====== Contours ======

===== Basic Processing=====

There are various contour extraction algorithms in QGIS, for example:
  * GDAL : Raster Extraction : Contour (same as Raster -> Extraction -> Contour...)

Below is an example of contours created without and with [[qgis_depressionless_dem|sink removal]]. The contours on the right have been derived from a DEM where the sinks (in yellow on the left) have been filled.
{{:2019_02_08_12_17_09_untitled_project_qgis.png?300|}}
{{:2019_02_08_12_17_57_untitled_project_qgis.png?300|}}

It is worth noting that a [[qgis_depressionless_dem|depressionless DEM]] does not guarantee that there will be no contour sinks. A deep diagonal channel combined with a small contour interval can still cause sinks. {{::qgis_raw_contours.png?nolink&600|}}

See [[https://gis.stackexchange.com/questions/390221/fix-unwanted-sink-contours-from-hydrologically-correct-dem/|GIS Stack Exchange]] for options of how to deal with this.

===== 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]]
V.generalize can also be used to smooth contours - possibly best done prior to simplificiation

===== 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. However, that shouldn't be a problem in most scenarios.

  * 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 =====

See separate page on [[[qgis_contour_labelling|QGIS Contour Labelling]]