It is often convenient to create a digital terrain model (DTM) from a contour map instead from raw survey data.

The procedure for creating a DTM from contours is the same as with raw survey data. It is useful to check that the DTM matches the input contours by generating contours from the DTM and comparing the two sets. In most cases, areas where the generated contours differ from the input contours can be resolved by making the some or all of the input contours into breaklines. However, some cases will require additional breaklines. The following example illustrates the problem.

Original input contours are shown above as blue dashed lines.

After the DTM is generated, the above screen capture shows the generated contours in red. Notice that the generated contours do not match the original contours in some areas. This is most common whenever a contour forms a 'nose shape'.

To correct the problem a breakline is created in the direction of the 'nose' as shown above. Creating this breakline is done after a terrain model has been created as follows.

1) Click on the pencil button on the tool bar to create a new NO ELEVATION feature (this feature will be 'draped' over terrain model to get its elevations ).

2) Using the mouse create the breakline with points located at the original contours.

3) After the breakline has been created, change it to an ELEVATION/BREAKLINE feature using menu Edit - Modify Selected Feature(s) - Properties.

4) When the terrain model is regenerated the contours should closely match the original contours.

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