Borstad Associates Ltd. - CSA Outreach Task 3

CSA Outreach Task 3

Project 4. If possible, make a three-dimensional model of the watershed that can be viewed from different angles on the computer.

Canadian Digital Elevation Data (CDED) are available at no cost from the GeoGratis web site. However, a more complete set of elevation data based on the 1:50,000 or 1:250,000 NTS map sheets is available for $25 (+ taxes) on the CTIS web site. For example, the 1:50,000 maps sheet for the Ottawa area is 031G05 and the elevation data can be downloaded as a zipped file for the east and west parts of the map sheet as 1.2 MB zipped file. Unzipped, the files are each 9.6 MB.

These data can be used in geographic information systems (GIS) for land-management applications. The basic elevation data can be used to determine the orientation and slope at elevation each point when used in GIS applications. CDED can also be used for terrain modeling, for inter-visibility studies, for satellite image rectification, for flood simulations, etc.

For the purposes of this demonstration project part of the Gatineau Park north of Ottawa has been used as it was decided the Barrhaven area was just too flat. The Manifold program used in the previous projects can be used to “drape” satellite imagery and vector layers over the digital elevation data. It makes things significantly more straight forward if all the layers are in the same projection, but re-projecting the data can be done. The more detailed the information in each layer, the better looking the result (although at the cost of increased processing time).

Once elevation data have been imported into Manifold it defines the default view of the surface with a 2D greyscale representation (Image #1).

By using the View > Display Options the user can change any number characteristics of the view from a colour ramp based on elevation (Image #2) to shading (with sun angle options). There are variety of built in standard colour ramps to allow for image creation. The simplest method is to use one of the standards, see what it looks like on your surface, then modify as you see fit. The image below was generated by using the defaults for height colour ramp.

Upon surface import, Manifold also generates 3D view of the same surface. A variety of overlays can be applied to either the 2 or 3D versions of the surface. The same display options are available as well as feature overlays. In this case we have overlaid the surface with the hydrology and contour layers (Image #3). Also, various aspects of clouds, fog, water, and lighting conditions can be used to modify the picture of the 3D surface at the user’s discretion.

The image to the right (Image #4) is a demonstration of vector being overlayed on the original elevation model surface. In this case the roads are in black and the waterbodies are in blue.

As well as colour and vector overlay options Manifold can also overlay raster imagery. In this case (Image #5) a Landsat TM scene for the same area is overlaid on the terrain model to provide a more realistic impression of the country. Manifold does have a limitation on the number of colours and pixels that it can display in this environment. Improved PC processing power and video card capabilities may improve this situation. For raster overlays there are software systems out there that are capable at providing a much better result, but they come with a much higher cost.

When viewing/working with the 3D representation of the surface there are navigation buttons that allow the user to position the viewpoint as they choose. Changing the surface properties to reduce the number or display tiles makes the screen refresh much faster, although on a smaller window. The up/down arrow keys can be used to tilt the view up/down, the S and W keys move the view point backwards and forwards, the Q and E keys move the view point up and down, and the A and D keys move the view point left and right. Holding the <Shift> key also accelerates the movement of the point of view. The top of the surface is a solid barrier. If you are moving in one direction and you encounter the surface (side of a hill) then you will continue to move in the same direction, but also up the surface.