[gdal-dev] Fast Pixel Access

[Jukka Rahkonen]

David Baker (Geoscience <david.m.baker <at> chk.com> writes:

> 
> Jukka,
> 
> Jukka wrote:
> >I was experimenting with something like a "GIS service without a GIS server)
> >and I have some examples online but...
> 
> I am looking to do as you have, a RESTful service to query the elevation
at a given location.  This will be used
> to in a DQM process as well as a geologic application that needs the
elevation of a proposed wellsite for
> data mining.  In both cases, 1000's if not tens of 1000's of calls will be
made so performance is an issue.

I don't believe that gdallocationinfo is the right thing for you. There are
only 60 seconds in a minute and our experiments show that each request takes
seconds or even tens of seconds. Your users will hang you.

Now what to do instead? A WMS/WCS service can send a piece of DEM with
thousands of pixels as GeoTIFF in a second and a heavy client like QGIS
could continue the analysis. Or you can use the existing vrt file and read
the region of interest with gdal_translate. This request is not very fast
either but it brings you one million height values faster than possible
users could find the rope
gdal_translate -of GTiff -srcwin 10000 10000 1000 1000
/vsicurl/http://latuviitta.kapsi.fi/data/dem10m/dem_10m.vrt test_1000by1000.tif

Or then WFS service could be used to send only thos DEM cells which
intersect the requested point/line/area. That could be nice for further
processing  but in that case your DEM should be converted into points or
polygons. I wonder if PostGIS raster has something to give for you. Or
perhaps there is some other clever way to do the job with light and powerful
tools. Ideas are welcome.

-Jukka Rahkonen-