[GRASS-user] v.to.db: obscenely large area values for tiny islands ?!

peter.loewe at gmx.de peter.loewe at gmx.de
Mon Mar 23 12:17:05 EDT 2009


I just observed the phenomenon of about totally exeragated area readings for islands, using GRASS 6.3.0 on SuSe with postgres (pg driver):

A location of county data for northern central europe was set up from a shape file [topology had been checked with v.build], including all the tiny islands dotting the southern shoreline of the Northern and Baltic Seas.

Eventually, a vector layer of subdivided counties _and all the separate islands_ was created.

  v.db.addcol foo col="area double precision" layer=1 
  v.to.db map=foo type=centroid option=area col=area unit=h

was used to derive the areas in hectar.

As said, above the areas for the islands remain way too large.
 The fair island of Helgoland should have a size of 2.076ha (according to Wikipedia). 
 The area calculated by v.to.db are stately 498.465 ha. Even considering eventual land reclamation actions, that's out of scale.

 Area readings for landlocked counties are fine.

 Has anybody also encountered this ? Are there workarounds ?

Dr. Peter Löwe
<peter.loewe at gmx.de>

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