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

[Steve Lime]

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.

Steve

Stephen Lime
Data & Applications Manager

Minnesota DNR
500 Lafayette Road
St. Paul, MN 55155
651-297-2937

>>> "Norman Vine" <nhv at cape.com> 08/01/02 10:29AM >>>
Jan Hartmann writes:
>
>Yes, I understand that it can be impossible to identify a 
>unique polygon for
>a point query, both in practice and theory. In that case, 
>however, I would
>prefer to have both polygons returned to me, instead of just 
>one chosen at
>random. At least I know that something IS indeterminate then.

Jan

I think that this is one of those attributes of a GIS that Mapserver
doesn't necessarily try to do for 'performance' reasons, also there
may be more then 2 poylgons sharing a point further complicating
the solution.

AFAIK many GIS applications will not give a determinate answer
for a simple point in poly request either,  however they will return
a list of polygons intersected by another polygon. 
  i.e. a point with area

FYI
I believe that there is work underway to include 'polygon algebra'
functionality in PostGIS which will allow you todo this kind of query 
see
http://www.vividsolutions.con/jts/jtshome.htm 

Cheers

Norman