User Tools

Site Tools


nsw_lidar

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

Both sides previous revision Previous revision
Next revision
Previous revision
Next revision Both sides next revision
nsw_lidar [2019/02/22 21:37]
bushwalking
nsw_lidar [2020/08/05 11:14]
bushwalking [Merge DEMs]
Line 24: Line 24:
 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 painful.+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 ==== 
 + 
 +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 steps, then the limitations from the screen redrawing may not be important. 
 + 
 +QGIS uses [[https://​gdal.org/​programs/​gdalbuildvrt.html|gdalbuildvrt]]. Uncheck "Place each input file into a 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://​gdal.org/​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 size) switch. 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 2 to force them to merge to a 2m DEM.
  
 ===== Fill Sinks ===== ===== Fill Sinks =====
nsw_lidar.txt · Last modified: 2020/09/09 10:36 by allchin09