[GRASS-user] Query: number of 'areas' reported in v.build output does not necessarily represent different geographical areas

Nikos Alexandris nik at nikosalexandris.net
Fri May 10 11:37:41 PDT 2013


Markus M:

> >> I assume the global mangroves map should not have overlapping
> >> polygons, therefore as long as there are overlapping areas and/or the
> >> number of centroids is not equal to the number of input polygons, more
> >> cleaning is needed.

Nikos A:

> > Unfortunately it seems there are overlapping polygons.

Markus M:
> Obviously so, and fairly large overlapping areas according to your
> output even for snap=1e-6:
> Overlapping area: 5.32085E+09 (41538 areas)
> Therefore there are probably logical errors in the input data, for
> example the same mangrove polygon might be present more than once.
> This can not be cleaned automatically with v.in.ogr or v.clean. You
> can investigate overlapping areas in the imported vector with d.vect
> type=area layer=2 (only overlapping areas have a category in layer 2).

It looks _not_ good at all. Layer 2 over layer 1 uploaded at: 

I have also found the relevant "factsheet" for the Global Mangrove 
Distribution (USGS, 2011), which does not help very much I think:



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