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

RichardC richtcooper at hotmail.com
Fri May 10 03:52:13 PDT 2013

Hi Markus,

> 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. 

How critical is it that the number of input polygons = number of centroids?
For example, I've imported a a different shapefile with 6842 input polygons
and after importing (with 1e-12 snapping threshold) there are 6800
centroids. Further cleaning does not change the topology. Also can I ignore
the areas without centroids?

Number of nodes: 17265
Number of primitives: 20687
Number of points: 0
Number of lines: 0
Number of boundaries: 13887
Number of centroids: 6800
Number of areas: 10521
Number of isles: 7099
Number of areas without centroid: 3721

Would I be correct in saying that a layer is topologically correct and needs
no further cleaning provided, there are no (i.e, zero):
1. overlapping areas;
2. incorrect boundaries;
3. centroids outside area; and
4. duplicate centroids.
5. areas without centroids can be ignored.

Many thanks, for your feedback.


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