DTM Contours

Track contours at any interval, through a regular grid of elevation points.

This function allows you to track contours at any interval, through a regular grid of elevation points created in DTM Grid Volumes. This implies that one of the DTM Grid routines has to be run first.

If the DTM grid was created on a portion of the site, this is the area that is automatically displayed for contouring. Only the contour interval needs to be entered, and fractions such as 0.5 are permitted. The contours then track through each DTM grid 'sheet' in the order in which they were created.

The type of contours created depends on the DTM grid generated. If either a Single Surface or Ground/Design Surface DTM was created, the contours conform to the relative terrain surface. If the Ground/Design Difference DTM was generated, the contours produced indicate depths of cut-and-fill (Isopachyte contours).

The area first has the original gridding squares, or sheets, plotted on it. Each square is scanned round its edges, and then quartered in the search for a contour start position. If a start point is not found, the contour is abandoned. It is therefore possible for highly localised contours to be omitted if the DTM grids are too large.

Always check your contours. Being a grid-based system, the contouring is almost infallible. The area can be any shape and even have holes in it. Any topographical feature, other than a reverse slope, is contoured as long as it is reasonably well defined by the spot heights.

However, as described below, small shifts in the contours from their theoretically ideal positions are possible. Furthermore, where there is insufficient information such as at the survey edges and in poorly defined banks, the computer may make erroneous assumptions. Particular attention should be given to these areas.

See Also Contours from Lines versus from DTM.

Procedure

1. You are prompted to specify:

2. Specify the contour interval you require and click OK.

Which Points Get Operated On?

The extents of the grid initially created, which will have been limited to the current view at the time.