Tech Tips

Creating a DTM from a Contour Drawing

This tech tip explains how to create a DTM from a contour drawing.


In the ideal situation, the survey data provided for a design is in ASCII format, provided by either the client or a surveyor. In some circumstances data in this format is not possible. Sometimes the only information that is available is old drawings that have contours on, and this information has been digitised or scanned and saved as a drawing. The contour lines on the drawings need to be polyline, i.e. if you select the line in CAD the entire contour needs to be selected. Civil Designer tracks the cad line and places points along the line.

Creating a New Project

We will open the dwg file and then add the dt7 file to the drawing.

File Menu Open

File Menu Project Settings

We will be adding a dt7 file to the drawing, the survey points will be added to the dt7 during the conversion process.

After adding the dt7 file, Click OK to continue.

Converting the CAD lines to DTM Points

Due to the many enhancements to the underlying CAD in Civil Designer 2011 , you can now have a cad drawing that has contours with elevations (3d contours) This will make the conversion process far quicker, as the elevations will be read from the cad drawing. if you do not have a 3d contour drawing, then the elevations have to be manually entered as you do the conversion.

As a quick test to see if the contour has a Z elevation, select one contour and then press F4, the properties dialog will appear

Under Geometry, the Z Property will be shown.

Applications Menu Terrain

Model Menu Polylines to DTM Indicated Polyines

Click on the polyline.

The Polyline to DTM dialog will appear, the point name can be assigned, the height to assign is automatically read from the 3d polyline, but can be modified if required.

Note on spacing:

If set to anything other than zero, then the spacing value is used to separate the generated points by an equal amount. The alternative of leaving it zero (as done prior to version 6.5) leads to a DTM points that more accurately reflects the polyline, but often with an overkill of points that then need to be thinned out using say Automatic Screen. Spacing gives you a more usable sequence of points.

The spacing is applied following exactly the polyline route and does not intend to be the final join distance between subsequent points.

In addition to your intended spaced points, you will also see the occasional point or pair of points that do not follow the equal spacing. This is done intentionally where the polyline has bends sharper than 45. The idea is that these are important deviations in the route and Civil Designer automatically inserts points to accommodate for the bend.

Continue clicking on each polyline until the entire drawing has been completed

If you switch on the dtm point heights, you can see which lines have been converted.

Once the entire drawing has been converted, depending on the contour interval of the drawing, a point grid can be created based on the interpolated points, this will create a uniform grid survey.

Creating a Point Grid

Applications Menu Terrain

Terrain Menu Point Grid

A section of the converted contours before generating the point grid.

The new grid will be stored into an empty surface, select the grid spacing required, and make sure "Don't store points with zero height" is selected. If this option is not selected the generated grid might contain points with zero elevation.

Area is defined for the new point grid.

Once the point grid is generated, triangulate the new surface

A zoomed in section of the new triangulated point grid.

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