[GRASS-user] v.surf.rst question

[Bulent Arikan]

Dear List,

I have 30m ASTER DEM from a region (UTM-37) which I am trying to interpolate
using ' v.surf.rst ' to obtain 10m resolution DEMs. I prefer this module
since I will also need additional maps such as slope, aspect, prof curv,
etc. I will try describing my problem below and I will appreciate any
suggestions/ideas. Since the original 30m res. image covers a large area, I
cropped it into several tiles using 'r.mapcalc'. Each tile has about 12
million cells in 30m res. and 115 million cells in 10m res. and takes an
average of 4 hrs to complete one run of v.surf.rst. I sample each tile using
'r.random' at 50%. Then I change the region settings in 'g.region': I align
region to resolution and set the resolution to 10m before running
v.surf.rst <http://v.surf.rs>. I ran the module at different tension
settings (55., 40., 25. and 10.) while keeping smoothing parameter (.1) and
other parameters at default settings. When I add legend to my 10m DEM from
the toolbar in Map Display, the elevation range is usually a couple meters
above or below the original map. After each run, I set my region resolution
back to 30m, and using r.mapcalc, I prepare a "difference" map [ (30m DEM) -
(10m DEM) ]. In this map, I get extremely high and low values (i.e., the
range in the difference map can be -70 to 183 suggesting to me that there is
both over- and undershooting), which seem to be limited to mountain tops and
some canyons. When I run 'r.report', I can see that actually these "extreme"
values are limited to very few cells (about 3-5% of 12 million cells) and
the main variation for 95% of cells is within more reasonable range such as
+/- 3 meters. I recently became aware of some accuracy issues related to
ASTER DEMs especially in mountainous areas however, I have the same issue
for flat parts of this landscape as well.

My question is: How can I fix the issue of having these few cells with
extreme values?

I am thinking that:

- I may be having this problem because I am trying to interpolate images
that are too large (crop tiles of smaller areas?),

- I may be able to fix the negative cells by making them NULL and then using
' r.fill.nulls' module but even if this is advisable, I do not know of a
solution for higher than normal value cells.

Again, I appreciate your ideas and suggestions. Thank you,


-- 
BÜLENT
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