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

[Jean-Claude Repetto]

Hello,

Moritz Lennert a écrit :
> 
> 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
> 
> Using cs2cs +init=epsg:4326 +to +init=epsg:31370, I get
> 
> 157201.51    171803.24
> 
> i.e. way off the official response.
> 
> 31370 is defined as
> 
> +proj=lcc +lat_1=51.16666723333333 +lat_2=49.8333339 +lat_0=90 
> +lon_0=4.367486666666666 +x_0=150000.013 +y_0=5400088.438 +ellps=intl 
> +towgs84=106.869,-52.2978,103.724,-0.33657,0.456955,-1.84218,1 +units=m 
> +no_defs
>

The correct parameters are :

+proj=lcc +lat_1=51.16666723333333 +lat_2=49.8333339 +lat_0=90 
+lon_0=4.367486666666666 +x_0=150000.013 +y_0=5400088.438 +ellps=intl 
+towgs84=-106.868628,52.297783,-103.723893,0.33657,-0.456955,1.842183,-1.2747 
+units=m +no_defs

(your 7th parameter is incorrect).

The results are :
157022.92	171745.23 -42.92

I spend several hours last week to clarify that problem !

Jean-Claude