Hi Helena, > 1) r.proj method=nearest good, the basic step has been fixed :-) > 2) r.resamp.rst >- you don't need this step if the default nearest neighbor works for >your application > - it is useful only if you needed smooth connection between the 20m >DEM and SRTM. ( r.resamp.rst does interpolation using a spline >function - you can make it pass exactly through the given data points >by setting smoothing =0.)
Actually I already thought that, in any case, I should at least fix the interferences among the two different maps. That means that I should at least apply r.resamp.rst along the line where the two maps collide. Having the 20m raster called, for example, 20mDEM, I figured out (and already tested) how to create a polygon matching the exact boudaries of that map (exactly an area vector layer matching the whole area covered by the 20mDEM map): create a raster from 20mDEM with NULL if NULL and 0 if >=0 transform that raster into a vector layer faeaturing the area covered by it r.mapcalc 20mDEM_surface='if(20mDEM,0)' r.to.vect feauture=area Then I could apply r.resamp.rst on a buffer along the polyline that describe the border of the above area... but I'm not sure how to do that, I still need to carefully read the online manuals (unfortunately I still don't have the book of Markus; GFOSS.it association kindly gave me a copy... but I still have to get it...) Thanks, Marco ________________________________ Da: Helena Mitasova [mailto:[EMAIL PROTECTED] Inviato: gio 08/05/2008 2.46 A: [EMAIL PROTECTED] Oggetto: Re: R: [GRASS-dev] Reproject, change resolution and patch: doubts.... On May 7, 2008, at 3:35 PM, Marco Pasetti wrote: > Hi Helena, > > Thanks for your reply. > >> just a few notes. If you set the resolution to 20m, GRASS will >> resample > your 90m SRTM to 20m using nearest neighbor, creating a DEM with > "steps" - > something line this (a) is automatic nn resampling, b) is > reinterpolation by > rst) http://www.grassbook.org/gallery/chapters/s050303f010_resamp.jpg > > Yes, yesterday I tried the described procedure and I noticed the > same result > NVIZing the reprojected dem. BTW I don't need to produce 3D maps, > but I need > to introduce the less data distorsion as possible. The NN method > should do > the job for me. I'm not sure that an interpolation would be a good > solution, > but I talk with a strict mathematic/electronic approach, that's my > professional training; probably, talking about topography, > interpolations > rarely produce remarkable errors. > > This said, I prefer the 20m data, against the SRTM ones, because I > analyzed > a very well known (small) area with NVIZ, using both SRTM and 20m > data, and > I noticed relevant errors with SRTM data (I cannot pretend much > from it, > it's a SAR shuttle mission... it's actually a miracle that we have > it, that > it works pretty fine and that is free). > > Summarizing: > 1) r.proj method=nearest > 2) r.resamp.rst - you don't need this step if the default nearest neighbor works for your application - it is useful only if you needed smooth connection between the 20m DEM and SRTM. ( r.resamp.rst does interpolation using a spline function - you can make it pass exactly through the given data points by setting smoothing =0.) Helena > > > Right? > > Thanks, > > Marco > > > > -----Messaggio originale----- > Da: Helena Mitasova [mailto:[EMAIL PROTECTED] > Inviato: mercoledì 7 maggio 2008 18.25 > A: [EMAIL PROTECTED] > Oggetto: Re: [GRASS-dev] Reproject, change resolution and patch: > doubts.... > > > Marko, > > just a few notes. If you set the resolution to 20m, GRASS will > resample your > 90m SRTM to 20m using nearest neighbor, creating a DEM with "steps" - > something line this (a) is automatic nn resampling, b) is > reinterpolation by > rst) http://www.grassbook.org/gallery/chapters/s050303f010_resamp.jpg > Dylan has a nice comparison of effects of different resampling and > reinterpolation methods on his web site, if you want to learn more. > > If you have a GRASS book and the nc_spm data set - it has both SRTM > data and > lidar based 10m and 1m resolution DEMs so you can see how SRTM > works, there > are several examples in the book - e.g. SRTM is consistently higher > and I > did viewshed from 30m SRTM and lidar-based 30m DEM - same > resolution but the > results were quite different. You need to keep in mind that SRTM > maps the > surface with vegetation on it while your 20m DEM is probably bare > earth > surface. Also look at the metadata for both to compare their vertical > accuracy, some smoothing of SRTM may actually be justified (unless > you want > the vegetation and have some means to add it to your 20m DEM). When > you > decide to reinterpolate you can compare your re-interpolated DEM > with the > original data (e.g. the 90m grid centers) and make sure that any > smoothing > does not exceed the accuracy of your original data. > > Just for illustration: here is a stream network derived from 10m > resolution > IFSARE patched with 90m SRTM where IFSARE data were missing, all > reinterpolated to 30m resolution to get the river flow seamlessly > back-and-forth along the patch line. > http://skagit.meas.ncsu.edu/~helena/measwork/panama/pacora30maccum.jpg > some SRTM data can be pretty noisy: > http://skagit.meas.ncsu.edu/~helena/measwork/nrc/mumbai_srtm.png > > Helena >> >> Do you agree? >> >> Thanks for all, >> >> Marco >> _______________________________________________ >> grass-dev mailing list >> [email protected] >> http://lists.osgeo.org/mailman/listinfo/grass-dev >
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