{"id":6011,"date":"2018-08-20T03:06:10","date_gmt":"2018-08-20T01:06:10","guid":{"rendered":"http:\/\/www.sigterritoires.fr\/?p=6011"},"modified":"2018-10-17T12:06:40","modified_gmt":"2018-10-17T10:06:40","slug":"qgis-2-8-wien-calculation-of-hypsometric-curves","status":"publish","type":"post","link":"https:\/\/www.sigterritoires.fr\/index.php\/en\/qgis-2-8-wien-calculation-of-hypsometric-curves\/","title":{"rendered":"QGis 2.8 Wien: calculation of Hypsometric Curves"},"content":{"rendered":"

The new version of QGis is available.
\nAmong the novelties announced in this version, the calculation of the curves hypsometric is included.<\/p>\n

Here is an article to discuss how this new algorithm works.<\/p>\n

A little reminder about the<\/strong> hypsometric<\/strong> curves<\/strong><\/p>\n

Given that most meteorological and hydrological factors are depending on altitude, it is interesting to study the hypsogram or frequency histogram of elevations of the watershed for each interval of altitude.<\/p>\n

The histogram is a graph on which the values of the altitudes are spread in intervals on the x – axis; and in the y-axis, the surface, in % of the total area between two successive altitudes .<\/p>\n

The hypsometric curve is a curve that can be built on the previous graph where the ordinates represent the catchment area which is located above the coast altitude defined in the abscissa<\/p>\n

\"\"
\nThe histogram and the hypsometric curve reflect the distribution of altitudes within the watershed and allow determining the following characteristic altitudes:
\n* average altitude: it’s the average abscissa of the hypsometric curve.
\nAverage altitude = ( alt.max + alt.min ) \/ 2, and also, following the weighting method
\n* Most frequent elevation: it is the maximum of the frequency of curve altimetry or, also, the altitude of the largest individual area between two successive contours.
\n* \u00bd frequency altitude: it is altitude corresponding to the ordinate point 50% of the hypsometric curve.<\/p>\n

Recent research have shown the general aspect of the curves according to the erosion state of the watershed:<\/p>\n

\"\"<\/p>\n

-for the most recent basins (3), the area is weak if compared with the initial altitude change l, which is characteristic of steep basins;
\n-the old basins (1) show the opposite ; that is to say, a gentle plain near a watercourse where the altitude varies very little in spite of being a large area;
\n-the third case (2), is approaching to the so-called \u00ab\u00a0mature\u00a0\u00bb state, at least according to this degree erosion classification associated with the age of the basin .<\/p>\n

The<\/strong> calculation<\/strong> of the<\/strong> hypsometric curve<\/strong> with<\/strong> QGis 2.8<\/strong><\/p>\n

We find the command in the treatment menu -> Toolbox to display the treatments window and, in the text box \u00ab\u00a0Geoprocessing QGis \u00a0 -> Raster tools -> Hypsometric curves<\/p>\n

\"\"<\/p>\n

To execute the command two layers are needed:<\/p>\n