[pdal] DEM Grids - p2g bounding box

[pete.gadomski at gmail.com]

Matthew,




We added the ability to specify a user-defined grid in Points2Grid a few months ago, thanks to work by Tristan Dyer. https://github.com/CRREL/points2grid/commit/b2ada6abfc85560ec10fb4f30333a1bf95be9025. Is that what you had in mind?




-Pete

On Tue, Mar 24, 2015 at 10:41 AM, Matthew Hanson
<mhanson at appliedgeosolutions.com> wrote:

> We've run into another little snag trying to create DEMs.    I made a
> commit last month to fix the half pixel offset in p2g output:
> https://github.com/PDAL/PDAL/commit/ea67d14f43b09e591ef94c7f03ef75d64dc786ed
> Which is correct, the output is not correctly aligned to how it's doing the
> gridding, however the gridding itself isn't really ideal.   There are 2
> problems:
> 1) p2g uses the upper left point (min x, max y) as the *center* of the top
> left pixel.   So this means that there are going to always be fewer points
> to choose from when on a corner of edge of the resulting raster.   How many
> fewer points depends on the search radius.    While this depends on the
> output of the raster (e.g., min, max, idw) this might not be noticeable at
> all, but there are certainly some edge effects from this.  Granted if p2g
> used half a pixel in from the minx/maxy point it could still suffer from
> this problem if given a search radius > 0.5 pixel, but I think we can agree
> that the using the top left available point as the center of the pixel is
> not the best way.
> 2) Related to the above problem is the more general one that the raster
> bounding box is completely reliant on the input point cloud.    When using
> point clouds that have been tiled, or otherwise have specifically chosen
> bounds (but perhaps points do not go all the way to the edge) the output
> raster may not have the same bounding box.   However, the even greater
> problem is that if you create a DTM from ground points and a DSM from
> non-ground...well, that top left point is going to be different between
> those and therefore you get two DEM rasters that are potentially on
> completely different grids.
> What is really needed, and would solve both issues in a general way, is to
> be able to specify the grid that should be used (really this just means
> being able to give it an exact top left coordinate).    This would allow
> alignment of DTM and DSM without requiring postwarping (really not
> desirable).
> It looks like this would mostly be a change within p2g to take in a
> starting x,y coordinate, and in PDAL the addition of another argument to
> pass to OutCoreInterp.
> Does this seem like desirable behavior?
> Matthew Hanson
> Applied GeoSolutions
> (603) 659-3363 x91
> http://appliedgeosolutions.com
> mhanson at appliedgeosolutions.com
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