Data Prep: Tips, Tricks & Techniques: Dont Take Contour from Strangers

Written by Chad Cooper

Created on Saturday, 04 July 2009 20:19

MachineControlOnline.com Exclusive Column by Chad Cooper, BSCE

At one point or another, most everyone who builds data has been tempted to do it. You’re under the gun to finish your model quickly. Time races toward the deadline, when all of a sudden a shimmer of hope; you feel like you have found a break and fortune has smiled upon you as someone provides you with an electronic format for the proposed grade contours. While you should spend hours detailing the exact elevations for the site, you decide to spend only a few moments building a surface out of only the proposed contours. Completing the project takes only a few more minutes as you add critical break lines. You have now met your deadline … but have you provided good data? The answer is “No”.

Allow me to explain. Proposed grade contours are the proverbial double-edged sword. You will find them in nearly every project that you work on. There is strong temptation to use them as the foundation for your model; and for some, perhaps standard operating procedure. The use of proposed grade contours are so important to the data building process that I am going to spend the next two articles addressing them and their proper use.

When building data, elevations for your project come from a number of sources: plan and profile, super elevations, typical sections, cross sections, proposed spot elevations, pad/finish floor elevations, detailed grading plans and of course, proposed grade contours. Since your goal is to recreate the engineer’s design exactly, we need to incorporate any and all grading information provided by the engineer. When contradictions arise, as happens frequently, the elevation with the greatest detail wins. Any given project, regardless of size, may include all of the above sources of elevations, or only one. However, regardless of the project type or size, the availability of  proposed grade contours is almost universal. It is natural then to see how one would be tempted to use contours as the basis of their project.
It is important to understand that proposed grade contours are but a single piece of the data prep puzzle.  I will share from my experience, that proposed grade contours are the very last thing I add to my model, and even then I may eliminate them entirely. As a data prep design engineer, proposed grade contours are based on my design; my design is not based on proposed grade contours. They take a secondary role to more accurate and detailed sources of grade. Again, the greatest detail wins and that is almost always not the proposed grade contours.

Contours are only as accurate as their interval. In other words, if you have a one foot contour interval you can only be certain of the exact elevation every one vertical foot. Low or high points, grade breaks and greater detail cannot be conveyed by contours alone. If your model is going to be used for rough, mass grading only then using only the contours to build your model may be sufficient. When this occurs, I notate the layer names, “The provided model is for rough, mass grading only!” In addition I email and convey by phone that all I have done is taken the engineers provided contours, built a surface, cleaned it up and shipped it out. Our price also reflects this work. I should add that rough grading models occur as one out of a hundred for my business.

In next months article I will detail my procedure for determining when to use and when not to use proposed grade contours. In latter articles, I will discuss what to do when proposed grade contours are your main or only source of grade.