[Qgis-user] Custom Projection for EPSG 27700 when opening shp and TAB files from other software

[James Stott]

I just wanted to check that this was working ok (using QGIS 1.9 Master).
I am a little confused by the different results I get from different
data.

The definition of a layer in QGIS using OSGB 1936 / British National
Grid (27700) is:

+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000
+ellps=airy +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489
+units=m +no_defs

I have data in MapInfo TAB files that are created in software other than
QGIS. When I open these they come in with the following custom
projection defined which is different from EPSG 27700 and as such not
recognised as it:

+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000
+ellps=airy +towgs84=375,-111,431,-0,-0,-0,0 +units=m +no_defs

I have data in a shapefile format not created by QGIS. When loaded into
QGIS these have the projection defined as a custom projection as shown
below which seems to match the EPSG 27700 definition but does not get
recognised as 27700:

+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012716 +x_0=400000 +y_0=-100000
+ellps=airy
+towgs84=446.448,-125.157,542.06,0.1502,0.247,0.8421,-20.4894 +units=m
+no_defs

Is this the correct behaviour and are all the custom projections giving
me the correct results?

James

James Stott BSc (Hons) MSc | Senior Professional
Nicholas Pearson Associates | 30 Brock Street | Bath | BA1 2LN
T: 01225 445548 | M: -
http://www.npaconsult.co.uk/ 


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