<HTML dir=ltr><HEAD><TITLE>Re: R: [GRASS-dev] Reproject, change resolution and patch: doubts....</TITLE>
<META http-equiv=Content-Type content="text/html; charset=unicode">
<META content="MSHTML 6.00.6000.16640" name=GENERATOR><BASE href=http://mailstore.rossoalice.alice.it/exchange/Alice000000001264700/Bozze/R:%20R:%20[GRASS-dev]%20Reproject,%20change%20resolution%20and%20patch:%20doubts.....EML/1_text.htm></HEAD>
<BODY>
<DIV id=idOWAReplyText11465 dir=ltr>
<DIV dir=ltr><FONT face=Arial color=#000000 size=2>Hi Helena,</FONT></DIV>
<DIV dir=ltr><FONT size=2></FONT> </DIV>
<DIV dir=ltr><FONT face=Arial color=#000000 size=2>> 1) r.proj method=nearest</FONT></DIV>
<DIV dir=ltr><FONT size=2></FONT> </DIV>
<DIV dir=ltr><FONT face=Arial color=#000000 size=2>good, the basic step has been fixed :-)</FONT></DIV>
<DIV dir=ltr><FONT size=2></FONT> </DIV>
<DIV dir=ltr><FONT face=Arial color=#000000 size=2>> 2) r.resamp.rst</FONT></DIV>
<DIV dir=ltr><FONT face=Arial color=#000000 size=2>>- you don't need this step if the default nearest neighbor works for <BR>>your application<BR>> - it is useful only if you needed smooth connection between the 20m <BR>>DEM and SRTM. ( r.resamp.rst does interpolation using a spline <BR>>function - you can make it pass exactly through the given data points <BR>>by setting smoothing =0.)</FONT></DIV><FONT face=Arial color=#000000 size=2>
<DIV dir=ltr><BR>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. </FONT><FONT face=Arial color=#000000 size=2>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):</FONT></DIV>
<DIV dir=ltr><FONT size=2></FONT> </DIV>
<DIV dir=ltr><FONT size=2>create a raster from 20mDEM with NULL if NULL and 0 if >=0</FONT></DIV>
<DIV dir=ltr><FONT size=2>transform that raster into a vector layer faeaturing the area covered by it</FONT></DIV>
<DIV dir=ltr><FONT size=2></FONT> </DIV>
<DIV dir=ltr><FONT face=Arial color=#000000 size=2>r.mapcalc 20mDEM_surface='if(20mDEM,0)'</FONT><FONT size=2></FONT></DIV>
<DIV dir=ltr><FONT size=2>r.to.vect feauture=area</FONT></DIV>
<DIV dir=ltr><FONT size=2></FONT> </DIV>
<DIV dir=ltr><FONT size=2>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...)</FONT></DIV>
<DIV dir=ltr><FONT size=2></FONT> </DIV>
<DIV dir=ltr><FONT size=2>Thanks,</FONT></DIV>
<DIV dir=ltr><FONT size=2></FONT> </DIV>
<DIV dir=ltr><FONT size=2>Marco</FONT></DIV>
<DIV dir=ltr><FONT size=2></FONT> </DIV></DIV>
<DIV dir=ltr><BR>
<HR tabIndex=-1>
<FONT face=Tahoma size=2><B>Da:</B> Helena Mitasova [mailto:hmitaso@unity.ncsu.edu]<BR><B>Inviato:</B> gio 08/05/2008 2.46<BR><B>A:</B> marco.pasetti@alice.it<BR><B>Oggetto:</B> Re: R: [GRASS-dev] Reproject, change resolution and patch: doubts....<BR></FONT><BR></DIV>
<DIV><BR>
<P><FONT size=2>On May 7, 2008, at 3:35 PM, Marco Pasetti wrote:<BR><BR>> Hi Helena,<BR>><BR>> Thanks for your reply.<BR>><BR>>> just a few notes. If you set the resolution to 20m, GRASS will <BR>>> resample<BR>> your 90m SRTM to 20m using nearest neighbor, creating a DEM with <BR>> "steps" -<BR>> something line this (a) is automatic nn resampling, b) is <BR>> reinterpolation by<BR>> rst) <A href="http://www.grassbook.org/gallery/chapters/s050303f010_resamp.jpg">http://www.grassbook.org/gallery/chapters/s050303f010_resamp.jpg</A><BR>><BR>> Yes, yesterday I tried the described procedure and I noticed the <BR>> same result<BR>> NVIZing the reprojected dem. BTW I don't need to produce 3D maps, <BR>> but I need<BR>> to introduce the less data distorsion as possible. The NN method <BR>> should do<BR>> the job for me. I'm not sure that an interpolation would be a good <BR>> solution,<BR>> but I talk with a strict mathematic/electronic approach, that's my<BR>> professional training; probably, talking about topography, <BR>> interpolations<BR>> rarely produce remarkable errors.