hi all, some comments in the text
>Hello, > >from Roland's (he was the person who brought first light for me in this >issue) and my/our experience (we work together, sometimes ;-) the >situation presents itself like that: > >* If you have an ESRI-generated .prj-file, the towgs84 - parameter is >missing: Programms using GDAL/Proj & Co. (like QGIS) need this parameter >to be able to make projections on the fly. So, you have to edit the .prj >file in question, or use the suitable one from the ones I sent earlier >today, and name it accordingly - if you make the jump from ESRI to QGIs >for example > >* If you stay just with ESRI, it is no problem, as far as I can tell, >ArcGIS does projections on the fly within its programming logic, it >seems (just a wild guess) in my work with ESRI ArcGIS 9.1 through 9.3 and my data with projections in wgs84, MGI, Monte Mario etc. i have to define manually for every projection the towgs-parameter that the on-the-fly-projection of all data together is correct. if you don't do this, there is for example a north-east-shift of about 70m in the on-the-fly-projection of Austrian and wgs84-data. So the situation is similar to QGis and other GDAL/Proj4-based GIS-software. And maybe for your interest, since version 9.2 ArcGIS uses also GDAL. > >* As a conclusion: The .prj files are not really wrong, they are >incomplete (Is there an official standard/definition somewhere?). To be >on the safe side, make your .prj-files complete, because if ESRI >stumbles over the towgs84-parameter it leaves it alone, so jumping back >and forth is no problem... I think the prj-file missing the towgs-parameter is not wrong, it's maybe the official standard? just try ogr2ogr with a projection-warp, in the prj-file of the new shapefile there is no towgs-parameter included. > >* From a kind of "scientific" point of view: If you have the parameter >in your .prj-files, it is documented what you intended to do with the >data... > that's sounds good, but be aware that the towgs-parameter can differ, also in Austria. So you can order from Bundesamt für Eich- und Vermessungswesen (http://www.bev.gv.at/) for Austria adopted towgs-parameter for 10 km x 10 km-grids with better accuracy than the epsg-towgs-data for the Austrian MGI. In my experience with Austrian govermental gis-data in the last years, beeing carefull and check all data saves nerves ;o) ... best regards Helli >Maybe the situation is a little clearer now... > >best wishes, > >Albin _______________________________________________________________________ DSL zum Nulltarif + 20 Euro Extraprämie bei Online-Bestellung über die DSL Freundschaftswerbung! http://dsl.web.de/?ac=OM.AD.AD008K15279B7069a _______________________________________________ Qgis-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/qgis-user
