Surface TOPO Files, Know When to Quit

We need a topo for all sorts of reasons. Initially the existing ground needs to be measured to verify your takeoff quantities. In progress topo’s are a great way to get paid for what you have done. As Built topo’s will verify you have done the job according to plan.In a perfect world, the topo will be done correctly in the field requiring little work to get it looking good in the office. Often that is not the case. Here is the take home message for this offering;

If the field does not do a complete topo and quits early, the office staff will need to do extra work and quit late.

You will see an article soon in the analog magazine that deals with this subject in depth, this is a primer to help educate users of GPS equipment in the mysteries of surface topo acquisition. This article will go over what to look for and how to deal with it. When the field hands you lemons, make lemon furniture polish. The topo at the right is from an existing roadway. A look at the triangle faces shows us what we need to know, but first a step back.

The lines we are looking at represent the TIN, (triangulated irregular network) topo points were shot in the field and imported into a software program. The points linked to make the TIN, all being flat triangles. Now that the TIN has been explained let’s look at this surface. The long lines represent the driving lanes. They are quite far apart along the road and will have the tendency to make the road surface modeling inaccurate. Outside the road, it appears there are ditches and culverts but still just a guess. Here is what a poor topo looks like in cross section;

The cross hairs in the lower part of the screen capture represent the middle of the road. Looks pretty bad, and not a representation of a 2% cross slope. The reason the topo is choppy is because the cross section points were taken at too long an interval to be accurate. At many areas in this topo the shots were taken ad 500 foot intervals.

In all fairness this may be the best that could have been done. It could be a busy road; no one wants to be killed by a driver while taking a topo. That requires a trip during a slow traffic period to correctly fill in the topo in order to make it look right. This was not the case and the field crews gave up too early. Now the office needs to work too late. Here is what is required they do.

The Office Work for a Bad TOPO

When we get a file like this, we are expected to make a good looking surface in spite of the poor data collection from the field. Every job will be different, but here are some guidelines for success;

 • Get some type of idea what the location looks like. Pictures or a look on Google earth is better than nothing.

 • Resign to yourself you are going to have to do some guessing. Points and breaklines added to a topo surface need to be stored on a different layer. NEVER do any additional work on the original topo. All your repair work needs to live somewhere else. When the field crew goes out and gets more data, you can bring it in and easily remove the guess work you did if it is on a different layer.

 • Use cross sections and contours throughout the process in order to check your work. The road job shown here is easier than a site. We at least hope the job looks a bit like a road and the assumptions are easier due to standard road parameters.

 • A site that has had eighty swimming pools worth of illegal dirt dumped on it is a different story. Google Earth will usually not help, the satellite images may be out of date rendering any visual inspection inaccurate.

 • Breaklines are the best way to connect points that seem to belong together. In other words, try to find the high point where you think the road is and connect the points found to make the centerline. The low points can be chased and connected with breaklines to try to assemble a ditch bottom. Same is true for the top of slope and swales that are often times a guess.

It will require many surface inspections and adjustment of breaklines. Every bit of work you do on a surface topo will make it look that much more like reality. I have spent many hours trying to fix bad topo’s, it is about my least favorite things to do.

While you are at it, talk to the field crews. Have them look at the article coming out in the magazine about the right way to do a topo.

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