What is the status of lib/gis/datumtransform.table ? Trying to do some reprojection between LL-WGS84 (epsg 4326) and belgian lambert 72 (epsg:31370), I see some problems due to the parameters in that file:
- in the proj epsg file (Rel. 4.6.0, 21 Dec 2007), the belgian lambert 72 projection 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
- this is confirmed by the info given by the Belgian geographic institute [1], which gives the same parameters with only differences in sign of which I don't know if they have any effect
- however in lib/gis/datumtransform.table one can find:
bel72 "towgs84=-99.1,53.3,-112.5,0.419,-0.830,1.885,-1.00" "Belgium" "Accuracy about 0.3m"
which gives quite different results when reprojecting compared, for example, to cs2cs.
What is the reason to have this datumtransform.table file ? Isn't most of this covered by proj / epsg these days ?
Moritz
[1] http://www.ign.be/FR/FR2-1-2.shtm (bottom of the page)
On Tue, 9 Jun 2009, Moritz Lennert wrote:
What is the status of lib/gis/datumtransform.table ? Trying to do some reprojection between LL-WGS84 (epsg 4326) and belgian lambert 72 (epsg:31370), I see some problems due to the parameters in that file:
- in the proj epsg file (Rel. 4.6.0, 21 Dec 2007), the belgian lambert 72 projection 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
- this is confirmed by the info given by the Belgian geographic institute [1], which gives the same parameters with only differences in sign of which I don't know if they have any effect
- however in lib/gis/datumtransform.table one can find:
bel72 "towgs84=-99.1,53.3,-112.5,0.419,-0.830,1.885,-1.00" "Belgium" "Accuracy about 0.3m"
This (like most of the parameters for European countries) comes from this site: http://crs.bkg.bund.de/crseu/crs/eu-countrysel.php?country=BE
(http://crs.bkg.bund.de/crs-eu/ for the top-level site, formerly crs.ifag.de as indicated in datumtransform.table).
Up to now it has been very accurate and reliable, so this is an interesting observation and I wonder where the different parameters come from. Belgium was covered in the ASPRS Grids & Datums column back in 1998 (http://www.asprs.org/resources/grids/10-98-belgum.pdf) and unfortunately there aren't any parameters given there that could be a clue.
which gives quite different results when reprojecting compared, for example, to cs2cs.
What is the reason to have this datumtransform.table file ? Isn't most of this covered by proj / epsg these days ?
Well a lot of countries have a choice of datum projection parameters and as far as I know the GDAL EPSG tables don't include any (to avoid making the wrong choice). Also if you're not creating a location using an EPSG code you'd need some way of interactively browsing the datum transformation parameters and making a choice anyway. This sort of thing is discussed on the MetaCRS list occasionally (http://lists.osgeo.org/pipermail/metacrs/) but I think a solution is still fairly far away from emerging.
Probably we should update datumtransform.table with the correct Belgium 72 parameters. But it would be reassuring to find out where the wrong ones have come from first...
Paul