[Proj] trying to understand results of reprojecting from epsg 4326 to epsg 31370

[OvV_HN]

----- Original Message ----- 
From: "Moritz Lennert" <mlennert at club.worldonline.be>
To: <Proj at lists.maptools.org>
Sent: Tuesday, July 07, 2009 4:14 PM
Subject: [Proj] trying to understand results of reprojecting from epsg 4326 
to epsg 31370


Hello,

I'm trying to reproject the following WGS84 Lat-Long (EPSG 4326)
coordinates to the Belgian 1972 projection EPSG 31370:

4.468493 50.856057

Using the official software from the Belgian National Geographic
Institute, I get the following coordinates (confirmed by a contact in
the Institute):

157022.870 171745.084


etc.


Reply:

With my software I get x,y:
157022.916, 171745.224
I used your official transformation parameters.
Note however the following:
(a) The scaling factor I used is ds=1.2747, since usually one gives the 
difference with 1.0 in units of 1e-6. The value you stated was: scale 
1.0000012747
(b) PROJ does a position vector datum transform, I did a coordinate frame 
transform with the values and signs of the parameters you stated. I  haven't 
checked whether the Belgian NGI (= IGN) actually uses this, but I got values 
very close to the needed values.
(c) I used the LCC version with fixed values of some projection constants, 
as stated by the Belgian NGI.
With the calculated projection constants I get for Y 171745.182 m; X is at 
the mm level the same.
(d) Note that my values are without grid interpolation.

Oscar van Vlijmen