<BR>><BR>> This said, I prefer the 20m data, against the SRTM ones, because I <BR>> analyzed<BR>> a very well known (small) area with NVIZ, using both SRTM and 20m <BR>> data, and<BR>> I noticed relevant errors with SRTM data (I cannot pretend much <BR>> from it,<BR>> it's a SAR shuttle mission... it's actually a miracle that we have <BR>> it, that<BR>> it works pretty fine and that is free).<BR>><BR>> Summarizing:<BR>> 1) r.proj method=nearest<BR><BR>> 2) r.resamp.rst<BR><BR>- you don't need this step if the default nearest neighbor works for <BR>your application<BR> - it is useful only if you needed smooth connection between the 20m <BR>DEM and SRTM. ( r.resamp.rst does interpolation using a spline <BR>function - you can make it pass exactly through the given data points <BR>by setting smoothing =0.)<BR><BR>Helena<BR><BR><BR>><BR>><BR>> Right?<BR>><BR>> Thanks,<BR>><BR>> Marco<BR>><BR>><BR>><BR>> -----Messaggio originale-----<BR>> Da: Helena Mitasova [<A href="mailto:hmitaso@unity.ncsu.edu">mailto:hmitaso@unity.ncsu.edu</A>]<BR>> Inviato: mercoledì 7 maggio 2008 18.25<BR>> A: marco.pasetti@alice.it<BR>> Oggetto: Re: [GRASS-dev] Reproject, change resolution and patch: <BR>> doubts....<BR>><BR>><BR>> Marko,<BR>><BR>> just a few notes. If you set the resolution to 20m, GRASS will <BR>> resample your<BR>> 90m SRTM to 20m using nearest neighbor, creating a DEM with "steps" -<BR>> something line this (a) is automatic nn resampling, b) is <BR>> reinterpolation by<BR>> rst) <A href="http://www.grassbook.org/gallery/chapters/s050303f010_resamp.jpg">http://www.grassbook.org/gallery/chapters/s050303f010_resamp.jpg</A><BR>> Dylan has a nice comparison of effects of different resampling and<BR>> reinterpolation methods on his web site, if you want to learn more.<BR>><BR>> If you have a GRASS book and the nc_spm data set - it has both SRTM <BR>> data and<BR>> lidar based 10m and 1m resolution DEMs so you can see how SRTM <BR>> works, there<BR>> are several examples in the book - e.g. SRTM is consistently higher <BR>> and I<BR>> did viewshed from 30m SRTM and lidar-based 30m DEM - same <BR>> resolution but the<BR>> results were quite different. You need to keep in mind that SRTM <BR>> maps the<BR>> surface with vegetation on it while your 20m DEM is probably bare <BR>> earth<BR>> surface. Also look at the metadata for both to compare their vertical<BR>> accuracy, some smoothing of SRTM may actually be justified (unless <BR>> you want<BR>> the vegetation and have some means to add it to your 20m DEM). When <BR>> you<BR>> decide to reinterpolate you can compare your re-interpolated DEM <BR>> with the<BR>> original data (e.g. the 90m grid centers) and make sure that any <BR>> smoothing<BR>> does not exceed the accuracy of your original data.<BR>><BR>> Just for illustration: here is a stream network derived from 10m <BR>> resolution<BR>> IFSARE patched with 90m SRTM where IFSARE data were missing, all<BR>> reinterpolated to 30m resolution to get the river flow seamlessly<BR>> back-and-forth along the patch line.<BR>> <A href="http://skagit.meas.ncsu.edu/~helena/measwork/panama/pacora30maccum.jpg">http://skagit.meas.ncsu.edu/~helena/measwork/panama/pacora30maccum.jpg</A><BR>> some SRTM data can be pretty noisy:<BR>> <A href="http://skagit.meas.ncsu.edu/~helena/measwork/nrc/mumbai_srtm.png">http://skagit.meas.ncsu.edu/~helena/measwork/nrc/mumbai_srtm.png</A><BR>><BR>> Helena<BR>>><BR>>> Do you agree?<BR>>><BR>>> Thanks for all,<BR>>><BR>>> Marco<BR>>> _______________________________________________<BR>>> grass-dev mailing list<BR>>> grass-dev@lists.osgeo.org<BR>>> <A href="http://lists.osgeo.org/mailman/listinfo/grass-dev">http://lists.osgeo.org/mailman/listinfo/grass-dev</A><BR>><BR><BR></FONT></P></DIV></BODY></HTML>