[GRASS-dev] GSOC Horizon based stratigraphy

Benjamin Ducke benducke at fastmail.fm
Thu Jun 27 02:41:06 PDT 2013

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).



> Cheers,
> P

Dr. Benjamin Ducke, M.A.
{*} Geospatial Consultant
{*} GIS Developer

   benducke at fastmail.fm

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