# incorrect convergence calculation for lcc projection. #16

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opened this Issue May 22, 2015 · 2 comments

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### proj4-bot commented May 22, 2015

 Reported by keesk on 22 Sep 2008 08:36 UTC Convergence calculation for lcc seems to have an incorrect sign. The convention for convergence is: `````` meridian convergence = bearing of grid north (measured clockwise from true north) so: true bearing = grid bearing + 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=lcc +datum=WGS84 +lat_0=52 +lon_0=3 +lat_1=50 +lat_2=54 #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 # +towgs84=0,0,0 2 52 -68634.11 472.08 <0.999392 0.999392 0.998785 0.000355057 0.999395 0.999389 0.788173> 4 52 68634.11 472.08 <0.999392 0.999392 0.998785 0.000355708 0.999395 0.999389 -0.788173> 4 52.1 68481.15 11591.10 <0.999393 0.999393 0.998787 0.000434814 0.999397 0.99939 -0.788173> `````` When i calculate two positions along a longitude east of the origin (point E4N52 and point E4N52.1) the easting reduces when going northwards. According te the convention the convergence should be positive and not negative. Testing with a UTM projection gives the expected results: ``````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 2 52 431350.30 5761510.32 <0.999658 0.999658 0.999316 1.20783e-006 0.999658 0.999658 -0.788041> 4 52 568649.70 5761510.32 <0.999658 0.999658 0.999316 8.54066e-007 0.999658 0.999658 0.788041> 4 52.1 568496.62 5772632.28 <0.999658 0.999658 0.999315 0 0.999658 0.999658 0.789115> `````` 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 which complies with the convention. Regards, Kees Krikke Last Modified by warmerdam on 22 Sep 2008 14:54 UTC Migrated-From: https://trac.osgeo.org/proj/ticket/16

### proj4-bot commented May 22, 2015

 Comment by warmerdam on 22 Sep 2008 14:54 UTC This is related to the thread: http://lists.maptools.org/pipermail/proj/2008-September/003804.html with the (current) conclusion: http://lists.maptools.org/pipermail/proj/2008-September/003821.html I'll wait for Gerald to complete his analysis and then look to apply the same fix he does.

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### dtutic commented Jul 12, 2017

 My findings also confirm that LCC gives opposite sign for meridian convergence if we accept the definition: "meridian convergence on a map, at a given point, is the angle measured clockwise from the tangent to the projection of the meridian to the northing coordinate line (grid north)—also called map angle or mapping angle." Solution is to replace the line: fac->conv = - Q->n * lp.lam; with fac->conv = Q->n * lp.lam; in PJ_lcc.c

### kbevers added a commit that referenced this issue Jul 13, 2017

``` Merge pull request #545 from kbevers/lcc-convergence ```
`Correcting convergence formula for LCC. Fixes #16.`
``` 2ec0759 ```