On 06/26/2013 06:48 PM, Pierre Roudier wrote:
Hi all,
This is an excellent point. While I like the mention of AQP in this context,
I totally support a GRASS-based implementation with as few dependencies as
possible.
+1 - I think a native GRASS implementation would make a lot of sense.
Yes, the thought of such "waffel voxels" is not exactly appealing.
However, they may be a smaller problem in practice, since the voxel
models themselves are often used to derive vertical slices
("profiles"), and those might look perfectly fine, even if derived
from malformed voxels. GRASS does allow for individual X, Y and Z
dimensions of voxels, so there is no technical problem with this.
The results of the interpolation don't need to be beautiful, they
just need to be as accurate and as true to the data as possible.
That's the very nature of soils data - we soil scientists often deal
with pixels of 10 to 500m resolution, to observe processes that occur
generally in the first meter in the z axis! It is not a problem, and
the challenge is to come up with tools that allow us to store, query
and interpolate such data.
This is a popular topic in the soils literature-- vertical anisotropy can be
an order of magnitude greater than what is found in the horizontal.
Restricted cubic splines have some desirable characteristics for dealing
with this kind of data-- however, these work best in the context of a
regression model. Also, there are the mass-preserving splines that are more
useful in the "interpolation along the soil profile" sense. For categorical
data, I would recommend the ordinal-ratio logistic regression model, which
generates class-wise probability estimates. I have found this quite useful
for generating probability depth-functions for categorical soil properties.
I can elaborate as needed.
The mass-preserving splines has become a key tool in the GlobalSoilMap
project. An implementation in R exists but is not very efficient. This
could be an opportunity to come up with a reference implementation! As
mentioned by Dylan, various interpolation methods are available,
restricted cubic splines look good as well.
But is that method suitable for categorized input data?
Or does it only work for continuous soil properties?
A spline-based interpolator from 3D vector to 3D raster
already exists in GRASS (v.vol.rst).
Best,
Ben
Cheers,
P
--
Dr. Benjamin Ducke, M.A.
{*} Geospatial Consultant
{*} GIS Developer
[email protected]
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