Topo Tactics—How to Collect Data in the Field to Make Quality Surfaces

A 1.057Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

I have discussed how to deal with bad topo collection in the past. When the office gets a file from somewhere in the ether, is the responsibility of the data engineer to make it work. This work is at least frustrating and at worst, dangerous. When you really don't know what the surface looks like, it's hard to connect the dots of a bad topo correctly.

I'm not discussing the quick point gathering for a pad site ready for development, or the former farm field that will soon be a shopping center. What we are discussing here is the site or road that has changes in elevation that when not closely detailed will cost you money.

We are all too familiar with the "pile of points" topo that comes from the field or other source. No descriptions of point types, everything on one layer and no guidance as to what the surface actually looks like. Where are the ditches and ridges, if any? Was there a slab that no one saw that needs removing, non-native spoils illegally dumped on the project?

There is a time difference between the quick point dash, and a correct topo. Follow along and I will show you the best way to gather points and get then to the office.

In our company, we use what I like to call the vacation principle. When you give me a file, I don't need to bother you if you are on a vacation to ask a question about what you did or what this layer is. You can make a text file with details, we just write notes on the screen in our CAD program. This helps us as well as others who are working on the same job.

I'll quickly go over what a bad topo looks like. I'm sure you have all seen them and there is no easy way to fix them. One needs to guess how the site really looks and the results are never as good as we like. I cannot provide hard and fast rules to detail a topo that needs breaklines added and bad points fixed, in my experience, here is the best approach.

• Turn on just the point marker and elevation text. Make it large enough so you can easily read at least a half a dozen points at a time.
• View the points in your 3D viewer, rotate the surface in all 3 axes and try to see what is happening.
• I find it easiest to connect low points of a ditch first. When you get started, you may need to move some points along the line around to get it to look right. This part takes time; it's also frustrating.
• Once you get some sort of low point established, the tops and other low areas become clearer.
• The job will need more work than you may think. Get good at doing breaklines, deleting all or part of them and re-doing. Get quick at this, you will do it a lot.

The Good Topo
The good news in this article is when you learn the correct way to perform a topo, the whole process is improved. There will be more field time, but the office will have good information as well as better dirt quantities. That is the primary reason for a topo so it makes sense to be as accurate as possible.

The topo has three specific areas that need to be addressed in order to be accurate; collection process; PCode management, and office detailing. Let's get to work:

Data Collection Process
You have a job site that needs to get a topo. What is the easiest and best way to do the job? Here are guidelines to help you with what to do before you actually go do it.

Drive, Don't Walk
When you need to collect data, do it the fastest way possible. This usually means driving. When it comes to driving, nothing beats a four wheeler of some sort. Here are some tips;
• Trucks are usually too bouncy and inaccurate for dirt. If you can drive slowly enough, they can work well for paved areas.
• When using a four wheeler, stiffen up the suspension and make the tires as firm as possible. This will increase the accuracy.
• Drive at a speed that allows the rover to take accurate measurements. Check tolerance residuals on the points occasionally to verify the quality of your information.

Walking Topos
No job can be completely done from the comfort of a vehicle. Sooner or later you need to get on your feet and do data collection. Here are some ideas;
• The biggest issue with walking topos is the stick is usually not vertical when the shot is taken, be careful and slow down.
• Shots done correctly with the pole are more accurate than those from a vehicle. I like to shoot some random pole shots to confirm the accuracy of the driving points.  
• Be sure you know your data collector well; they will take quick accurate manual shots with the press of a button. There is no need to go through menus every time. Get to know the equipment.

PCode Management
PCode stands for Point Code. The PCode is the description attached to a point when it is generated. It can also be added after the fact when editing a point. Of all the things I talk about here, correct use of PCodes will make the biggest impact on the quality of a topo. The reason is that with good point codes, office staff can tell where you were on a job and fill in the blanks where information may be missing; more on that later.

