[mapserver-users] Point inside or outside polygon???

[Jan Hartmann]

Steve Lime wrote:

> And MapServer can do this to. Just use mode=nquery and give your polygon
> layer a non-zero tolerance. The point query capability was originally
> built for web apps where a user clicks on a feature (imagine that) and
> this is really a non-issue. If you think about how hard it is to
> actually come up with a point exactly on a line segment for most
> purposes the current setup it fine, but not foolproof but as mentioned
> about there are workarounds. For a discussion of the current algorithm
> and alternatives check out subject 2.03 in the comp.graphics.algorithms
> FAQ at www.faqs.org.


OK Steve, this solves most of my problem. And yes, I was thinking of a
non-interactive MapServer application. I'm working on a project to compute
maps and statistics for regions around the Dutch railroad and roads
networks, using buffers of different size (1, 2, 5, 10, 25 km etc).
Ultimately these should be rendered not only as static maps, but also as
Flash movies, showing what happens when a parameter changes. I can use his
University's Beowulf cluster, so lots of intermediate maps can be produced
to get flowing maps. I'm using MapScript and PostGis for this (it already
has a - non documented - distance function), and I was afraid of the kind of
instabilities in the algorithms mentioned by Norman Vine and Ed McNierny.
For interactive use this doesn't matter much, but with the repeated
computations required for a movie you can very easily get flip-flop
behavior. Of course I am very much interested in the topological extensions
to PostGis.

As for Paul Ramsey's words:

> Combining PHP with mapscript with postgis
> should allow some very complex web apps to be built with functionality
> which was previous considered desktop-GIS-only

I'd rather say the MapServer3.7/MapScript/PostGis-JTS combination will be in
the ArcInfo/ArcIMS/OracleSpatial range, while also offering the
accessibility of a desktop GIS. Not to mention the possibilities of linking
in with other Open Source projects like Beowulf, Grass and R for
multiprocessing, raster mapping and statistics. You already can combine
PostGis with Grass and R and use GDAL image files with R. This gives some
very interesting prospects for the near future!

Jan Hartmann
Department of Geography
University of Amsterdam
jhart at frw.uva.nl