[GRASS-user] v.to.db area calculations

[Moritz Lennert]

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