Here is the dirty secret about PCodes and why it's tough to use them; you either need to do more driving/walking or PCode changing to make it work correctly. Let's go over the details of PCodes for a topo;
• People get hung up on names. Here is the short answer. Pick a code, best if it's 2 characters; write it down along with a description of what it is. Some examples, DB=Ditch Bottom, EP= Edge of Pavement, GS= Ground Shot, TB= Top of Bank.
• If you are doing a road, there may be ten different PCodes for each cross section. You need to decide if you want to do the topo along or across the road. Along allows you to keep the PCode and walk/drive to collect. Across requires you to take a shot, change the code and shoot the next.
• In Figure 2, the road topo shown has PCodes for the edges of pavement and the shoulders in addition to the road centerline. With this information the office will know where they are on the job and help them to fill in any missing areas. When it comes time to generate contours, if there are spikes in the elevation, knowing where they are will make it easier to change the elevations to make sense or delete the point.
• Try to set up a PCode naming system before you need it. Have the office and field agree on what it should be and stick with it.
• A notebook taken along will allow you to keep track of things you will want to convey to the office. You will forget that there was a pile of concrete rubble, if you write it down, you won't.

Office Detailing
No matter how well a topo was done, post processing is necessary. If there are no mistakes to fix, the office work serves to tighten up the surface and make it perform better. Here is what needs to be done in the office;
• Review the surface for obvious spikes. There may be a 20 foot pit in the middle of the freeway, but not likely. Initially you can make the point 2D and possibly interpolate the elevation later. If the field did a good dense topo, the point can probably be ignored.
• Check the PCodes. Make sure the field staff did not forget to change PCodes during their work. Now is the time to make all the codes correct. When you are uncertain about what a code may be, remove the PCode. It is better to have no name than a bad one.
• Group the points. It is easier to look at all of the same type of point as opposed to the mass of all the different codes in the topo. In Figure 3, the TIN lines are shown connecting the points. Without PCodes, all we have to deal with are points that seem to go along the road, then take in some type of drainage pipe. Too many guesses for a good surface.

The single most important thing that can be done in the office is the production of breaklines for the topo. With that being said, let's review what a breakline is and what it does; A breakline connects points of a similar type and does not allow any link to cross it.

Let's use our road as an example. If there were not breaklines along the grade breaks of the road features, the TIN may miss part of the road and not correctly show the proper surface. In this example the road is missing the crown. Remember that all the triangles in a TIN are flat. Obviously this would not affect the quantities greatly, but if that were a ditch or a berm the quantities could be off substantially.

In a perfect world, the field used the agreed upon PCodes and everything is correctly tagged. The points are brought into your office program and initial review shows points that seem to be out of tolerance. They are made 2D and at this point you can commence with adding breaklines. Things are never perfect so I will work on a job with no PCodes and show you the process. Our client is replacing a culvert and took a topo of the area where the old pipe was removed. Here is the process I will use to build the surface;
• The field named all the points "TOPO" no help with PCodes.
• Upon importing the points, I generated contours at a tenth of a foot interval to see if there were any vertical spikes. Everything looked good.
• I need to generate breaklines in order to maintain the ditch cross section and stop any TIN lines from crossing grade breaks.
• To do this, I isolated the point marks and elevations in order to keep the screen clutter down. I have an idea this is linear, but several areas were confusing about what really is the low point.
• A few runs at creating breaklines and I have a workable surface. Figure 6 shows the breaklines in one of the easier to follow areas. There are some areas where information is missing and could look better with additional points. This is what we had to work with so we did the best we could.
• Figure 7 shows a 3D rendering of the ditch after the breaklines were added and the surface cleaned up. The slopes are straight and clean. Remember, this was the removal of an old pipe, the surface here represents the ditch cleaned up and ready for bedding for the new culvert. Without breaklines the ditch would not look this way, points would link in random order, there would be spikes everywhere and you would never know this was supposed to be a ditch.

Things to Remember
Very seldom do we get a perfect field collection file. This is tough work compounded by the elements, traffic and the conditions on the site itself.

Have the office and field staff meet and come to an understanding as to what each expects of the other. I have been in these meetings and the field thought that PCodes were just a way for the office to make more work for them. I opened a bad, then a good field topo and once they discovered the difference good collection methods and PCodes made, they changed their methods.

It is also common to process a topo in the office long after the situation on the ground has changed. If the field can give the office what they need the first time, the bottom line will benefit.

Marco Cecala is the owner of Take-off Professionals in Arizona and is a nationally-known expert and teacher of CAD, automated surveying and machine guidance.

A 1.057Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

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