[GRASSLIST:3121] Re: How to correct SRTM data

Ian Macmillan ian_macmillan at umail.ucsb.edu
Thu Apr 8 13:46:51 EDT 2004

hey there, there are a few different things that you can try.  You can use the
gtopo30 dataset to define your coastline.  Everything that is null in gtopo30
can be made null in you SRTM dem using mapcalc

r.mapcalc srtm2 = 'if(isnull(gtopo30),gtopo30,srtm1)'

This gives a blocky coastline though (gtopo30 is ~900m resolution compared with
~30 or ~90 for SRTM).  You can also get free coastline vector data from
gisdatadepot.com.  You can then use this vector coastline to trim you srtm
data.  There are a number of ways to do this I think.  One way (mebbe not the
best, but it works for me) is to convert the coastline vector to a raster
(v.to.rast) and then do a similar method described above.

As far as filling holes in your SRTM dem on land, if you are worried about
resolution problems (the filled holes might be 900m blocks if you use gtopo30
data), you can also use r.surf.idw to fill the holes (at whatever resolution
you set with g.region).  It is fake data, but at least it is at the right
resolution, and for holes that are generally small, I have found that it
doesn't pose too much of a problem.

Good luck.


Quoting tatel <tatel at euskalnet.net>:

> Hi,
> I'm looking for ways to correct srtm data. I found that r.mapcalc can 
> correct mountain holes by interpolating GTOPO and SRTM data. Great.
> Can you gis gurus give me some pointer about correcting speckled sea? 
> And what about sea level and coastline?
> Thanks in advance,
> Roman

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