Contour DTM in layer

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

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

In the Function Chooser, type GRAPHICS CONTOUR EXTRACT, 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 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, your 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.

Fields on the Extract Contours From a DTM form

Field	Description
Define the DTM to be contoured
Layer name	The DTM to be contoured. You select the layer using the list of available DTM layer names. 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	The contours extracted can be defined using an interval with associated minimum and maximum contour values, or by entering a range of contours that are required. 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 will be 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 therefore 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.
Contour Layer	The layer that is created to contain the contour strings. This must be a new layer. If you want to keep the data, you must save this layer.

Output

The contour strings are saved to the contour layer as determined by the inputs made. 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
`There are no valid DTMs loaded in Graphics`	This function cannot be performed because there are no valid DTMs from which contours can be extracted.
`Some strings are open`	Some of the contour segments in the resultant layer are not closed . This is a warning, in case you expected all the contour strings to be closed.