[GRASS-dev] GSOC Horizon based stratigraphy

[Pierre Roudier]

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.


Cheers,

P