Mmm, sorry but I don't understand it.<br><div class="gmail_quote">The topological correctness (in a general meaning, beyond GRASS) states that two polygons *cannot* overlap.<br>GRASS topological model admits overlapping areas (the build tool doesn't complaint), but some modules produce wrong results with these areas. E.g., v.rast.stats collects data as if one of the areas were clipped and not overlapping. I can explain it better, but it was just an example where GRASS model admits a not clean area, but it silently fails...<br>
So, GRASS is a bit "fuzzy" (*should* is an arbitrary proposition)... I comment between your answer rows:<br><div class="gmail_quote"><div class="im"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div><div>
<br>
</div></div>In some cases, overlapping polygons can be converted to a valid grass<br>
vector, e.g. Landsat tiles (WRS2 coverage). The conditions are that<br>
polygons are only partially overlapping, each polygon has a part that<br>
does not overlap with any other polygon, the centroids are located in<br>
that non-overlapping part, boundaries are not broken at intersections,<br>
and it helps if nodes of boundaries are not not shared with nodes of<br>
boundaries of other areas.<br></blockquote></div><div><br>Overlapping areas *are* valid vectors, even if they aren't topologically correct, right?<br>You say "partially overlapping". What does it mean? My three polygons overlap almost entirely (it is a synthetic layer for testing), the centroids are located on the not-overlapping regions (i.e. the overlaps aren't areas, nor have correctly broken boundaries) and v.build still accept them without errors... <br>
I don't understand where the rules you're listing are evaluated (and described).<br></div><div class="im"><blockquote class="gmail_quote" style="margin:0pt 0pt 0pt 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<br>
Nevertheless, even though these areas can be maintained in GRASS,<br>
these areas are per definition topologically not clean.<br></blockquote></div><div><br>Ok, this confirms that GRASS model isn't strictly topological. It suggests to be, to guarantee correct results (see the problem with v.rast.stats).<br>
<br>giovanni<br> </div><div class="im"><blockquote class="gmail_quote" style="margin:0pt 0pt 0pt 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<br>
Markus M<br>
<br>
><br>
> thanks,<br>
> giovanni<br>
><br>
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