Friends,
If I understand correctly (based on experience with my own data),
when GRASS calculates the topology for vector area data it doesn't
actually ensure that the centroid lies within the area it represents.
If the area is a torus, or at least hollow in a general way, then the
centroid might actually lie in the void in the center of the polygon
rather than within the interior of the polygon itself. It seems like
it would be preferable to run a point-in-polygon check on the calculated
centroid and move it of necessary to achieve correct point-in-polygon
correspondence.
This seems like it may also influence the results of v.what and
d.what.vect as well, and not always identify the correct polygon if
another centroid is nearer. I assume this is a well-known situation,
and that perhaps the solution for complex (sometimes hollow) polygons
(following advice from Achim Kesseler)is to do a v.to.rast and sample
the raster. Is that considered best practice for complex area data, or
are there better approaches? Is v.what reliable in these situations,
including in point-in-polygon mode?
Thanks, Dave
--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
David W. Roberts office 406-994-4548
Department of Ecology email drobe...@montana.edu
Montana State University
Bozeman, MT 59717-3460
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