTM)
# Cyl, Sph
# zone= south
# +proj=utm +datum=WGS84 +lon_0=3 +ellps=WGS84 +towgs84=0,0,0
431350.30 5761510.32 <0.999658 0.999658 0.999316 1.20783e-006
0.999658 0.999658 -0.788041>
568649.70 5761510.32 <0.999658 0.999658 0.999316 8.54066e-007
0.999658 0.999658 0.788041>
568496.62 5772632.28 <0.999658 0.999658 0.999315 0 0.999658
From the above results it seems that for utm the convergence is
negative when west and positive when east of lon_0.
If you calculate two utm positions along the same longitude east of
lon_0, you see that the easting reduces when going northwards.
From this is seems that TrueNorth = GridNorth + convergence
I used the same points for a lcc projection:
C:\proj-4.6.1\src>proj -vS +proj=lcc +datum=WGS84 +lat_0=52 +lon_0=3
#Lambert Conformal Conic
# Conic, Sph&Ell
# lat_1= and lat_2= or lat_0
# +proj=lcc +datum=WGS84 +lat_0=52 +lon_0=3 +lat_1=50 +lat_2=54 +ellps=WGS84
-68634.11 472.08 <0.999392 0.999392 0.998785 0.000355057 0.999395
68634.11 472.08 <0.999392 0.999392 0.998785 0.000355708 0.999395
68481.15 11591.10 <0.999393 0.999393 0.998787 0.000434814
0.999397 0.99939 -0.788173>
The convergence sign west and east of the origin are opposite of the
signs found with the utm. But what confuses me is that when i calculate
two positions along a longitude east of the origin, the easting reduces
when going northwards just like with the utm projection.
From this is seems that TrueNorth = GridNorth - convergence
I think that either the convergence for lcc or utm has the wrong sign.
But i can't tell which one because i don't know how convergence is defined.
Any thoughts about this?
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