[GRASS-dev] [GRASS GIS] #3356: v.to.db: incorrect area calculations in lat-long location

[GRASS GIS]

#3356: v.to.db: incorrect area calculations in lat-long location
--------------------------+-----------------------------------
  Reporter:  mlennert     |      Owner:  grass-dev@…
      Type:  defect       |     Status:  new
  Priority:  normal       |  Milestone:  7.4.0
 Component:  Vector       |    Version:  svn-trunk
Resolution:               |   Keywords:  v.to.db area lat-long
       CPU:  Unspecified  |   Platform:  Unspecified
--------------------------+-----------------------------------

Comment (by hellik):

 Replying to [comment:1 hellik]:
 > Replying to [ticket:3356 mlennert]:
 > > {{{
 > > $area in field calculator in QGIS:
 > > 425.1112801
 > > }}}
 >
 > just tested it with
 >
 > {{{
 > QGIS version 2.18.9 (OTF enabled)
 > }}}
 >
 > {{{
 > 1     34.815138348493
 > }}}
 >
 > and when I do manual measure in QGIS (OTF on)
 >
 > {{{
 > 32,4 m²
 > }}}

 now tested

 {{{
 g.region -p vector=training_single at test
 projection: 3 (Latitude-Longitude)
 zone:       0
 datum:      wgs84
 ellipsoid:  wgs84
 north:      0:13:32.187577N
 south:      0:13:32.175318N
 west:       73:12:48.504873E
 east:       73:12:48.518218E
 nsres:      0:00:00.012259
 ewres:      0:00:00.013345
 rows:       1
 cols:       1
 cells:      1
 }}}

 {{{
 v.in.region output=reg_training_single
 }}}

 {{{
 v.report map=reg_training_single at test option=area units=meters
 cat|area
 1|0.155378405506674
 }}}

 then export reg_training_single to a shapefile, reprojet this shapefile to
 EPSG:32743, creating a EPSG:32743-location by ogr2ogr, import the
 reprojected shapefile into the new location

 {{{
 v.report map=myreg2_32743 at data option=area units=meters
 cat||area
 1|0.155406186822802
 }}}

 now area of the bbox of training_single in wgs84 and EPSG:32743 seems to
 be similar.

 check this bbox vector area also in qgis (OTF enabled).

 {{{
 cat     areawgs84       areareproj
 1       0.1574797419    0.157479741
 }}}

 QGIS and GRASS area seems similar and reasonable, when you're looking at
 the kml-file in real life conditions.

 these numbers are similar to the number in the original report

 {{{
 v.report training_reproj_grass op=area u=me
 cat|id|area
 1|1|0.126369981910102
 }}}

 but not similar to

 {{{
 v.report training op=area u=me
 cat|id|area
 1|1|38.2256243922775
 }}}

 or

 {{{
 $area in field calculator in QGIS:
 425.1112801
 }}}

 any idea?

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Ticket URL: <https://trac.osgeo.org/grass/ticket/3356#comment:3>
GRASS GIS <https://grass.osgeo.org>