[GRASS-user] Recommendations for v.surf.rst parameters for huge region

[Eric Patton]

Hello list,

I am trying to interpolate raster values for a single massive dataset that
represents dozens of multibeam bathymetry surveys over a few decades.

The region is pretty big: all of Eastern Canada; at 100m resolution there are 
~ 464M cells. 

The raster data has a wide range of completeness; in some areas, there is
near 100% coverage - these areas were surveyed with modern sonars and 100%
overlap between survey lines. In other areas, the data is very sparse, with
~1km or more between tracklines. These areas would represent surveys from the
70's to 90's using singlebeam echosounders.

Firstly, would v.surf.rst be the best module for a massive interpolation job
like this?  If not, could you recommend what would be the optimal method?

If v.surf.rst is the right module to use, I was wondering if anyone could help
with what parameters to use for an area this size, at least as a starting
point. I have read the manual several times, but I still don't have a good
intuition for how parameters like npmin, segmax, dmin, dmax, smooth all work
together.

At the moment, I have a script written that accepts a user-supplied number of
random positions all over the input raster. For each random point, I obtain
the east and north coordinates with v.to.db, feed these to g.region, and grow
the region around the point in all four cardinal directions by some value like
10,000m to create an analysis window around each point. I create a polygon
vector of this region with v.in.region, and use this polygon to clip my
vectorized raster bathymetry with v.clip, and then do v.surf.rst on this
clipped vector. 

I have no other way of knowing what v.surf.rst parameters to use other than
trial and error, so I have a 4-level nested 'for' loop written to basically
traverse through all permutations of the parameters within the ranges I
chosen. So for example, I am exploring all combinations of parameters within
these ranges:

Tension: 10, 20, 40, 80, 160
npmin: 100, 200, 400, 800, 1000
smooth: 1, 2, 4, 8, 16, 32

With 7 random points selected around the full region, this script will produce
a few hundred maps per point, and quite a long time to run. And even more time
to look at the results.  I know this isn't the best approach.

I am looking for help to find some workable set of parameters to use for the
entire dataset from other users who have more experience using this module.

Thanks,

-- 
Eric Patton
Marine Geoscience Technologist
Geological Survey of Canada (Atlantic)
Natural Resources Canada
Bedford Institute of Oceanography
1 Challenger Drive, Dartmouth NS, Canada