[GIS] Issues using geographic and projected coordinate systems QGIS 2.0

coordinate systemqgiswgs84

I am having issues with QGIS 2.0.1 working between local projected coordinate systems (such as OSGB36, NAD83) and Geographic systems (such as WGS84).

The stock answer I have been provided is 'load the data and "save as"' – this doesn't cut it, the files are always 50m out. The data requires a Helmert or NTV2 transformation but I cannot see how to appy this in QGIS 2.0

Unfortunately I have gone back to ArcGIS and QGIS 1.9 (using the transformation tool plugin), but I cannot be alone in my issues, so has anyone else had this issue and what is the work -around?

Best Answer

Helmert transformation is done using the +towgs84 value in the proj definition, and ntv2 requires to specify the .gsb transformation file (except for NAD27). If your data comes with a less accurate CRS transformation definition, you have to assign (and not reproject with Save As ...) a user CRS to it.

Any CRS using NAD83 should not have any offsets, as this geodetic CRS uses the same ellipsoid and datum as WGS84 (unless you are using the NA2011 or similar methods which are not yet supported in QGIS).

For NAD27, the ntv2 adjustment is built-in in GDAL and QGIS.

For OSGB 36, you have to create a custom CRS. A ntv2 grid file can be downloaded from Ordnance Surveys website See also the questions here:

Raster incorrectly reprojected to OSGB(27700)

How to reproject a raster file in QGIS with datum transformation?

I had no problem doing that in QGIS 1.8.0 Lisboa, without using the transformation tool plugin at all.

If you encounter problems in QGIS 2.0, you could reproject your data in QGIS 1.8.0 to another CRS like UTM based on WGS84, and work with that in QGIS 2.0.

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