lasheight

[Gordon Frazer]

Hi Martin,

 

The following is a likely scenario faced by lidar analysts and forest scientists interested in computing accurate canopy heights for lidar data collected in time series over the same region. In some situations, you may have previously developed or been given a DEM that was painstakingly built to produce a very low RMSE. Rather than constructing a separate DEM from each new lidar acquisition, you’d like to reuse an existing DEM of known accuracy to compute canopy heights for all repeat lidar acquisitions. These repeat lidar datasets may have been collected at a much lower point density strictly to support a forest inventory update or to assess forest growth and site productivity, and therefore not optimal for DEM construction. Is it possible for lasheight to compute canopy heights based on a pre-existing DEM stored as a regularized grid or TIN?

 

Thanks,

 

Gordon

 

p.s., thank you for the modifications to lastile – wonderful!

 

[Etienne B. Racine]

Gordon, maybe a quick fix could be to vectorize your DEM (using countours or points -- you'll need points ultimately) and merge it with your other file to compute lasheight ?

Etienne

2012/3/28 Gordon Frazer <gfr...@islandnet.com>

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[Alicia]

Hi Gordon,

The situation you described can be handled within FUSION, you can use a DEM you have already built with different lidar acquisitions. You might check out the documentation for that, I know this is meant for lastools, but that might address your issue if there is not an option within lastools.

Hope that helps,

Alicia

--

[Martin Isenburg]

Hello Gordon,

one option that comes to mind is make use of the fact that as of the
120327 release all LAStools can read ESRI's ASCII rasters *.asc as if
they were LiDAR point files. So simply convert your current raster DEM
(the one that was painstakingly built to have a low RMSE) to the *.asc
format and run

lasheight -i lidar.laz -ground_points dem.asc -replace_z -o
lidar_above_dem.laz

cheers,

Martin @lastools

[Gordon Frazer]

Thanks to all for helpful suggestions. I knew that Martin would have a
more direct solution hidden up his sleeve. Yes, that would work
perfectly!

Thanks again,

Gordon

[Mike Gleason]

I've been reviewing the lasheight documentation and I'm wondering if
Martin's proposed solution might provide slightly different results
than expected. The documentation suggests that the input -
ground_points will first be triangulated and then heights will be
calculated off of the resulting TIN. This makes sense when the ground
points themselves are lidar (or other spot heights), but not
necessarily when they are based on a pre-built DEM. In the latter
case, the tool would essentially re-interpolate a DEM that was already
built to represent the best possible model. I'm not sure exactly what
the effect would be or if it would be desirable. Nonetheless, it would
most likely be preferable to calculate the height of each point
directly from the "ground elevation" of the containing DEM cell.

Best,
Mike