[gdal-dev] Fast Pixel Access

[Even Rouault]

Le samedi 01 février 2014 00:23:13, Brian Case a écrit :
> evenr
> 
> 
> what about the use of a tileindex?

You really mean a tileindex as produced by gdaltindex ? Well, that's not 
exactly the same beast as a VRT, but yes if it was recognized as a GDAL 
dataset then you could potentially save the cost of XML parsing. One could 
imagine that the VRT driver would accept a tileindex as an altenate connection 
string.

Anyway it would be interesting to first profile where the time is spent in David 
use case. If it's in the XML parsing, then I can't see what could be easily 
improved in that area. If it's the intersection, then there's potential for 
improvement.

> seems an intersection with a set of
> polys first would be quick


> 
> brian
> 
> On Fri, 2014-01-31 at 19:30 +0100, Even Rouault wrote:
> > 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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