Hello, Thank you for your investigations.
Is there anything else I can do about this? I trust my QGIS measurement because I know the size of the object I have drawn around. However I would really like to use GRASS to automate my workflow... Thanks James On 19 May 2017 at 13:41, Moritz Lennert <[email protected]> wrote: > On 19/05/17 11:25, James Duffy wrote: > >> Just to clear a few things up. >> >> The shapefile contains only a single polygon (I created it as a simple >> test to work out this discrepancy). >> >> OTF is turned off in QGIS, I am working in EPSG 4326. >> >> The 0.127m2 comes from using the Measure tool (OTF off) in QGIS. And it >> is set to display m2. >> >> Using the $area calculator in the field calculator in QGIS does indeed >> show something that is more likely degrees squared. >> > > Below some results from quick tests. > > Summary: personally I would trust GRASS GIS a bit more, but I'm biaised... > > *************************** > Test 1: > > GRASS trunk in NC demo location (EPSG 3358) > > g.region n=200100 s=200000 w=500000 e=500100 -ap > v.in.region out=nc_testpolygon > v.to.db -p nc_testpolygon op=area --q > 1|10000 > > GRASS trunk in EPSG 4326 location > v.proj location=nc_spm_08 mapset=user1 input=nc_testpolygon > v.to.db -p nc_testpolygon op=area --q > 1|10002.503642603 > v.out.ogr nc_testpolygon out=nc_testpolygon.shp (i.e. shapefile is in > degress, EPSG 4326) > > QGIS (using the exported shapefile) > > No OTF > measuring tool: 12.324,737 m² > $area in calculator: 12320.2346026 > > OTF to EPSG 4326, with datum WGS84 > measuring tool: 10.072,355 m² > $area in calculator: 10046.3679510 > > > ******************************** > Test 2: > > GRASS trunk, in ESPG 4326 location > > g.region n=74 s=73 w=0 e=1 res=1 -p > v.in.region region_1degree_73_74N > v.to.db -p region_1degree_73_74N op=area --q > 1|3539348689.40454 > > v.out.ogr region_1degree_73_74N out=region_1degree_73_74N.shp > > > GRASS trunk in EPSG 3575 location > v.proj location=LL_WGS84 mapset=mlennert input=region_1degree_73_74N > v.to.db -p region_1degree_73_74N op=area --q > 1|3539337483.97153 > > > QGIS (using the exported shapefile) > no OTF projection > > measuring tool 1,002 m² (sic) > export geometry column: 1 > $area in calculator 12392029030.5000000 > > with OTF projection, datum for measurements WGS84 > > measuring tool 3.503.322.774,337 m² > export geometry column: 1 > $area in calculator: 3499702798.3400998 > > reproject to EPSG 3575 > with OTF, datum WGS84 > > export geometry column 3539169026.18628 > export geoemtry column (column named otf) 3499702798.00000 > $area in calculator: 3499702798.6579294 > > > -- *James Duffy* PhD Researcher Environment and Sustainability Institute Penryn Campus University of Exeter Penryn Cornwall TR10 9FE
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