Geographic Information Systems Stack Exchange is a question and answer site for cartographers, geographers and GIS professionals. It only takes a minute to sign up.
Sign up to join this community
I have a very large feature layer consisting of polygons of different sizes. I need to select or subset a certain amount of features (say I need 4 polygons of size x, 6 polygons of size y, and 10 polygons of size z).
The problem is that I can't use the subset feature because I need these selection to be all bundled together, sharing at least one vertex, so that it is a somewhat representative sample of what the larger dataset looks like.
Any suggestions?
This is extremely challenging thing to do.
This is what I suggest:
Add field P2013 and populate it like that, using 3 classes of areas:
Important is proportion of polygons count in each class, it should be 4:6:10, i.e. exactly as shown on artificial dataset in the picture above. If this is not the case you need to weed some polygons on the edges. So compute total of P2013 (P) and do modulo division:
P % 1064
The answer is to be converted to number in each class subject for removal. For example if it is 35, you need to remove 5 from first class and 3 from second, or, perhaps 35 from 1st class. Number of 1064 came from:
4*1+6*10+10*100
I applied method described here to split 200 polygons into 10 groups. Results:
As one can see I've managed to locate only 2 groups, that are exact match to the criteria:
Method does not guarantee that you find ideal candidates (depending on a spatial mixture of your classes), but at least it will give you few, that are very close to it, similar to what table shows.
INon subquery of IDs in a lookup table. And third is a Query Definition with a simpleINlist of IDs. The table should be indexed on the query term.