[pdal] DEM Grids - p2g bounding box

[Matthew Hanson]

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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