Hi there. I'm a fairly new user of PostGIS, but I think I finally have
it figured out and I'm finding it *very* useful and productive.
Now I'm ready to tackle one of my larger problems. I'm working with a
cadastral layer and forecasting future development patterns. I have a
need to simulate subdivision, where a large vacant parcel (say, 640
acres) will realistically be developed in the future in smaller chunks
(say, 1 acre). Right now I ignore the spatial part of it, and end up
having multiple future "pseudo-parcels" associated with one current-year
parcel. So, for instance, if the simulation has two 1-acre development
events in year 2010 and one 1-acre development event in year 2012, all
occurring on the same original 640 acre parcel, then in year 2013 I have
4 records associated with the original 640 acre parcel: a 637 acre
record that is still vacant, and three 1 acre records representing the
development that has occurred. All four of these records are still
associated with the same polygon.
For the general predictive power of the model and for the mathematics
behind it, this is ok, at least for now. But for visualization, this
isn't ok. For visualization, I'd like a separate (non-overlapping)
polygon for each of these records. I need an algorithm that takes off
an arbitrary 1acre chunk of the large parcel each time a development
event is simulated for it.
The algorithm I'd like to use (unless someone suggests a better one) is.
1) find the east edge of the bounding box for the polygon.
2) draw a north-south line that is sqrt(43560) ft to the west of the
east edge of the bounding box, and consider the areas on either side.
3) if the area on the east of the line is less than 1-acre, move the
line west until the area on the east of the line is exactly one acre,
done (stop).
4) else, draw an east-west line through the polygon far enough south so
that the area north-east of the intersection of the two lines is exactly
1 acre.
I'm pretty sure this algorithm will grab 1-acre chunks of the original
parcel starting in the north-east, and working down the east side, and
when the east side is completely separated, it will start again at the
new north-east corner and work down that side. So the parcel will be
slowly "subdivided" as development occurs from the east to the west in
strips. Since there are 43560 sqft in an acre, most of the chunks will
be square. (If the original polygon is non-convex, then the subdivided
pieces may end up being multi-polygons, but that's ok, I can deal with
multi-polygons, so long as the subdivision algorithm can handle it.
There will also be some rather long-and-thin slivers, I think; it would
actually be better to spiral around the parcel, working towards the
inside in a circle, instead of always coming from the east side. But...
I think this algorithm will work, and once it's working I can "tweak" it
later.)
Can anyone give me some pointers as to how to code this up in PostGIS?
I have a few clues as to how to draw a line sqrt(43560) ft from the east
side, but I don't have any clues about how to draw a line "far enough
south so that the resulting area is exactly 1 acre". I could try some
sort of trial-and-error bisection algorithm, I suppose, but I expect
there might be a much easier way.
Any and all comments appreciated, thanks
--
John Abraham
[EMAIL PROTECTED]
_______________________________________________
postgis-users mailing list
[email protected]
http://postgis.refractions.net/mailman/listinfo/postgis-users
