A.2 More about Grdcon

Program Grdcon will only allow 100 contours to be derived in each execution. If you wish to have more than 100 contours, you will need to run multiple executions and concatenate the result files. Listed below is an example of a .gcn data file.

Line Data 01 ... 700.0 1190.0 5.0 02 ... 1 03 ... 0.0 04 ... 1 05 ... 2 06 ... 0 07 ... 6000.0 16000.0 08 ... 50000.0 68000.0 09 ... 20.0 20.0 10 ... 6000.0 16000.0 11 ... 50000.0 68000.0 12 ... 20.0 20.0 13 ... burra-all.grd 14 ... (10f8.2) 15 ... burra-all.con

Line 01	These are the lowest contour, highest contour and contour interval to derive. There is an alternative form where these 3 arguments are all 0.0, then you supply the number of contours to derive and a list of the contour elevations one per line. If the contour interval is positive, then contours are derived arithmetically, e.g. 2.0 64.0 2.0 would derive contours at 2.0, 4.0, 6.0, 8.0, 10.0, ..., 60.0, 62.0 and 64.0. If the contour interval is negative, then contours are derived geometrically, and the modulus of the contour interval must be greater than 1.0, e.g. 2.0 64.0 -2.0 would derive contours at 2.0, 4.0, 8.0, 16.0, 32.0 and 64.0.
Line 02-03	The first line is the Special Value Option. It has 2 possible values that are 0 and 1. 0 means that the entire area was gridded, i.e. no polygon was used in interpolation. 1 means that some polygon was used to cut-off areas not of interest and the next line will contain the value assigned to all these non-gridded points. The supervisor program will automatically calculate the special value of the grid if you select this option.
Line 04	This is Label Spacing. It describes how often contours are labelled. This is not accessible through the supervisor programs and is automatically set to 1, i.e. every contour has a labelled elevation.
Line 05	This is the Label Fraction Length. This is not accessible through the supervisor programs and is automatically set to 2, i.e. 2 decimal places on the contour label.
Line 06	This is the Grid Type. It is automatically read from the Splin2h parameter file and not accessible in the supervisor programs.
Line 07-09	These lines are the bounds and resolution of the input grid. These are automatically read from the Splin2h parameter file and are not accessible in the supervisor programs. If these numbers do not match exactly with those in the .spl file, your derived contours will be unintelligible.
Line 10-12	These lines are the bounds and resolution of the output grid. These are set by default to the same as for the input grid, but may be changed to a subset of the grid. You can also change the grid resolution to an integral multiple, to increase or decrease the final resolution of the derived contours.
Line 13	This is the name of the DEM file. It is automatically read from the .spl file and is not accessible in the supervisor programs.
Line 14	This is the format of the grid file. It is automatically read from the .spl file and is not accessible in the supervisor programs.
Line 15	This is the name of the file you want derived contours to be written to. With multiple runs of Grdcon, you can derive different contour sets and paste them together afterwards.

Normally contour files are created by specifying a minimum and maximum contour along with a fixed contour interval. If you desire irregular contour spacings you must specify the total number of contours you want and then list each elevation in sequence. For example, to produce 2 meter contours between 100m and 120m you would write:

100. 120. 2.

However, to get 2m contours from 100m to 110m and then 5m contours up to 120m, you would instead write:

0. 0. 0. 8 100 102 104 106 108 110 115 120

The data triplet of 0. 0. 0. at the beginning of the block is a flag which indicates to Grdcon that variable contour intervals are to be used. The easiest way to implement these instructions is to get inside the .gcn file with the screen editor and simply replace the line which deals with a uniform contour interval.