Contour DTM file

You can use this function to extract contours from a DTM.

To run this function: Choose Surfaces > Contouring > Contour DTM file, or...

In the Function Chooser, type EXTRACT CONTOUR, and press ENTER.

Contours are created by interpolating line segments at nominated values for every triangle in the DTM. The points are then joined to form continuous contour strings, which are written out to a new layer. The contours produced honour the original data from which the DTM was produced.

Contours are extracted from the DTM and stored in a new layer. You can use the contour layer for:

plotting
volume calculations and polygon intersection with other string files

If contours are required for volumes, or polygon intersection, they must be closed strings. To make sure the contours are closed strings, the original DTM must have a closed boundary string. If the DTM represents a pit, the boundary string must also have a Z value higher than the highest Z value in the data. If the DTM represents a stockpile, the boundary string must also have a Z value lower than the lowest Z value in the data.

Field	Description
DTM from which contours will be extracted
Location	The DTM to be contoured. When you select a DTM, the range and contour values are updated to the extents of the DTM. Note: The DTM does not have to be displayed in Graphics to be selected.
Field to contour	The field of the DTM to be contoured. This can be the elevation (Z value) or any description field.
Object ID	The ID number of the object in the DTM that you want to contour.
Trisolation ID	The ID number for the trisolation of the object in the DTM that you want to contour.
Define contours by
Interval	The contours to be extracted are defined by entering an interval with a minimum and maximum contour value. If you choose this method then the contours will always be at integral intervals of the contour interval. For example, if the minimum contour value is 147.39 and the contour interval is 2 then the first contour will be extracted at 148.
Range	The contours to be extracted are defined by entering a range. Unlike the interval method which guarantees that contours will be at integral intervals of the contour interval, this method allows you to define the contour levels of interest precisely. It is possible to create contours at intervals of two but starting from a non-integral interval of two. For example, 147.5, 149.5, 151.5. You could also use a range file to define the range. This method also allows you to extract contours at irregular intervals, making it possible to extract contours at values of particular interest.
Minimum contour	Available only if Define contours by is set to Interval. The minimum contour value.
Maximum contour	Available only if Define contours by is set to Interval. The maximum contour value.
Contour interval	Available only if Define contours by is set to Interval. The interval between contours.
Contour range	Available only if Define contours by is set to Range. The range of contours that are required. The contour range can contain only a single value or it can contain any simple or complex range expression.
Define the contour string file
Location	The string file created to contain the contour strings.
Produce contour annotations
Define the contour annotation details	Selected: A string file is created that contains the contour value at the midpoint of each contour. You can use the contour value to label the contour strings for plotting. String one in the string file contains the data for normal contours. String two in the string file contains the data for index contours. Cleared: No string file is created for plotting.
Location	The location to save the annotation string file.
Annotation range	The range of contour values for which contour annotations are generated. Note: For information on how contour annotation string files should be processed by the plotting module, refer to the Contour Annotate function.
Define plot enhancement requirements
Create index contour file	Selected: A string file is created that contains the index contours. You can use this file to plot the index contours in a different colour for plotting. Cleared: No string file is created for plotting.
Location	The location to save the index contour file.
Index value	The index contour value to determine which contour strings should be saved to the index contour file. Note: If you choose to create an index contour file, then the index contours are not saved to the contour string file, they are saved only to the index contour file.

Output

The contour strings and annotation details are saved to the string files as described. Each discrete contour level consists of one or more string segments all with the same string number. The contour segments are open or closed depending on the nature of the DTM.

If the DTM you are contouring was created on a non-horizontal plane, and you chose to contour by a description field, the contour strings are generated on the plane used to create the DTM, and the contour value is stored in the D1 field of each point in the string.

If the DTM you are contouring was created on a non-horizontal plane, and you chose to contour by elevation, the contour strings are generated parallel to the plane used to create the DTM, and the contour value is stored in the D1 field of each point in the string.

If the DTM you are contouring was created on a horizontal plane, the contour strings are created on the horizontal plane, and the Z value of the contour string represents the value of the contour.

Troubleshooting

Message	Description
`Warning: Some strings are open`	Some of the contour segments in the resultant string files are not closed. This is intended to warn you in case you expected all the contour strings to be closed.