[Gdal-dev] Numerical differences ogr -sql versus postreSQL

[Thomas Windholz]

Numerical differences in, for example, buffer:

 

I noticed numerical differences between an sql statement through the ogr library versus directly through postgreSQL (I thought the sqlexecute goes through?). Has anyone noticed these differences as well? In the following dataset these numerical differences produce sort of ears that are equal to the radius of the buffer (so not really negligible and quite visible).

 

Here is an example (other number combinations are less visible, but still produce errors):

 

Buffer of Point(3.0 -3.7) results:

 

C:\Program Files\FWTools0.9.9>ogr2ogr -f "ESRI Shapefile" -sql "SELECT buffer(Ge

omFromText('POINT(3.0 -3.7)'), 1.0,4)" buffer.shp "PG:user=postgres password=xxx 

dbname=xxx host=localhost port=5432"

 

In numbers:

4.0000000000000000,-3.6999999999999957

3.9238604590292496,-4.0820349338195641

3.6289817765446712,-4.2508567811706746

3.3826834323961990,-4.6238413855257363

3.0000000000437552,-4.7000000000000002

2.6173153755938636,-4.6238413855600982

2.2928932188610727,-4.2508567812341678

2.0761204675187801,-4.0820349339023796

2.0000000000000000,-3.7000000000716033

2.0761204674499845,-2.0673165677282341 
 ear 1 
 should be around 2.07.., -3.06..

2.2928932187340854,-2.9928932188928199

2.6173153754279466,-2.7761192754430666

2.9999999998653095,-2.6999999999999957

3.3826834322302819,-2.7761192753399788

3.6289817810754332,-2.9928932187023385

3.9238604586694903,-2.0673165674793625 
 ear 2 
 should be around 3.92.., -3.06..

4.0000000000000000,-3.6999999998204118

4.0000000000000000,-3.6999999999999957

 

Initially, I thought this might be a PostGIS or Geos issue, so I tested the same sql 

statement directly in PostgreSQL using:

 

SELECT AsText(buffer(GeomFromText('POINT(3.0 -3.7)'), 1.0,4))

 

"POLYGON((

4 -3.7

3.92387953251558 -4.08268343235472

3.70710678120242 -4.40710678117067

3.3826834323962   -4.6238795324984

3.0000000000449   -4.7

2.61731656768676 -4.62387953253276

2.29289321886107 -4.40710678123417

2.07612046751878 -4.08268343243768

2                              -3.70000000008979

2.07612046745006 -3.31731656772824

2.29289321873409 -2.99289321889282

2.61731656752084 -2.77612046753596

2.99999999986531 -2.7

3.38268343223028 -2.77612046743287

3.70710678107543 -2.99289321870234

3.92387953244686 -3.31731656747936

4                              -3.69999999982041

4                              -3.7

))"

 

The PostGIS buffer function did not reproduce the ears on the buffer. Also, 

overall there are some differences between the datasets created through ogr2ogr 

versus directly in PostgreSQL. The shapefile is not really an issue since I also tested reading the results directly through ogr ...

 

...any hints/help is appreciated!

 

Thanks,

Thomas

 


		
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