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nsw_lidar [2019/02/10 21:28]
bushwalking
nsw_lidar [2019/08/30 09:35] (current)
bushwalking
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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 a virtual raster, I have had poor performance with this. Any operation seems to result in screen redrawing, so moving around and zooming in and out is quite slow and painful.+While this can be done in theory using a Virtual Raster, I have had poor performance with this. Any operation seems to result in screen redrawing, so moving around and zooming in and out is quite slow and painful. Possibly more recent versions of QGIS have resolved these issues.
  
 Instead, I generally use the the Raster- > Miscellaneous -> Merge... function Instead, I generally use the the Raster- > Miscellaneous -> Merge... function
 +
 +Note that while most of the eastern ranges, where a 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 (reproject) tool with an Output resolution of 2.
  
 ===== Fill Sinks ===== ===== Fill Sinks =====
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 The raw stream data is very jagged. Smooth using  The raw stream data is very jagged. Smooth using 
   * v.generalize   * v.generalize
-  * Algorithm = Hermite (there are other options which can be used) +  * Algorithm = Hermite (there are other options which can be used, but Hermite has the smoothed line passing through the points of the original
   * Maximal tolerance value = 20 (in m, obviously scale dependent)   * Maximal tolerance value = 20 (in m, obviously scale dependent)
  
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 ===== Clifflines ===== ===== Clifflines =====
  
-The steps below have been tested ​in the Blue Mountains, a region that has a significant number of relatively vertical sandstone cliffs. It may be less effective in different terrain.+The steps below are being developed for use in the Blue Mountains, a region that has a significant number of relatively vertical sandstone cliffs. It may be less effective in different terrain.
  
 This is more a set of ideas than a fully fledged process. The main aims are to get a set of steps that can largely be automated, and that create cliffline vectors that are running in the correct direction. There is still some way to go on this! This is more a set of ideas than a fully fledged process. The main aims are to get a set of steps that can largely be automated, and that create cliffline vectors that are running in the correct direction. There is still some way to go on this!
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 using DEM and [1] Maximum Triangle Slope (Tarboton (1997)). I haven'​t tested any other algorithms. ​ using DEM and [1] Maximum Triangle Slope (Tarboton (1997)). I haven'​t tested any other algorithms. ​
   
-Cliff areas can be identified using a range of 60-90 and 70-90 degrees on the Slope file. Using 60-90 degrees helps connect logical cliffs and avoid small breaks.+Cliff areas can be identified using a range of say 60-90 and 70-90 degrees on the Slope file. Using 60-90 degrees helps connect logical cliffs and avoid small breaks.
  
 ==== Initial Cleaning ==== ==== Initial Cleaning ====
  
 Next convert data to 1 bit (1,2 not 0,1, as Sieve ignores 0s) using Raster Calculator. Next convert data to 1 bit (1,2 not 0,1, as Sieve ignores 0s) using Raster Calculator.
-Formula is: (Slope > 0) + 1+Formula is: (Slope > 60) + 1
  
 Then Sieve resulting data using a Threshold of 100 and 8-connectedness to get rid of small non-connected cliffs. Note above that Sieve doesn'​t like 0s. Then Sieve resulting data using a Threshold of 100 and 8-connectedness to get rid of small non-connected cliffs. Note above that Sieve doesn'​t like 0s.
nsw_lidar.1549834102.txt.gz ยท Last modified: 2019/02/10 21:28 by bushwalking