[GRASSLIST:1027] Re: srtm

[Ian Macmillan]

thanks for the suggestions, I thought of that too, but I don't think that would
work for srtm data.  The data in the oceans is not necessarily negative, the
values sometimes even get up into the hundreds of meters.  It is actually
pretty difficult to pick out the coastline with that data set.  I was thinking
I might be able to somehow turn to null everything outside of a vector (or
raster) coastline file.

-Ian

Quoting Kevin Slover <Kevin.Slover at noaa.gov>:

> Ian,
> 	just a thought, and I think I have done this before, but can't 
> remember.. it has been a while since I have worked with the srtm data...
> 
> but.. there are two ways that I can think about doing this.
> the first, take out all values of the srtm data that are less then a 
> certain number, say, -1... this would get rid of all data over the 
> water...and then you would be able to merge the data sets together... 
> the way I usually do this is r.out.xyz, then push it through a perl 
> script, and then reimport using s.in.ascii, s.to.rast... recreates the 
> raster image...
> 
> the second, is to use the r.mapcalc function... use the newmap = 
> if(map<5, null(), 5)... this example makes all data less than 5 null 
> values.  so, for you, use a negative value to make them null...
> 
> the reason you do not really want the mask in place is that the hydro 
> data then would not be displayed.  The mask "masks" everything below it...
> 
> Hope this helps...
> 
> Ian Macmillan wrote:
> > Hi all, you all have been very helpful in the past, so I am hoping you keep
> your
> > patience with me.
> > 
> > I have downloaded some SRTM data from the USGS web site, and it so happens
> that
> > my data is near the coast.  My trouble is that SRTM actually records some
> > anomalous data out in the oceans.  I have bathymetry data that i would like
> to
> > view along with topography, but the ocean data is a bit difficult to get
> rid of
> > from the SRTM.  My thought is that I might be able to take the coast lines
> > generated from GMT, import those into GRASS, and then somehow mask out all
> data
> > from the SRTM that lies outside of those coastlines.
> > 
> > Does anyone have any experience with this?  Is there another way that I am
> > ignoring?
> > 
> > I thought I saw an email about this recently, but I couldn't find it in
> the
> > archives.
> > 
> > thanks, Ian