[gdal-dev] Fast Pixel Access

Even Rouault even.rouault at mines-paris.org
Fri Jan 31 10:30:02 PST 2014

Le vendredi 31 janvier 2014 17:15:53, David Baker (Geoscience) a écrit :
> Dev's,
> I have a set of 55,501 bil files in a single directory.  They are DEMS data
> that cover the US in 7.5 minute tiles.  I would like to randomly access
> elevations at a given lat/lon's from the whole dataset.  I created a vrt
> file from the directory of bil files, and have been able to access the
> elevation at a given lat/lon using gdallocationinfo, but because of the
> size of the dataset, this operation is somewhat slow.  Can the vrt be
> indexed?

No, it isn't currently, although I think it could be improved to have a in-
memory index with moderate effort.

But are you sure the slowness is due to the lack of index ? 55,000 is a big 
number, but not that big. Maybe the slowness just comes from the opening time 
(XML parsing) of such a big VRT. That would need to be profiled to be sure 
where the bottleneck is.

> Or, is there a faster, better way to access the pixels?  I would
> first like to do this with the utilities before diving into code (C#). 
> The files are regularly named base on their location within a 1 arc-second
> grid.
> Thanks,
> David
> David M. Baker
> Senior Advisor - Geoscience Technology
> Chesapeake Energy Corporation
> david.m.baker at chk.com<mailto:david.m.baker at chk.com>
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