[Liblas-devel] Indexing for libLAS

[Mike Grant]

Hi Howard & Gary,

On 15/08/10 15:15, Howard Butler wrote:
> this week.  This email is to solicit feedback on the concept, as
> well as attempt to attract some ideas on what incorporating spatial
> indexing should mean for the design of libLAS.  How would you use a
> spatial index of LAS data?  Do you already have some experience with

We've written a Linux viewer to optimise our processing workflow (gotta
check the derivation - might even be virally GPLed :) )  It incorporates
a quadtree structure to allow us to use a tiling-type strategy for
bigger-than-RAM datasets.  Currently, building the quadtree structure
takes ages as it has to read all the points..  If liblas starts
including indexing functionality, we'd be interested in using some of
the features to improve startup / load time.

The main things we'd like are:
 - simple window queries
 - optionally, more complex windowing (transects using an arbitrarily
oriented rectangle / polygon intersection test)
 - direct access to a hierarchical structure (without loading points) so
we can see roughly where points are bulked and pick tiles
 - a way to query if a file has a stored index and a way to cause one to
be built (+ stored?)
 - the index to dynamically update if points are added (non-optimally
balanced tree may be ok, as people can rebuild the index from scratch
for optimal results)

Things that are possibly interesting in future but not directly relevant
for indexing:
 - we deal with multiple LAS files, so combining the trees would be
handy (this sounds hard!)
 - we don't currently do z binning, but may sort (z, t, flightline, ..)
points in a tile
 - we also implemented reduced resolution versions of the tiles and disk
caching ; the main relevance would be if liblas implemented caching -
it's useful to indicate which areas are in cache

Cheers,

Mike.

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