Re: [GRASS-user] polygons and centroids redux
Thanks Hamish. It seems only likely that such a routine would exist. Here's the part where I need a little help though. It appears that v.distance from=pnt to=area dmax=0 upload=cat column=areacat will find and report the cat number of the polygon that points lie within, and I can use that in a database operation which is extremely handy. So, I have done that, and now the area cat for each point is available in the database. That works for points in a point vector feature. Using d.what.vect or v.what, however, I cannot control whether the function reports a boundary or centroid. This makes it hard to replicate the behavior of v.distance for a few points on the fly. Is there some way to access the existing point-in-polygon routine from an existing GRASS function to simulate what I'm after? Looking at the progammer's manual, writing something like v.pip (= to d.what.vect reporting always area cat) appears to be a bit more than I want to bite off at present. Perhaps one approach would be d.where file (edit file as necessary for separator) v.in.ascii in=file out=points fs=whatever v.distance from=points to=area dmax=0 upload=cat column=areacat That works but seems like a lot of work for what I'm after. Thanks much, Dave Hamish wrote: Dave wrote: I think it would indeed be handy if a switch was added to v.what and d.what.vect in GRASS 7. In the meantime it might be handy to write a v.pip command for GRASS 6. I have written a point-in-polygon routine for other software (including R), but I would have to really study the file structures of GRASS vectors to attempt that in GRASS. If any GRASS programmers are interested let me know and perhaps we could work it out as an add-on. point in polygon is a standard function in the vector library for years. Hamish ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
[GRASS-user] polygons and centroids
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 ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] polygons and centroids
Dave Roberts wrote: 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. No, grass does ensure that a centroid lies within an area when building topology (attaching centroids). After building full topology, the number of centroids outside any area is reported as Number of centroids outside area: Markus M ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
[GRASS-user] polygons and centroids redux
Friends, This is the second time in two days I have had to post an apology and retraction for hasty questions. I apologize for testing everyone's patience. Thanks Markus; I was composing this message when I saw your post. I mistook a centroid of a polygon too small to plot at the current resolution as the centroid of the surrounding hollow area. My mistake; sorry. It does raise a question, however, of what the conditions are that cause d.what.vect to return a feature type of Boundary rather than Area when querying a map. Is it simply proximity to the boundary, and can that behavior be suppressed in preference to always returning the data associated with the centroid? Thanks, Dave -- David W. Roberts office 406-994-4548 Department of Ecology email drobe...@montana.edu Montana State University Bozeman, MT 59717-3460 ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] polygons and centroids redux
Dave Roberts wrote: It does raise a question, however, of what the conditions are that cause d.what.vect to return a feature type of Boundary rather than Area when querying a map. d.what.vect and v.what search for points and centroids first. If none is found, they continue and search for lines, boundaries and faces. If none is found, they continue and search for areas. This search order causes any centroid or boundary to be returned if found, and not the corresponding area. Currently there is no way to select the feature type (point, centroid, line, boundary, area, face) to be searched. This option could be added for v.what in 7.0 if desired. Markus M ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] polygons and centroids redux
Thanks Markus! I think it would indeed be handy if a switch was added to v.what and d.what.vect in GRASS 7. In the meantime it might be handy to write a v.pip command for GRASS 6. I have written a point-in-polygon routine for other software (including R), but I would have to really study the file structures of GRASS vectors to attempt that in GRASS. If any GRASS programmers are interested let me know and perhaps we could work it out as an add-on. Thanks, Dave R Markus Metz wrote: Dave Roberts wrote: It does raise a question, however, of what the conditions are that cause d.what.vect to return a feature type of Boundary rather than Area when querying a map. d.what.vect and v.what search for points and centroids first. If none is found, they continue and search for lines, boundaries and faces. If none is found, they continue and search for areas. This search order causes any centroid or boundary to be returned if found, and not the corresponding area. Currently there is no way to select the feature type (point, centroid, line, boundary, area, face) to be searched. This option could be added for v.what in 7.0 if desired. Markus M ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] polygons and centroids redux
Dave wrote: I think it would indeed be handy if a switch was added to v.what and d.what.vect in GRASS 7. In the meantime it might be handy to write a v.pip command for GRASS 6. I have written a point-in-polygon routine for other software (including R), but I would have to really study the file structures of GRASS vectors to attempt that in GRASS. If any GRASS programmers are interested let me know and perhaps we could work it out as an add-on. point in polygon is a standard function in the vector library for years. Hamish ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
