Chapter 2 ....continued
Running the Digital Elevation Model

Sections 2.4 & 2.5

2.4 Running Splin2h and Grdcon using Topog_Demgen

To run Splin2h or Grdcon simply depress the button labelled 'Run' in the menu window and select one of two available options in the scroll-down menu. Neither application will run unless you have the appropriate parameter files (.spl and .gcn) in your local directory. Both applications run in the foreground (even though diagnostic messages are written to the screen in the background window). This means that you will not be able to perform any function in Topog_Demgen (like displaying data or building new parameter files) until the program has finished.
Splin2h produces the following output files:

basename.grd the grid of elevations basename.snk sink diagnostics file (optional) basename.str stream diagnostics file (optional) basename.sys data bounds file

Grdcon produces the following output file:

basename.tcn topographic contours

Of the above files, only the .grd and .tcn files can be viewed using Topog_Demgen

2.4.1 Generating a contour map with irregular contour intervals

Some times it is not appropriate to generate a contour map with a regular contour interval. To overcome this problem, run Grdcon several times with different low and high contour and contour interval values. By doing this you will generate multiple .tcn files that can then be concentrated together to form a single .tcn file. An example is shown below.

Example 2.1 Generating a .tcn file with multiple contour intervals.
The problem:
The user has a catchment with an elevation range of 560-730 m. The terrain is fairly flat towards the bottom of the catchment, but gradually steepens towards the top of the catchment. Using a single contour interval for the whole catchment produced a map in which contours were sparse at the bottom and tightly packed at the top.
The solution:
Run Grdcon three times, changing the contour range and interval on each occassion.

How to do it?

For run #1 set the low and high contour values to 560 and 620 m. Set the contour interval to 1 m. Once Grdcon has run, rename the output file from basename.tcn to basename.tcn1.

For run #2 set the low and high contour values to 622 and 700 m. Set the contour interval to 2 m. Once Grdcon has run, rename the output file from basename.tcn to basename.tcn2.

For run #3 set the low and high contour values to 703 and 730 m. Set the contour interval to 3 m. Once Grdcon has run, rename the output file from basename.tcn to basename.tcn3.

Concatenate all three .tcn files by typing:
$ cat basename.tcn1 basename.tcn2 basename.tcn3 > basename.tcn

2.5 A final word on the DEM

Some of the output files from Splin2h and Grdcon can also be displayed in Topog_Display. Skip ahead to Chapter 6 to learn about this program. Use Topog_Display to plot the DEM contours on screen along with high points and saddles (see Chapter 3), and consider the following:

Have you generated the right number of contours, and are they distributed in the appropriate manner? If not, edit the your .gcn file and rerun Grdcon. If your catchment has a mixture of very flat and very steep sections, consider rerunning Grdcon with variable contour intervals.

Do the contours generated by Grdcon adequately represent the shape of the catchment? You may find that the input data supplied to Splin2h was either insufficient or erroneous. If this is the case, check the data files, repair or append the data, modify the .spl file and rerun Splin2h.

Are the high points and saddles (as defined by your original .hpt and .sad files) in correct positions? If high point or saddle elevations conflict with contour heights, then edit their values in the .hpt or .sad files.

Is there sufficient information to resolve a catchment boundary? This is one of the most important questions. Often, elevation data sets have very few data points located around the catchment boundary. If insufficient information is provided in this area, then Splin2h and Grdcon may have difficulties in properly interpolating the contour shape. If the contours near the boundary are poorly defined, then Topog_Element (see Chapter 3) might not be able to resolve a catchment boundary automatically.

Take me out of frames

.... on to Chapter 3

last modified on 15 June 1997

Chapter 2 ....continued Running the Digital Elevation Model

Sections 2.4 & 2.5

2.4

Running Splin2h and Grdcon using Topog_Demgen

2.4.1 Generating a contour map with irregular contour intervals

2.5

A final word on the DEM

Chapter 2 ....continued
Running the Digital Elevation Model