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nsw_lidar [2019/08/30 09:35]
nsw_lidar [2020/08/05 11:14] (current)
bushwalking [Merge DEMs]
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 The NSW DEM data is supplied in 2km squares. The squares need to be merged into a single DEM for further operations. The NSW DEM data is supplied in 2km squares. The squares need to be merged into a single DEM for further operations.
-While this can be done in theory using Virtual RasterI have had poor performance with thisAny operation seems to result in screen redrawingso moving around ​and zooming in and out is quite slow and painfulPossibly ​more recent versions of QGIS have resolved these issues.+Most of the eastern ranges, where lot of bushwalking happensare 2m DEMsThe coast is typically 1m, and the western slopes ​and plains are 5m (with major rivers 1m!)Be careful if you need to merge DEMs with different resolutions - see below for more details.
-Instead, I generally use the the Raster- > Miscellaneous -> Merge... function+==== Virtual ​Raster ​====
-Note that while most of the eastern rangeswhere lot of bushwalking happens, are 2m DEMs, the coast is typically 1m, and the western slopes and plains are 5m (with major rivers 1m!). QGIS appears to merge DEMs to the highest ​resolution ​(ie a combination ​of 1m and 2m DEMs will be merged to a 1m resolution output ​file). This may not always ​be desired. ​The 1m DEMs can be downsampled to say 2m in QGIS using the Warp (reprojecttool with an Output resolution ​of 2.+While merging can be done in theory using a Virtual Raster (Raster- > Miscellaneous -> Build Virtual Raster... ), I have had poor performance with this (including recent version 3.12 Bucuresti). Any operation seems to result in screen redrawing, so moving around and zooming in and out is slow and painful.  
 +That said, if you are just using the Virtual Raster for future stepsthen the limitations from the screen redrawing may not be important. 
 +QGIS uses [[https://​​programs/​gdalbuildvrt.html|gdalbuildvrt]]. Uncheck "Place each input file into separate band"and if you are using files with different resolutions (eg 1m and 2m)change ​the Resolution to Highest or Lowest as appropriate. You can also change the Resolution to User, and set the optional -tr switch ​(eg -tr 1 1 or -tr 2 2 
 +==== Merge Raster ==== 
 +I generally use the the Raster- > Miscellaneous -> Merge... function 
 +QGIS uses [[https://​​programs/​gdal_merge.html|gdal_merge]],​ which defaults ​to using the resolution of the first file. This is not always desired. ​It can be controlled by using the optional -ps (pixel sizeswitch. For example, if you have a combination ​of 1m and 2m DEMs, you can use -ps 1 1 to force them to a merged 1m DEM, or -ps 2 to force them to merge to a 2m DEM.
 ===== Fill Sinks ===== ===== Fill Sinks =====
nsw_lidar.1567157759.txt.gz · Last modified: 2019/08/30 09:35 by bushwalking