[gdal-dev] Buffer unit

[Jukka Rahkonen]

Iván Sánchez Ortega <ivan <at> sanchezortega.es> writes:

> 
> El Miércoles, 9 de octubre de 2013 07:33:43 Federico Jurio escribió:
> > Dear all, I am using WGS84 (4326) and would like to buffer based on meters
> > and not degrees. [...]
> > Can someone please point me in the right direction?
> 
> If you have a working PostGIS instance, you can get your geometries as
PostGIS 
> geographies, then run st_buffer on them. PostGIS will take care of the dirty 
> work of finding a more-or-less appropiate projection for buffering the 
> geography.
> 
> > [buffering in degrees] is the easy way and i'm sure that the accuracy isn't 
> > the best.
> > [reprojecting] probably doesn't have the best performance.
> 
> This begs the question: What do you want to achieve? Accuracy, or performance?
> 
> Personally, if PostGIS geographies aren't available, I'd rather reproject 
> whole geometries into WGS84 UTM (EPSG:326xx) depending on the centroid's 
> longitude, then buffer, then project back into WGS84 lat-lon (EPSG:4326).
> 
> Best,

Hi,

How about using all that on-the-fly with the amazing OGR Spatialite dialect?
 
ogrinfo -dialect sqlite 
-sql "select  
transform(buffer(transform(geometry,26915),10000),4326) as geometry2
from states limit 4" states.shp

So, transform 4 features from the Geoserver demo shapefile "states.shp" from
EPSG:4326 into 26915, create 10000 metre buffer while in 26915 and transform
the buffered features back to EPSG:4326.

Try it, it's fast. If you think that there are too many vertices in the
result, add "SimplifyPreserveTopology" into SQL. Buffer can sometimes make
self-intersections into the resulting polygon but that's no problem either
because you can correct them in-th-fly by adding "MakeValid" into SQL. The
hardest part is to keep trac of all the paranthesis :)))))

-Jukka Rahkonen-

-Jukka Rahkonen-