Re: [GRASS-user] v.generalize for polygons?
Hamish, I played around with the map you gave me, and I think I found a way around the problem (though I'm still not very sure about what the real problem is, perhaps too many small areas or too many shorter segments (the boundary seems to be built of a number of smaller lines). I'll try to create another problematic map. Anyway, here is how I was able to generalize it: # First I fuse the short segments into one long boundary v.build.polylines --o input=rc_merge_coast3 output=fused # Then I clean away some (relatively) small islands v.clean --o in=fused out=clean tool=rmarea thresh=50.0 # finally generalize v.generalize -r --o input=clean output=gen method=douglas_reduction reduction=20 The generalized map contains about 20% of the original points. --Wolf On 28.02.2008 06:16, Hamish wrote: Hamish: I have a high-res vector area map of regional districts which I wish to generalize. I am having trouble with finding the correct method in v.generalize to use. Currently every thing I try tends to break the area topology and leave only a portion of the now- open boundary. I have now tried with a related vector, linked below, and it worked (very!) nicely for that. But it fails with a derivative vector map. v.digit shows no problems with topography. Wolf: What methods did you try? many of them.. mainly douglas with a number of threshold values. What exact commands have you tried that fail? at the simplest: v.generalize in= out= but some areas are missing. Can you share the problematic map? (you can email it to me directly) sure, starting with: http://www.stats.govt.nz/statistics-by-area/regional-statistics/geography-mapping/download-digital-boundaries.htm -- Census based NZMG 2006 (37mb shapefile .zip) I am looking at regional boundaries (RC) from REGC06_LV2.shp this map generalizes nicely, but it includes the 12 nautical mile territorial buffer around the coastline. When I overlay that map with a detailed coastline is when I see the problem. I'll send a sample of the v.overlay output off-list. v.generalize does preserve nodes, and as long as the input map is topologically correct so should the output map be. ok. (confirmed, it does a very nice job simplifying the above shapefile) Perhaps your threshold is way off? Possible, as I am just learning. But I did try a number of ranges and slowly increase. All would be ok for slight generalization then big breakage. e.g. it has a big jump between thresh=0.4865 and 0.487 Good: v.generalize in=rc_merge_coast3 out=rc_gen thresh=0.4865 --o ... Number of vertices was reduced from 569815 to 521969 [91%] Bad: v.generalize in=rc_merge_coast3 out=rc_gen thresh=0.487 --o ... Number of vertices was reduced from 569815 to 336380 [59%] Daniel: However, there is a flag(-r?) which prevents the module from removing them. Flags: -c Copy attributes -r Remove lines and areas smaller than threshold thanks, Hamish Looking for last minute shopping deals? Find them fast with Yahoo! Search. http://tools.search.yahoo.com/newsearch/category.php?category=shopping -- :3 ) Wolf Bergenheim ( 8: ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] v.generalize for polygons?
Hi, I am not sure that I understand correcly what you are trying to do, but as far as I remember, the boundaries are generalized as polylines. That is, if the boundary contains some corners, they might be removed which might create the holes. Also, ovesimplified lines are removed by default. These are the lines that are shorter than the threshold. However, there is a flag(-r?) which prevents the module from removing them. Daniel On Wed, Feb 27, 2008 at 2:19 AM, Hamish [EMAIL PROTECTED] wrote: Hi, I have a high-res vector area map of regional districts which I wish to generalize. I am having trouble with finding the correct method in v.generalize to use. Currently every thing I try tends to break the area topology and leave only a portion of the now-open boundary. i.e. it should preserve nodes, only generalize (remove) non-node vertices. We can assume there is only a single boundary line between areas, so there shouldn't be topological issues. (or at least only in tiny corner cases where the new generalized line overlaps another feature, but that should be easy to fix with v.clean) the idea is to simplify the map before running v.extrude to get something like this: http://grass.osgeo.org/grass60/screenshots/images/inc_employ_usa_2002.jpg but without the 600,000 extruded faces created from every little twist in the coastline. Hamish ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] v.generalize for polygons?
Hamish: I have a high-res vector area map of regional districts which I wish to generalize. I am having trouble with finding the correct method in v.generalize to use. Currently every thing I try tends to break the area topology and leave only a portion of the now- open boundary. I have now tried with a related vector, linked below, and it worked (very!) nicely for that. But it fails with a derivative vector map. v.digit shows no problems with topography. Wolf: What methods did you try? many of them.. mainly douglas with a number of threshold values. What exact commands have you tried that fail? at the simplest: v.generalize in= out= but some areas are missing. Can you share the problematic map? (you can email it to me directly) sure, starting with: http://www.stats.govt.nz/statistics-by-area/regional-statistics/geography-mapping/download-digital-boundaries.htm -- Census based NZMG 2006 (37mb shapefile .zip) I am looking at regional boundaries (RC) from REGC06_LV2.shp this map generalizes nicely, but it includes the 12 nautical mile territorial buffer around the coastline. When I overlay that map with a detailed coastline is when I see the problem. I'll send a sample of the v.overlay output off-list. v.generalize does preserve nodes, and as long as the input map is topologically correct so should the output map be. ok. (confirmed, it does a very nice job simplifying the above shapefile) Perhaps your threshold is way off? Possible, as I am just learning. But I did try a number of ranges and slowly increase. All would be ok for slight generalization then big breakage. e.g. it has a big jump between thresh=0.4865 and 0.487 Good: v.generalize in=rc_merge_coast3 out=rc_gen thresh=0.4865 --o ... Number of vertices was reduced from 569815 to 521969 [91%] Bad: v.generalize in=rc_merge_coast3 out=rc_gen thresh=0.487 --o ... Number of vertices was reduced from 569815 to 336380 [59%] Daniel: However, there is a flag(-r?) which prevents the module from removing them. Flags: -c Copy attributes -r Remove lines and areas smaller than threshold thanks, Hamish Looking for last minute shopping deals? Find them fast with Yahoo! Search. http://tools.search.yahoo.com/newsearch/category.php?category=shopping ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
[GRASS-user] v.generalize for polygons?
Hi, I have a high-res vector area map of regional districts which I wish to generalize. I am having trouble with finding the correct method in v.generalize to use. Currently every thing I try tends to break the area topology and leave only a portion of the now-open boundary. i.e. it should preserve nodes, only generalize (remove) non-node vertices. We can assume there is only a single boundary line between areas, so there shouldn't be topological issues. (or at least only in tiny corner cases where the new generalized line overlaps another feature, but that should be easy to fix with v.clean) the idea is to simplify the map before running v.extrude to get something like this: http://grass.osgeo.org/grass60/screenshots/images/inc_employ_usa_2002.jpg but without the 600,000 extruded faces created from every little twist in the coastline. Hamish ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
