Re: [GRASS-dev] Some doubts about GRASS topology
Benjamin Ducke wrote: Dear all, in an attempt to better understand the GRASS vector and topology model, I imported a set of 3 polygons from an ESRI Shapefile (see attachment). The polygon in the upper left has 4 holes (called islands for some reason by GRASS), the lower one consists of 3 parts (QGIS calls this a polygon with islands -- good to know we understand each other in the GIS world!). Yes, maybe the way the term islands is used in GRASS is a bit misleading. According to simple feature specifications, GRASS islands are (more or less, not sure if 100%) equivalent to holes. The third is a simple, convex shape. Displaying the imported map shows all geometries exactly as it should. So far so good. Now, when I run v.info on the imported map, I get: Number of lines: 0 Number of boundaries: 9 Number of centroids: 5 Number of areas: 9 Number of islands: 9 This completely baffles me! The GRASS documentation consistently states that an area is a boundary + a centroid + any number of islands. That's an error in the documentation. An area is a closed ring of boundaries (can be only one boundary) + any number of islands (holes) within + *optionally* an attached centroid. An area without centroid can not have a category but as far as topology is concerned, it's a valid area. Now, assuming that the lines around the four islands count as boundaries, I understand why there are 9 boundaries altogether. 5 centroids also check out, given that there is no 1:1 equivalent for a shapefile multipart polygon in GRASS. But how in the (GRASS) world can there be 9 areas if there are only 5 centroids? See above, an area in GRASS topology does not need to have a centroid attached. And why 9 islands? Every area is also an island if no boundary is shared with another area. If a boundary is shared with another area, these two areas together form one island. In your example, the area in the upper left with the four islands: the four islands are also areas, but without centroid attached. When attaching islands during topology building, the internal IDs of all islands falling inside the outer area are added to the topology information of that outer area. If one of these four islands would share a separate boundary with two other islands each, and only one islands would be completely isolated, that thing in the upper left would still consist of five areas (four inside, one outer), but of only three islands, one consisting of three connected areas, one for the remaining isolated inside area, one for the outer area. When building topology, areas and islands are constructed first, islands are not yet attached to areas. Only in the next step are islands attached to areas, areas get holes. In the last step, centroids are attached to areas, or more precisely: for each area a not yet attached centroid is searched for, if found attached, if already attached, it's a duplicate centroid. There may also be several centroids falling inside the current area and only inside this area, these will also become duplicate centroids. AFAICT, GRASS vector topology is very much based on simple feature specifications, but not strictly, it deviates here and there in the usage of terms and in the methods to build topology. The methods are not a problem, they are consistent even though not 100% following simple feature specifications, but the usage of terms can be confusing, particularly with misleading documentation and sometimes different meanings in closely related applications (QGIS). Markus M ___ grass-dev mailing list grass-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-dev
Re: [GRASS-dev] Some doubts about GRASS topology
Well, that clarifies it (finally)! Thanks very much for taking the time to write up all this detail. It's much appreciated. Ben - Original Message - From: Markus Metz markus.metz.gisw...@googlemail.com To: Benjamin Ducke benjamin.du...@oxfordarch.co.uk Cc: GRASS developers list grass-dev@lists.osgeo.org Sent: Thursday, September 24, 2009 9:04:33 AM GMT +01:00 Amsterdam / Berlin / Bern / Rome / Stockholm / Vienna Subject: Re: [GRASS-dev] Some doubts about GRASS topology Benjamin Ducke wrote: Dear all, in an attempt to better understand the GRASS vector and topology model, I imported a set of 3 polygons from an ESRI Shapefile (see attachment). The polygon in the upper left has 4 holes (called islands for some reason by GRASS), the lower one consists of 3 parts (QGIS calls this a polygon with islands -- good to know we understand each other in the GIS world!). Yes, maybe the way the term islands is used in GRASS is a bit misleading. According to simple feature specifications, GRASS islands are (more or less, not sure if 100%) equivalent to holes. The third is a simple, convex shape. Displaying the imported map shows all geometries exactly as it should. So far so good. Now, when I run v.info on the imported map, I get: Number of lines: 0 Number of boundaries: 9 Number of centroids: 5 Number of areas: 9 Number of islands: 9 This completely baffles me! The GRASS documentation consistently states that an area is a boundary + a centroid + any number of islands. That's an error in the documentation. An area is a closed ring of boundaries (can be only one boundary) + any number of islands (holes) within + *optionally* an attached centroid. An area without centroid can not have a category but as far as topology is concerned, it's a valid area. Now, assuming that the lines around the four islands count as boundaries, I understand why there are 9 boundaries altogether. 