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nsw_lidar [2019/02/08 14:15] bushwalkingnsw_lidar [2019/02/10 13:59] bushwalking
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-Page for documenting NSW lidar processing 
 ====== Intro ====== ====== Intro ======
 NSW Spatial Services have undertaken a program to map all of NSW using lidar (light detecting and ranging) NSW Spatial Services have undertaken a program to map all of NSW using lidar (light detecting and ranging)
 For details, see information on their [[http://spatialservices.finance.nsw.gov.au/mapping_and_imagery/environmental_spatial_programs|elevation program]]. For details, see information on their [[http://spatialservices.finance.nsw.gov.au/mapping_and_imagery/environmental_spatial_programs|elevation program]].
  
-Elevation data can best be accessed through the [[http://elevation.fsdf.org.au/|Geoscience Australia ELVIS program]], and then processed with a GIS such as [[https://www.qgis.org/en/site/index.html|QGIS]] to create useful topographic maps.+Elevation data can best be accessed through the [[http://elevation.fsdf.org.au/|Geoscience Australia ELVIS program]]
 + 
 +It can then processed with a GIS such as [[https://www.qgis.org/en/site/index.html|QGIS]] to create useful topographic maps. Instructions below are specifically for use with QGIS, though the general outline may be useful for other GISs.
  
 ====== Resources ====== ====== Resources ======
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 ===== Contours ===== ===== Contours =====
 +==== Basic Processing ====
 There are various contour extraction algorithms in QGIS, for example: There are various contour extraction algorithms in QGIS, for example:
   * GDAL : Raster Extraction : Contour (same as Raster -> Extraction -> Contour...)   * GDAL : Raster Extraction : Contour (same as Raster -> Extraction -> Contour...)
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 {{:2019_02_08_12_17_09_untitled_project_qgis.png?300|}} {{:2019_02_08_12_17_09_untitled_project_qgis.png?300|}}
 {{:2019_02_08_12_17_57_untitled_project_qgis.png?300|}} {{:2019_02_08_12_17_57_untitled_project_qgis.png?300|}}
 +
 +Even with sink removal, small 
 +
 +==== Simplifying ====
 +
 +Vectors can be compressed by using something like:
 +  * Vector geometry : Simplify
 +A tolerance of 1(m) seems reasonable for 1:25000 mapping. Smaller tolerances may be appropriate for larger scale maps (eg 1:10000, 1:5000).
 +
 +For more options in compression, look at:
 +  * GRASS : [[https://grasswiki.osgeo.org/wiki/V.generalize_tutorial|v.generalize]]
 +
 +==== Cleaning ====
 +
 +Once simplified, it is worth removing small closed loops, such as those in the image below.
 +{{:contour_loops.png|}}
 +
 +Here is one approach, which involves adding a length attribute to each contour, and removing those that fall below a certain length. It may cause issues if you have short sections of contour near the edge of the map that you need.
 +
 +  * Open Attribute Table (F6)
 +  * Open field calculator (Ctrl+I)
 +  * Add new attribute length, calculated as $length
 +{{::qgis_add_field.png|}}
 +  * Select all features and filter on length < 25 (or whatever length is appropriate for your scale)
 +{{:qgis_filter_field.png|}}
  
 ==== Contour Labelling ==== ==== Contour Labelling ====
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 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 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.
 +
 +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!
  
 ==== Initial analysis of slope, aspect ==== ==== Initial analysis of slope, aspect ====
nsw_lidar.txt · Last modified: 2023/06/02 12:33 by allchin09

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