Hi Jose, it certainly would be nice to have a relief even for larger areas. However your approach will not work due to limitations in GDAL. To access the values the API function RasterIO is used. RasterIO can even scale the requested data. However it does a poor job. It simply uses the "nearest neighbor" method. This is kind of acceptable for maps. For elevation data this is bad, as the shading it the differential function of the data and reveals every non-linear disturbance.
In other words: If I scale the data via RasterIO stripes will get visible in the relief shading. And that is the reason why the code in CMapDEM looks a bit different from what I do for normal maps. I read all data covered by the viewport unscaled, calculate the shading and scale the result with the advanced scaling algorithm used in QImage. And that is the reason for the 10000x10000 limit. Things get really slow if you hit that amount of points. Oliver > Hello all, for the last few days I have been dealing with large to very > large DEM for Spain, as a source of survey-grade quality data was released > some time ago, and I took the time to create a 25x25-meter resolution for > all mainland Spain, which is about 500.000 square kilmeters in area. The > resulting file is some 700 MB in size, and has 52786x30003 pixels. > > First, the good news. QLGT does a great job handling such a big file. It > doesn't crash, it has some code there to avoid taking too long processing > the DEM (IIRC, skips processing it if viewport needs more than 10000x10000 > points), and when the DEM is set to "none" (instead of "shading" or > "contour"), QLGT works as fast as without the DEM loaded for any zoom level. > > Now, the improvement I think would be reasonably easy to implement, and > which would allow QLGT to deal with showing DEM at any zoom level with > little additional processing needed, enhancing the end user experience > (redrawing the screen would be dramatically reduced). GeoTiff files can have > an arbitrary number of overview images in them, which are smaller versions > of the original file. If QLGT could be adapted to look at the GeoTiff file > looking for the available overviews, and use the smallest one that covers > the viewport to be drawn, if the DEM is created with (or modified to > include) scaled overviews, users could have the option to leave the DEM > visible on screen at all zoom levels, with little effect on UI reponse times. > > This is how a GeoTiff DEM with several overview images look like: > > $ gdalinfo mainland_balearic/mainland_balearic_dem.tiff > Driver: GTiff/GeoTIFF > Files: mainland_balearic/mainland_balearic_dem.tiff > Size is 52786, 30003 > Coordinate System is: > GEOGCS["unnamed ellipse", > DATUM["unknown", > SPHEROID["unnamed",6378137,298.257223563]], > PRIMEM["Greenwich",0], > UNIT["degree",0.0174532925199433]] > Origin = (-9.448178185470384,43.906719781655518) > Pixel Size = (0.000269437866598,-0.000269437866598) > Metadata: > AREA_OR_POINT=Area > Image Structure Metadata: > COMPRESSION=DEFLATE > INTERLEAVE=BAND > Corner Coordinates: > Upper Left ( -9.4481782, 43.9067198) ( 9d26'53.44"W, 43d54'24.19"N) > Lower Left ( -9.4481782, 35.8227755) ( 9d26'53.44"W, 35d49'21.99"N) > Upper Right ( 4.7743690, 43.9067198) ( 4d46'27.73"E, 43d54'24.19"N) > Lower Right ( 4.7743690, 35.8227755) ( 4d46'27.73"E, 35d49'21.99"N) > Center ( -2.3369046, 39.8647476) ( 2d20'12.86"W, 39d51'53.09"N) > Band 1 Block=2048x2048 Type=Int16, ColorInterp=Gray > Overviews: 13197x7501, 6599x3751, 3300x1876 > > > Those overviews were added to the DEM after being created, and increased the > file size from 704392704 to 1055199158 bytes, although increases in size will > depend on the number and sizes of the overviews. But I think that from the > point of view of the user, hard drive space is affordable, but an unresponsive > UI is unacceptable. Sure, the user can be told to just hide the DEM, > although it will be there an usable, but they will demand their nice > mountain shadings. > > > I _believe_ that changes needed to QLGT shouldn't be very intrusive or > complex, but I can only guess, as my coding skills are next to null, and by > reading the code I can only have a general impression on how it works. The > constructor for CMapDEM (CMapDEM.cpp) should be change, to include code to > make use of GDAL C++ binding functions GDALGetOverviewCount and > GDALGetOverview > and create a list of "bands", instead of a single band for the "main" file. > And then (if the API for the rest of the application can stay the same), > under CMapDEM::draw some logic should be implemented to always choose the > best overview, according to the viewport map coordinates. > > I can't explain it better, but I hope someone has understood it. I'm sure I > have missed a lot of internal API dependencies and the change, which seems > conceptually simple, is harder once you try to implement it. But maybe > there is someone out there that would like to have a look at it, provided I > am perfectly aware this is not exactly a top priority, and though it may > help for some people, it may not be worth the effort. > > By the way, the GDAL command to add overviews to an existing GeoTiff is > "gdaladdo", well documented in the man page. And if there is interest, I > can provide a DEM for the Canary Islands including overviews (at a size of > less than 20 MB, it is easy to make it available, although large enough to > make rendering at low zooms noticeably slower). > > Hope it helps. > > > > > ------------------------------------------------------------------------------ > Try before you buy = See our experts in action! > The most comprehensive online learning library for Microsoft developers > is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, > Metro Style Apps, more. 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