[GRASSLIST:15] Re: SRTM data transformation

[Maciek Sieczka]

On Tue, 07 Mar 2006 08:52:30 +0100
Wolfgang Zillig <wollez at gmx.net> wrote:

> OK, one final question: are the z-Values also adapted to the new
> region?  I was told, that in my case the difference should be about
> 40-50m.

I'll try to share what (I thik) I know. ANYBODY PLEASE CORRECT ME. Paul?

In SRTM "Heights are in meters referenced to the WGS84/EGM96 geoid"[1],
which is roughly an equivalent of the mean sea level [2,3] (the
difference between MSL and geoid could reach several meters, but it is
not an issue considering SRTM accuracy). If you reproject SRTM into a
coordinate system based on ellipsoid other than WGS84 or GRS80 (like
Bessel in this case) you still want your elevation to be referenced to
the mean sea level - the same DEM in different coordinate systems
should have the same elevation (cosider elevation as an attribute,
rather than as a coordinate in this case). And this is well achieved
using 2d only transformation.

Now please note that if you use 3d transformation for your SRTM,
your Z coordinate will be shifted down about 40 m (as you are
transforming to from WGS84 to Bessel ellipsoid - EPSG 31285), which you
propably don't want. You propably would rather preserve the SRTM's
elevation, the "real one".

What 3d transformation would improve however, is be the 2d positional
accuracy - reprojectiong in 3d would give you few centimeters better.
Thus if you need it, the way to go could be: extract SRTM rasters'
midpoints with elevation to a text file (r.stats -g > output.txt), use
cs2cs to transform it, then replace the output's Z coordinate (as it is
now shifted circa 40m down after 3d transformation) with an original
elevation (UNIX text tools), import such an ascii data set into Grass
(v.in.ascii) and trasform into raster (v.to.rast) or interpolate
(v.surf.rst). But, as the actuall 2d position accuracy improvement
would be circa 3-4 cm for mountainous area and even less for lowlands,
it might be not worth the effort, at least not in case of SRTM.

Again, I only wrote what I think I know. Please correct me if anything
is wrong with my thinking. I'd be glad to sort this issue out for good
for me too.

Maciek

[1]
ftp://e0srp01u.ecs.nasa.gov/srtm/version2/Documentation/Quickstart.pdf
[2]
http://en.wikipedia.org/wiki/Sea_level
[3]
http://www.esri.com/news/arcuser/0703/geoid1of3.html

--------------------
W polskim Internecie s? setki milion?w stron. My przekazujemy Tobie tylko najlepsze z nich!
http://katalog.panoramainternetu.pl/