[gdal-dev] Fast Pixel Access

[Jukka Rahkonen]

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

> 
> 
> 
> 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? 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.

I was experimenting with something like a "GIS service without a GIS server)
and I have some examples online but because of http connection the speed
comparison does not make sense.

Vrt combining biomass data from 13 single band tif files
gdallocationinfo
/vsicurl/http://latuviitta.kapsi.fi/data/metla/puuston_tilavuus.vrt -geoloc
 389559 6677412

DEM of Finland with 10x10 m grid through vrt
gdallocationinfo /vsicurl/http://latuviitta.kapsi.fi/data/dem10m/dem_10m.vrt
-geoloc  389559 6677412

The same from a single BigTIFF
gdallocationinfo /vsicurl/http://latuviitta.kapsi.fi/data/dem10m/dem_10m.tif
-geoloc  389559 6677412

Feel free to download the originals if you want, they are all made from open
data. Just mention the National Land Survey of Finland, 2013 for the DEM and
Finnish Forest Reserch Institute, 2013 for the biomass data if you publish
data somewhere. The DEM datasets are about 10 GB each (Bigtiff + the
original small ones).

My tiffs have tiles but for this usage where only the value of a single
pixel is interesting striped tiffs could be as fast to read than tiled
tiffs. A trial would tell everything

-Jukka Rahkonen-