View source for Exercise: Mapping storm damaged forest areas

==Definition of storm damage==

Mapping of forest damages caused by storms using visual interpretation of very high resolution digital ortho photos (DOP) need clear definitions and characteristics which are usually documented as interpretation key. 
Here, clearings are defined with a
minimum area = 0.2ha,
minimum width = 20m,
minimum tree crown cover = 10%.

Characteristics of windthrow areas depend on the time between storm events and the availabiltiy of optical remote sensing images but also on the status and progress of logging and clearing operations at the time of the image aqcuisition.
Typical characteristics for recognizing storm damages are: 
horizontal complete trees with root plates, root plates, stumps with fresh cross section, piles of logs for storage / skidding, piles of branches, skid roads and tracks, no or few ground vegetation.

==Digitizing of a windthrow area==

Load and display German Geoadata following the [[Exercise: Displaying German Geobasis data (GBD)]] or load the saved project file ''GBData\display_gbd.qgs'' where the project coordinate reference system (CRS) is set to ETRS89/UTM32N (EPSG:25832).

# Create a new shapefile layer
## Select {{mitem|text=Layer --> New --> New Shapefile Layer}}. 
##* As layer type, select {{button|text=Polygon}}. Click the {{button|text=Specify CRS}} button and select ETRS89/UTM32N (EPSG:25832). 
##* To add an attibute: 
##** For the attribute's name type ({{typed|text=Class}} into the {{button|text=Name}} field of the {{button|text=New attribute}} section.
##** Select {{button|text=Whole number}} as data type.
##** Confirm with {{button|text=Add to attributes list}}. 
##* Confirm with {{button|text=OK}} and enter the path and name (e.g. {{typed|text=clearing.shp}}) in the following menu.
# Digitizing areas
## Select the new shapefile in the Layers window. Start the edit mode by clicking the {{button|text=Toggle editing}} button [[file:QGIS_2.0_Edit.png|25px]].
## Zoom in on a cleared area in the forest (figure '''A'''). Click {{button|text=Add feature}} [[file:QGIS_2.0_Addpolygon.png|25px]] to start digitizing. Draw polygons according to the definition of clearings. Simply click the desired point on the canvas to start. After selecting a second point, the third point will construct a triangle and each additional point will add another vertex to a polygon. Finish the geometry by right-clicking and entering the attributes in the appearing window (just use an increasing number for the ID, and Class = 1 in case of a windthrow area). Take your time to get used to this way of digitizing -- you can always delete the last polygon by simply clicking {{button|text=Cancel}} in the attributes dialogue, or by selecting it [[file:QGIS_2.0_Select.png|25px]] and deleting with the {{button|text=Delete selected}} [[file:QGIS_2.0_Delete_selected.png|25px]] button. An example of a finished sample point can be seen in figure '''E'''.

==Estimating area and perimeter==