I am facing a dilemma while trying to use v.generalize to simplify polygons which are similar in size and structure to US Counties or States. Often, I lose polygons from the data set after running either a simplification or smoothing method on them. What troubles me is that there does not seem to be any consistent pattern to this behavior. In some cases the same set of commands applied to different data sets causes causes completely different results. As a result, I am unable to develop a repeatable workflow and am reluctantly starting to look for other simplification tools that preserve topololgy. Here is an example:
# LOAD AND CLEAN v.in.ogr dsn=counties.shp output=county v.build.polylines input=county output=county_polylines v.clean input=county_polylines output=county_clean tool=bpol,rmdupl type=boundary # SMOOTH AND GENERALIZE v.generalize input=county_clean output=county_smooth method=sliding_averaging look_ahead=7 slide=1 v.generalize input=county_smooth output=county_general method=douglas threshold=.005
Each of the v.generalize steps results in some of the polygons disappearing. I'd like to understand why this happens. Is it due to features being loaded into insufficient memory? Are my parameters "tight" enough to cause features to be excluded because they've met some sort of geometry criteria? (Note that I'm losing some very large features.) Is there a problem with the topology of the dropped features which causes them to get erased? Related to that last question, I assume that build.polylines and clean operations that I do initially should take care of this, but I'm not entirely certain, and I can't seem to find good instructions that address this. Are there any?
Appreciate any insights people can give, especially those based on personal experience with the v.generalize tool.