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--- nsw_lidar [2019/02/09 22:41] – bushwalking
+++ nsw_lidar [2019/02/10 11:29] – bushwalking
@@ Line 44: / Line 44: @@
 ===== Contours =====
+==== Basic Processing ====
 There are various contour extraction algorithms in QGIS, for example:
   * GDAL : Raster Extraction : Contour (same as Raster -> Extraction -> Contour...)
@@ Line 51: / Line 51: @@
 {{:2019_02_08_12_17_09_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 ====