Differences

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--- nsw_lidar [2019/08/30 19:35] – bushwalking
+++ nsw_lidar [2020/08/05 18:50] – bushwalking
@@ 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.
-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.
+While this can be done in theory using a 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 quite 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.
-Instead, I generally use the the Raster- > Miscellaneous -> Merge... function
+<insert function here>
-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.
+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 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.
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