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

Markus Metz markus.metz.giswork at gmail.com
Fri May 10 07:33:07 PDT 2013

On Fri, May 10, 2013 at 12:52 PM, RichardC <richtcooper at hotmail.com> wrote:
> 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?

It depends on what is (supposed to be) in the input. If the input
polygons are supposed to overlap, e.g. buffers created with
non-topological tools, then the number of input polygons is most
likely not identical to the number of centroids which is perfectly ok.

> 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.

It could be that some of the input polygons are exact duplicates.

> Also can I ignore
> the areas without centroids?

Yes, these are typically holes in polygons, e.g. a patch of
non-mangrove surrounded by mangrove.

> 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.
> While:
> 5. areas without centroids can be ignored.

Yes, with the exception of overlapping areas. They may or may not be
correct, depending on the input.

Markus M

> Many thanks, for your feedback.
> Richard
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