[GRASSLIST:3940] Re: Merging topographic data

[Ian MacMillan]

Hi there, as I see it, there are a couple of ways that you can do this. 
  Although I don't recognize the .lld format, so you might have to 
figure out how to import that into GRASS first.  I am assuming that you 
have imported both data sets into one GRASS location (careful with your 
projections!).

1) If your bathymetry has a well defined shoreline (i.e. it is always 
at 0 meters, or there is no land data), then you can just do a simple 
r.patch input=bathymetry,srtm output=combined.  r.patch uses all data 
from the first file, then fills in holes with the second file.  Make 
sure your region is set correctly too.

2) If your shoreline is difficult to pick out in your bathymetry file, 
then it is a little trickier.  You can download a shoreline vector file 
from here (it's free, you just have to create a user).  It should be 
the political/ocean network file, you'll have to surf around on the 
site.

http://data.geocomm.com/

Then import that into grass using m.in.e00.  Bring it into the location 
with your other data using v.proj (usually the vector files are nad27, 
lat-long).  Turn it into a raster with v.to.rast.  Subtract out the 
ocean data from SRTM and patch in the bath file with
r.mapcalc newfile = 'if(isnull(shoreline_file),bathymetry,srtm)'

If there is a gap at the shoreline (the vector files aren't always 
exact), you can make a buffer with r.buffer input=shoreline_file 
output=shoreline.buff distances=500 units=meters.  Then do the mapcalc 
command.


Good luck,
Ian


On Jul 16, 2004, at 1:16 AM, lawrence moran wrote:

> Hi,
>
> I have two data sets, one SRTM covering Saudi Arabia and the Red Sea,
> the other digitized bathymetry of the red sea (an .lld file) and need 
> to
> merge them. How do I go about this?
>
> I guess I first need to get rid of the SRTM coverage of the sea, but
> how?
>
> I'm using Grass 53, though I have 57. I also have GMT, so I could use
> that as well as or instead of Gras to accomplish this. However I should
> point out that I'm a beginner with both Grass and GMT!
>
> Any ideas? Many thanks in anticipation...
>
> Lawrence
>