topic Re: Project transformation from WGS84 to 31370 (Belgium Lambert 72) in ArcGIS JavaScript Maps SDK Questions

Project transformation from WGS84 to 31370 (Belgium Lambert 72)

JohnnyPenet — Sun, 15 Jul 2012 16:59:59 GMT

I am trying to add Google Places at a map with a basemap having spatial reference 31370. Because Google gives the result in WGS 84 coordinates, a project transformation is needed to convert the result into spatial reference 31370. For the project transformation I use the project method of the geometry service of an ArcGis 10.1 server (REST service).
The results have a displacement of some 100m in x and y, probably due to the fact that no datum correction has been done in the project method.
In the list of possible transformation for 31370, i found several possibilities :

1609,1610,15749,15929

It is not clear what the differences are, but all give me the same results.

I also use the same method for doing the reverse transformation, from spatial reference 31370 to WGS84. This works fine, and give me the correct coordinates.

Conclusing, in one direction it works, the other way not. I also used different transformationForward parameter options, but same errors.

When I use a basemap with spatial reference 102100, no problems (but I don't need using the geometry service).

Is this a limitation of the geometry service project method, handling the transformation in one direction in case of the spatial reference 31370?

Johnny

Re: Project transformation from WGS84 to 31370 (Belgium Lambert 72)

JohnnyPenet — Sat, 04 Aug 2012 10:33:24 GMT

Solved the problem by writing a transformation in Javascript, not an easy task. I had to read a lot of pages concerning projection transformations, lucky I am a mathematician.

For doing a transformation of WGS 84 to Lambert 72 (31370) I had to program the following steps :

1. Convert WGS 84 geographic coordinates to Geocentric coordinates

2. Perform Bursa-Wolf transformation based on Helmert 7 parameters of Lambert 72 for geographic transformation

3 Convert geocentric coordinates to geographic coordinates

4. Perform a Lambert conic conformal SP2 (Belgium) transformation to get Lambert 72 projection coordinates.

Doing so has the advantage that no async operation is required on a rest service. The most tricky point is step 4. It depends on the local projection required.

Re: Project transformation from WGS84 to 31370 (Belgium Lambert 72)

JohnnyPenet — Thu, 16 Aug 2012 14:19:19 GMT

For those who are interested, you can find my projection transformation adventure here, contains also some hints to write your own local transformation. :

http://jpenet.blogspot.be/2012/08/transformation-between-datums-in.html