[Gdal-dev] downsampling shapefiles ?

[Ari Jolma]

Chapman, Martin kirjoitti:

>I agree.  It would be nice to be able to render fewer points when my map
>is zoomed way out.  I think arcmap does that kind of thing.  How does a
>geometry simplification algorithm work?  Does it look for linear
>patterns in the geometry and then somehow eliminate points based on some
>sort of angle/distance tolerance?
>  
>

This might be one of the best available proprietary technologies: 
http://www.idevio.com/ravegeoinfo.htm

It is interesting that there is no references on this website. I made a 
quick search for these algorithms and this quote is telling: "There 
exists many different approaches and plenty of methods." (Andujar. 1999 
Geometry simplifacation.) and this is just for curve simplification. For 
curve simplification I glanced at this: "Near-Linear Time Approximation 
Algorithms for Curve Simplification in Two and Three Dimensions. by 
Agarwal et al 2002." and it is difficult for me to understand quickly 
what is the principle but the algorithm is there and it has been 
implemented at least by this guy: http://www.cs.duke.edu/~wys/curvesimp/

I think the first question to ask is: precompute the simple geometries 
or compute them in real-time? The problem is also not just point 
elimination, for example you might want to collapse several linestrings 
or polygons into one linestring or polygon. But I see the need when I 
look at how slowly large vector datasets are rendered.

Ari

>Martin
>
>-----Original Message-----
>From: gdal-dev-bounces at lists.maptools.org
>[mailto:gdal-dev-bounces at lists.maptools.org] On Behalf Of Frank
>Warmerdam
>Sent: Friday, September 09, 2005 8:16 PM
>To: Christian G. Tveen
>Cc: gdal-dev at lists.maptools.org
>Subject: Re: [Gdal-dev] downsampling shapefiles ?
>
>
>On 9/9/05, Christian G. Tveen <cgt at navicon.dk> wrote:
>  
>
>>Hi,
>>
>>I have a number of very fine grained shape files. Is it possible to 
>>have gdal in some way downsample these files to be more usable for 
>>overview purposes ?
>>    
>>
>
>Christian, 
>
>No, there is no support for geometric simplification in OGR.  I just
>checked GEOS's API documentation and I don't see anything there either.
>I think I have heard there is something in PostGIS for generalization
>but I didn't check into it. 
>
>I do think a "textbook" geometry simplification algorithm would be an
>asset to GEOS, and I would be willing to expose it in some fashion
>through ogr if it were available. 
>
>Best regards,
>  
>


-- 
Prof. Ari Jolma
Kartografia ja Geoinformatiikka / Cartography and Geoinformatics
Teknillinen Korkeakoulu / Helsinki University of Technology
POBox 1200, 02015 TKK, Finland
Email: ari.jolma at tkk.fi URL: http://www.tkk.fi/~jolma