Tutorial: the use of free online data to determine catchment boundaries
This tutorial explains the posibilities of freely available worldwide elevation maps for hydraulical and hydrological projects.
Click on
Zoom in to your area of interest (using
)Define the area of interest (using
or 
) -> a bounding box will appear defining the selected area
Download data for the selected area by clicking on this button:
Select te desired source of the elevation data (by default select “Aster DEM 30m (Global)”)
Ignore the warning by clicking on “Ok”
Select “GeoTIFF” as output format, “Lat/Lon” as projection
Click “Submit”
The data is processed and you will get a preview.
Click on the “Color Relief” button
A preview map is generated.
Click “Export as KML”
Google Earth will open showing you a Color relief map (purple/white is high, dark green is low).
This together with Google Earth’s own elevation tool (move your mouse over the map and in the bottom Google Earth will show the elevation:
) it is possible to determine the catchment boundaries.The latest version of Google Earth (version 5.2 and higher) has some additional tools which can be useful to determine the catchment boundaries and size. The latest version can be downloaded here: http://earth.google.com/
If you are unsure about a catchment boundary, or you want more information on the elevation, you can draw a line on you map and get the elevation profile along that line.
Click
to start drawing a line.You can select multiple points on the map to create a line between them. Click “OK” when you are finished. The line will show up on the map.
Right-click on the name of the line element in the “Places” panel on the left side of the screen and select “View Elevation Profile”.
A profile-view will pop up in the bottom of the window. If you move your mouse around the graph, it will show the exact elevation in the graph and the location on the map is marked by a big red arrow.
This graph and the elevations are based on the elevations in Google’s database. Most of the times they correspond with the Aster GDEM data (the data from the color relief map), but it can be different (more or less detailed).
If you have determined the boundaries of the catchment you can draw the catchment into the map by clicking on this button
and clicking on the map to draw a polygon of the catchment.Once you have finished drawing the catchment, click the tab “Measurements” to display the catchment size. Click “OK” to finish the drawing.
Example of the catchment for the location Yelbouga in Burkina Faso