5 centroids also check out, given that there is no 1:1 equivalent for a shapefile multipart polygon in GRASS. But how in the (GRASS) world can there be 9 areas if there are only 5 centroids? See above, an area in GRASS topology does not need to have a centroid attached. And why 9 islands? Every area is also an island if no boundary is shared with another area. If a boundary is shared with another area, these two areas together form one island. In your example, the area in the upper left with the four islands: the four islands are also areas, but without centroid attached. When attaching islands during topology building, the internal IDs of all islands falling inside the outer area are added to the topology information of that outer area. If one of these four islands would share a separate boundary with two other islands each, and only one islands would be completely isolated, that thing in the upper left would still consist of five areas (four inside, one outer), but of only three islands, one consisting of three connected areas, one for the remaining isolated inside area, one for the outer area. When building topology, areas and islands are constructed first, islands are not yet attached to areas. Only in the next step are islands attached to areas, areas get holes. In the last step, centroids are attached to areas, or more precisely: for each area a not yet attached centroid is searched for, if found attached, if already attached, it's a duplicate centroid. There may also be several centroids falling inside the current area and only inside this area, these will also become duplicate centroids. AFAICT, GRASS vector topology is very much based on simple feature specifications, but not strictly, it deviates here and there in the usage of terms and in the methods to build topology. The methods are not a problem, they are consistent even though not 100% following simple feature specifications, but the usage of terms can be confusing, particularly with misleading documentation and sometimes different meanings in closely related applications (QGIS). Markus M -- Files attached to this email may be in ISO 26300 format (OASIS Open Document Format). If you have difficulty opening them, please visit http://iso26300.info for more information. ___ grass-dev mailing list grass-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-dev
Re: [GRASS-dev] Some doubts about GRASS topology
Benjamin Ducke ha scritto: Well, that clarifies it (finally)! Thanks very much for taking the time to write up all this detail. It's much appreciated. Ben Hi Ben. Would you mind documenting this a bit? It would be good to have in the official grass-doc. Thanks. -- Paolo Cavallini: http://www.faunalia.it/pc ___ grass-dev mailing list grass-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-dev
Re: [GRASS-dev] Some doubts about GRASS topology
Hi Paolo, I am working on it, but will be busy with other things for the next two or three weeks. Hopefully towards the end of the month I will be able to focus on GRASS and its vector model again. I will post something on the Wiki as soon as I feel that I have fully grasped all details. We can then discuss the text and add it to the official GRASS docs when its quality will be good enough. Cheers, Ben - Original Message - From: Paolo Cavallini cavall...@faunalia.it To: Benjamin Ducke benjamin.du...@oxfordarch.co.uk Cc: GRASS developers list grass-dev@lists.osgeo.org Sent: Thursday, September 24, 2009 2:11:15 PM GMT +01:00 Amsterdam / Berlin / Bern / Rome / Stockholm / Vienna Subject: Re: [GRASS-dev] Some doubts about GRASS topology Benjamin Ducke ha scritto: Well, that clarifies it (finally)! Thanks very much for taking the time to write up all this detail. It's much appreciated. Ben Hi Ben. Would you mind documenting this a bit? It would be good to have in the official grass-doc. Thanks. -- Paolo Cavallini: http://www.faunalia.it/pc -- Files attached to this email may be in ISO 26300 format (OASIS Open Document Format). If you have difficulty opening them, please visit http://iso26300.info for more information. ___ grass-dev mailing list grass-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-dev
