[Proj] GGRS87 TO WGS84 PROBLEM(?)

[Clifford J Mugnier]

Are you certain about the "FROM" coordinate system?  The Hatt Azimuthal
Equidistant used to be the common system used for Greece, and inadvertenly
using that system will indeed produce spurious results.

C. Mugnier
LOUISIANA STATE UNIVERSITY
---------------------------------
From: "Andreas T"
Date: Wed, 6 Dec 2006 23:44:19 +0200
Subject: [Proj] GGRS87 TO WGS84 PROBLEM(?)

> I have trouble trying to reproject a point from greek grid to WGS84.
> ....

> Supposing that these params are official and accepted enough,
> ....
I can't find better ones at the moment, so one might assume they're
correct.

> ....
> [CODE]
> // Projections declaration
> $projInObj=ms_newprojectionobj
> ("proj=tmerc,ellps=GRS80,towgs84=-199.87,74.79,246.62,0,0,0,0,units=m,
> lon_0=24,k=0.9996,x_0=500000,no_defs")
> $projOutObj=ms_newprojectionobj(
> "proj=latlong,ellps=WGS84,DATUM=WGS84,no_defs");
> // Shape to be reprojected
> $myshape = ms_newpointobj();
> $myshape->setXY(552000, 4098077);
> ...
> The expected result (according to very reliable software like ArcGIS and
> others...) is:
> lon=24,351076881,lat=37,014812887
> Though! the result that I get from mapscript is:
> lon=24.585568772,lat=37.028433901

>...
> I can't relize any systematic error.
>...

This is a tough problem indeed! I have no clue.
Going from x, y = 552000, 4098077 through Transverse Mercator in the GGRS87
system should produce lat, lon = 37.027448, 24.584634 deg. A geocentric
shift as mentioned should produce lat, lon = 37.030039, 24.586333.
The ArcGIS values seem very unlikely; on the GRS80 globe they are nearly 21
kilometers away from the regular TM values. Datumshifting from GRS80 to
WGS84 cannot result in such large differences; the ellipsoids are nearly
the
same and the cartesian shifts are less than 250 meters.



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