[postgis-users] PostGIS analogues to ArcInfo geoprocessing functions

[Paul Ramsey]

Well, for sparse vs non-sparse, Martin's still-dreamy
PreparedIntersection() enhancement might actually make a good effect.

P

On Thu, Jan 29, 2009 at 5:44 PM, Chris Hermansen
<chris.hermansen at timberline.ca> wrote:
> 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
>>>>>> well-understood
>>>>>> 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
>>>>>> existing
>>>>>> 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).
>>>>>
>>>>> http://grass.itc.it/grass64/manuals/html64_user/v.overlay.html
>>>>>
>>>>> P.
>>>>> _______________________________________________
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>>>>>
>>>>>
>>>
>>>
>>>
>>
>
>
> --
> Regards,
>
> Chris Hermansen         mailto:chris.hermansen at timberline.ca
> tel+1.604.714.2878 · fax+1.604.733.0631 · mob+1.778.232.0644
> Timberline Natural Resource Group · http://www.timberline.ca
> 401 · 958 West 8th Avenue  · Vancouver BC · Canada · V5Z 1E5
>
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