[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
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  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
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Comment (by mmetz):

 Replying to [comment:7 mmetz]:
 > Replying to [comment:6 mlennert]:
 > > Replying to [comment:5 annakrat]:
 > >
 > > > But the area computation problem must be somewhere in
 [https://grass.osgeo.org/programming7
 > > >/area__poly1_8c.html#af6f1f53bacc34249be98006c95369695
 G_ellipsoid_polygon_area], probably related
 > > > to very small numbers, but I couldn't pinpoint the problem. There is
 something specific about this
 > > > polygon, when I draw a similar one, it gives more reasonable
 results.
 > >
 > > So, yes, this definitely seems to be a precision issue, but at this
 stage I can't really see where in the source code, and don't have much
 time to delve into it in greater detail. Maybe MarkusM has an idea ?
 >
 > The problem seems to be in G_ellipsoid_polygon_area() at
 [https://trac.osgeo.org/grass/browser/grass/trunk/lib/gis/area_poly1.c#L161
 L161]. If dy is much smaller than dx, dx / dy becomes very large and
 erroneus values might sneak in. A simple solution could be to set dy to
 zero if dy is very small, but then how to define "very small"?
 Interestingly, dx must not be snapped to zero, otherwise I get complete
 nonsense results for the test shapefile.

 Apparently small differences in latitudes of adjacent vertices can cause a
 wrong latitude correction in G_ellipsoid_polygon_area(). Please try trunk
 r71167.

 Note that this does not affect areas created with v.in.region, but this
 fix also affects larger areas such as country boundaries.

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Ticket URL: <https://trac.osgeo.org/grass/ticket/3356#comment:8>
GRASS GIS <https://grass.osgeo.org>