[Qgis-user] Recognition of SRS in MapInfo-Files

[Fischer, Andreas]

Dear Community,
thanks for your replies. I was very busy this week, so I could make further tests not until today. Unfortunately after my tests today I'm more confused than before. But I will try to explain...

First I figured out that Bo Victor is right in my case too. After I opened a TAB-File with EPSG:25832 the layer SRS definition is exactly as he posted. So this problem will address gdal/ogr library.

Then I tried out ogrinfo for a TAB file with EPSG 31467 as Jürgen mentioned. The output shows TOWGS84 parameters! These are used within the automatically user defined layer SRS definition in QGIS as well, after I open a MapInfo TAB file with 31467. It seems that ogr always assumes that TOWGS84 parameters have to be used for EPSG 31467. Even though the CoordSys-String in MapInfo MIF file (CoordSys Earth Projection 8, 104, "m", 9, 0, 1, 3500000, 0) does not point at such parameters and I changed the build in SRS definition in QGIS.  In my opinion this is not the best way, because it seems to be impossible to get the modified build in SRS for EPSG 31467 matched. The purpose for the modification was to make use of grid shift transformation with BETA2007 / NTv2 instead of using parameter transformation in general.

In former times I had trouble with ogr and MapInfo too. But these problems were more easy to solve. Using ogr2org to get PostGIS tables to MapInfo TAB files with -s_srs EPSG 31467 parameter caused trouble. The same happens with EPSG:25832. In both cases MapInfo Tab files didn't get the right projection in MapInfo. The reason are different DATUM parameters. To get around these problems I used ogrinfo for MapInfo TAB with EPSG 31467 and 25832 to save suitable SRS definition in text files first. After that I used ogr2org with -a_srs 31467.txt or -a_srs 25832.txt and everything went fine. 

But in case of my recent problem I don't have a chance to do the same, because everything happens automatically. Even though if the next QGIS-version will make use of the latest GDAL-library, the problem of detecting the proper build in SRS I my case isn't solved. Maybe this will be difficult because MapInfo and ogr do not use equal IDs for projections ;-(

My workaround is to assign the proper SRS after opening a MapInfo TAB file. But users have to do this every time a MapInfo file is opened in QGIS. Some do not like this, some don't understand this and some do both. 

I would be glad, if you have new ideas!

Best regards
Andreas

Ogrinfo output for MapInfo TAB file with EPSG:31467:

	Layer SRS WKT:
	PROJCS["unnamed",
	    GEOGCS["unnamed",
	        DATUM["DHDN_Potsdam_Rauenberg",
	            SPHEROID["Bessel 1841",6377397.155,299.1528128],
	            TOWGS84[582,105,414,1.04,0.35,-3.08,8.3]],
	        PRIMEM["Greenwich",0],
	        UNIT["degree",0.0174532925199433]],
	    PROJECTION["Transverse_Mercator"],
	    PARAMETER["latitude_of_origin",0],
	    PARAMETER["central_meridian",9],
	    PARAMETER["scale_factor",1],
	    PARAMETER["false_easting",3500000],
	    PARAMETER["false_northing",0],
	    UNIT["Meter",1.0]]

User defined layer SRS definition for the same MapInfo TAB file opened in QGIS:

	+proj=tmerc +lat_0=0 +lon_0=9 +k=1 +x_0=3500000 +y_0=0 +ellps=bessel +towgs84=582,105,414,1.04,0.35,-3.08,8.3 +units=m +no_defs


Im Auftrag

Andreas Fischer
Sachgebietsleiter

Kreis Unna - Der Landrat
Zentrale Datenverarbeitung
Sachgebiet DV-Organisation
Friedrich-Ebert-Straße 17
59425 Unna

Fon 02 3 03 27-44 16
Fax 0 23 03 27-28 96
andreas.fischer at kreis-unna.de
www.kreis-unna.de


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