This plugin is designed for contouring points datas to a line or a polygon vector layer. Thanks to the matplotlib library, it's now trivial to contours any point layer with datas gridded or not with the Delaunay triangulation algorithm.
So, this plugin requires
The plugin installs a menu item (Plugins|Contour|Contour) and a button in the plugins toolbar. To contour data in a point layer, select the menu item or click the button.
The contour dialog is used to contour point data. To generate contours, select the required options, then press OK. More than one set of contours can be generated. When all the required contours have been generated, press Close to quit the dialog. It's now possible to reopen a previously contoured layer, all options will be automatically imported, and a contoured layer can be modified in place without multiple resulting layers.
The input box is used to select the data to be contoured. This requires choosing the map layer containing the data (only point layers are available), and the data field of the layer containing the values to be contoured.
Contours can be displayed either as line features along the contour levels, or as polygon features (defining the area between each set of contour lines), or both.
The number of levels, minimum and maximum values and the computation method define levels at which contours will be drawn. When the data are read, the minimum and maximum values of the data field are calculated and displayed. They can then be edited to choose different minimum and maximum contour levels. The number of levels includes the minimum and maximum values. So for example choosing minimum 200.0, maximum 600.0, and 5 levels equally spaced will generate contours at 200, 300, 400, 500, and 600. An extend option can be selected to generate closed filled contours (in our example, if a levels of 600 to infinity is required, then select extend=max will compute this level).
When the minimum and maximum values and the number of levels have been set the calculated contour levels will be displayed in a list. These can be individually edited by double clicking on them. You can also edit the resulting layer name and the labelling precision in the "Outpout" box. The "label precision" is a convenient way to format the "label" attribute of the resulting layer, which then can be directly used and displayed in the "Categorized" renderer.
The contour plugin may generate a number of warnings. Most are self expalanatory, requesting information that is not supplied or is not correct. Some less obvious messages are:
No contours will be drawn for the requested contour values because they are beyond the range of the data. Another case is that the values are on unsatisfying number of points, because a contour (line) needs at least 2 points and a filled contour (polygon) 3 points. A workaround is to select a lesser maximum value and/or a bigger minimum value.
Thanks for trying this plugin
Authors : Lionel Roubeyrie, Chris Crook