# [postgis-users] line simplication

Paul Ramsey pramsey at refractions.net
Wed Jan 8 09:54:50 PST 2003

```There should be lots of examples in the source for prospective
developers of this. Check the length functions for examples of using
vertices from features, and check the constructor functions for example
of returning geometries as a function result. Given that info, some C
knowledge, and a paper on D-P, you should be off to the races.

P.

strk wrote:
> I think postgis should do this. I'd like helping with it.
>
> For the sorrouding environment problem:
>
> "The use of spatial context in linear feature simplification"
> http://www.geovista.psu.edu/sites/geocomp99/Gc99/020/gc_020.htm
>
> --strk;
>
> andy wrote:
>
>>Douglas Peucker is a useful algorithm, but you need to be careful with
>>it in a mapping setting. The algorithm basically removes points from a
>>linestring that don't change it too much.
>>
>>The problem is that the algorithm has no knowledge of the surrounding
>>environment. Intersections are a good example. If you have a normal
>>intersection of two street segments, moving the point of intersection a
>>little doesn't harm the map much.
>>
>>However, D-P may take T-intersection and transform it either into two
>>disconnected lines or two lines that cross each other. Neither of these
>>looks correct visually and may cause other problems in interpretation.
>>
>>Also, D-P operates in a cartesian space and applying it to raw lat/lon
>>data probably wouldn't work right.
>>
>>On Mon, 2003-01-06 at 16:47, Paul Ramsey wrote:
>>
>>>Precisely correct (I think). The best way to the desired result should
>>>be to implement a Douglas simplification function, and then create a
>>>view which (a) uses attributes to reduce to just the features of
>>>interest (interstates, etc) and (b) uses the simplify() function to
>>>remove unnecessary vertices from the features.
>>>
>>>P.
>>
>>
>>
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>
>
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```