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nsw_lidar [2019/02/09 11:41]
bushwalking
nsw_lidar [2019/02/10 00:29]
bushwalking
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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 ====
nsw_lidar.txt ยท Last modified: 2020/09/09 10:36 by allchin09