[postgis-users] PostGIS analogues to ArcInfo geoprocessing functions
chris.hermansen at timberline.ca
Thu Jan 29 17:44:06 PST 2009
I too would hope for someone to spend some quality time on this
problem. Here are a few other related thoughts:
* GOAI (that's Good Old ArcInfo, you know the one with AML, not the
one that only runs on that funny O/S from WA) has pretty archaic
geoprocessing stuff built in, generally. For example, CLEAN
(which will sometimes take a collection of points and linestrings
and form a polygon network) has some serious limitations on the
amount of stuff it can process and its ability to deal with small
infelicities in the data. Also, I don't think the code for UNION
has changed much since, oh, 1987 or so... or maybe before that
even. I wonder if it's still FORTRAN...
* I don't know if there is a canonical use case for the geometric
union of two polygon themes. however, it's often used to overlay
a sparse theme of "areas of interest" or "planning units" or
"special habitat zones" or "steep slopes" over a non-sparse theme
of "forest" or "soils". In the GOAI context of course this is
done on a map-sheet by map-sheet basis, which is a wonderfully
rudimentary form of spatial indexing. nevertheless, the sparse
polygons are probably of similar size to the non-sparse polygons,
so a given "steep slope" polygon might overlap a few, or a few
tens of, "soils" polygons. Lots of opportunity to reduce the
amount of intervention required in these cases, rather than
blindly unravelling all the polygons from both themes into topology.
* In the case of two non-sparse inputs, st_intersection() works as
well as a geometric union would, so another possibility is to
"complete" the sparse polygon layer with a "world polygon" and
then use st_intersection()
Maybe in my copious spare time I'll come up with a couple of test data
sets and do some testing on them...
Martin Davis wrote:
> Chris, generally I agree (especially if the code is written in
> Java... 8^). After I posted this I started thinking that probably a
> few more minutes/hours/days splunging through the C code would no
> doubt reveal much about how it actually works. At the moment however
> I can't even tell if it is memory-only or whether it uses external
> files to hold intermediate results!
> I look forward to some cleverer programmer than me figuring out how to
> integrate the Grass code with PostGIS...
> Chris Hermansen wrote:
>> Martin, Martin, Martin: code is the best documentation there is! and
>> besides, it's not written in Perl so how bad can it be :-)
>> Martin Davis wrote:
>>> Have you looked at the code? I rest my case....
>>> Paul Ramsey wrote:
>>>> On Thu, Jan 29, 2009 at 8:42 AM, Martin Davis
>>>> <mbdavis at refractions.net> wrote:
>>>>> The basic approach to computing polygon overlays has been
>>>>> for a long time (although this does not imply well-documented!). The
>>>>> implementation however is quite tricky, especially if performance and
>>>>> robustness is required. There is a notable lack of open-source
>>>>> implementations - which should be an indication of just how
>>>>> difficult this
>>>>> really is.
>>>> I believe the vector component of GRASS includes overlay (and of
>>>> course the raster component does too).
>>>> postgis-users mailing list
>>>> postgis-users at postgis.refractions.net
Chris Hermansen mailto:chris.hermansen at timberline.ca
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