This is how I can tell who knows what they are doing and who does not when it comes to existing ground Digital Terrain Models (DTM).

Profiles

Profile views of road centerlines are a good place to start. Most have a surface profile of the existing ground. When the existing conditions are pavement just ask yourself. Can you drive a car on this road or is it too bumpy?

Contours

When looking at contours visualize the slope between two contours. Is it uphill or downhill? Do the contours form a ridge or a swale (ditch). A crowned road should form a ridge, the center being higher than the edge.

The crowned road shown above has contours in the red oval that do not represent a crowned road.

Triangles

DTM displayed as triangles are the most revealing of flaws. When I use the term flaws it is not the source data that is bad it is the interpretation of that data that is flawed. Triangle edges show how the source data connects to form a Triangulated Irregular Network (TIN).

The crowned road shown above has triangles in the red oval that do not connect the centerline points correctly. The edge of pavement points are not correctly connected.

Correcting errors in interpolation

The best way to correct problems is to add breaklines where you want to form triangle edges.

Do not flip faces

Look at other work you have done when you flip faces (a surface editing technique). Can you follow what you did if you needed to recreate those edits? Not easily right? The “I don’t care” part of the audience can continue editing in this manner. I will continue to bristle each time I encounter “Bad CAD” because I am uncertain of how the end result was achieved.

Breaklines

Breaklines enable you to “follow the footsteps of the original surveyor”. It helps you figure out what you did to force triangles to represent existing conditions accurately (as accurate as the source data allows you to).

Conclusion

Make a better DTM

Learn the warning signs of bad data interpolation.

Profiles that should be smooth but are not is a good clue that something is wrong. Practice displaying a surface using a smaller contour interval so flat areas have more contours to help you spot errors.

Change the surface display to triangles and start looking for flat areas in crowned roads and/or ditch bottoms.

Adding breaklines to a surface help you correct triangulation to better represent existing conditions. They also help you keep track of what areas have been checked and corrected, and thru the absence of breaklines, show you what areas may still need attention.