[GRASS-user] about resampling DEMs and DEM-derived parameters

[Glynn Clements]

Carlos Grohmann wrote:

> The other day something popped on my mind, which I think could end
> in some research paper, but before I venture into it I decided to
> ask you guys about it. I did some googling and got nothing.
> 
> So, considering a large area DEM: if I want its derivatives (slope,
> aspect, etc) in large-scale (larger than the original DEM), which
> one is best, to resample the DEM and then calculate the derivatives,
> or to calculate the derivatives and resample them? Much difference
> expected? The same?

You get better results if you know what you're actually trying to
measure. The concept of a derivative is only meaningful for a
continuous surface, which a DEM isn't. Do you want the actual
derivative at a specific point (which may be the side of a rock), or
the slope of some "best-fit" plane?

It also helps if you know what the DEM represents. E.g. is the
elevation a sample at a specific point within the cell, or an average
over the area of the cell?

As a general guide, you probably want to apply some form of low-pass
filter to the data, so that the derivatives at each point are
calculated for a local best-fit curve.

7.0 has r.resamp.filter, which performs resampling with an analytic
(continuous) kernel. In 6.x, you can obtain similar results using a
combination of upsampling, filtering with r.neighbors or r.mfilter.fp,
then downsampling.

-- 
Glynn Clements <glynn at gclements.plus.com>