Create A Topographic Survey With A Dual-Frequency GPS and Total Survey Station

Introduction
The ultimate goal of the first topographic activity was to conduct a topographic survey with a dual-frequency GPS and a Total Survey Station. We were to learn how to set up and connect the equipment, and be able to troubleshoot issues as they arose. After collecting data we were compare the two survey methods, analyze the advantages and disadvantages of each, and be able to apply each to appropriate projects.

Study Area
The study area for this activity was the UWEC campus mall between the Davies Center and Schofield Hall (Figure 1.) The area can be found behind 105 Garfield Avenue, Eau Claire, WI 54702.

Figure 1. Google Earth image of the UWEC campus mall.

This location was chosen because it was a large enough area (1 hectare) for our survey criteria and had adequate microtopography change with which to create a significant survey. It's recent construction in 2013 meant that it may not yet have been adequately surveyed using this equipment for this exercise. The wide open space allowed us to obtain 100 topographic points without the need to create break lines which would complicate the project beyond the scope of this exercise.

Methods For Dual-Frequency GPS
This exercise and the following topographic exercise seemed to be a hard lesson in "buttonology," or the maddeningly specific series of actions one must take in order to work with the equipment. In class Prof. Hupy treated us to a demonstration for the Topcon Tesla.(Figure 2.) The Tesla connects to a Topcon Hiper which is the dual-frequency GPS which gives survey grade accuracy.(Figure 3.) The Hiper attaches to the top of a standard height tripod. The height is standard so the GPS can account for the distance to the ground. We also employed the use of a portable wifi hotspot called Mifi. (Figure 4.) This ensured that our connection would remain steady and stable. This also meant we could collect data in more remote locations without wifi. If we had used the campus-wide wifi, the signal would be unstable and could drop which would severely impact the data. It was important that we didn't "wander off" with the portable wifi after connection lest we lose our signal and corrupt our data.

Figure 2. Topcon Tesla similar to the one used in the exercise.

Figure 3. Hiper Dual Frequency GPS mounted on a tripod of known height.

Figure 4. Mifi portable hotspot.

 We then had a lecture outside where each of us set up our project, specified collection criteria, and practiced connecting and disconnecting the device safely. We were warned that an incorrect disconnect could cause a whole host of issues so we were very conscious of our disconnection method.

My partner Grant and myself were to collect 100 data points. We opened up the job that we had created in the class demonstration and set the Tesla to take 15 points at each spot. The Tesla would then average the locations of those points to select the final data point. This is to account for any minor movement or error that can occur with any handheld device. We chose 15 because we felt it was accurate enough for the purposes of this exercise, but time efficient enough to enable us to collect 100 final data points within an hour.

In order to obtain a point, we moved the tripod over a desired spot and used the attached bubble level and adjustable legs to obtain as precise of a level as we could. We followed a back and forth meandering pattern in order to achieve a microtopographically diverse dataset.

After we completed our survey, we exported the data from the Tesla in the lab. After opening our project file we exported the data as a text file using Exchange. (Figure 5.) We had to "clean up" the data by removing extraneous text from the top line in order to make it more useable and clear when importing it into ArcMap. (Figure 6.)

Figure 5. Grant exporting the data into a .txt file.

Figure 6. "Cleaned up" text file.

In ArcMap, we chose 'Add XY data'  and selected our X field to specify for the Easting, the Y field to specify for the Northing, and the Z field to specify for our height. (Figure 7.) Because of licensing issues we were in Demo mode and could therefore only collect 25 points at a time so we had to repeat these steps three more times to get all 100 of our points into ArcMap (Figure 8.)

Figure 7. Add XY Data window. Here we specified the X,Y,and Z fields to the Easting, Northing, and Height fields.
 

Figure 8. Map of data from the Dual-Frequency GPS survey. Because the data had to be broken into four different files, I wasn't able to create an elevation map for this section.

Methods For Total Survey Station
A similar technique was used for the Total Survey Station survey, however some more equipment was used. This included a Topcon Total Survey Station (Figure 9.), the associated tripod, and a survey prism.(Figure 10.)

Figure 9. Total Station similar to the one used in the exercise.

Figure 10. Prism similar to the one used in the exercise.

In the class demonstration we learned how to properly level the total station through increasingly precise adjustments. The data collection demonstration proved to be an issue even for Prof. Hupy as buttononlogy claimed even a seasoned expert. (Figure 11.)

Figure 11. Prof. Hupy expressing visible anguish as he experiences 'buttonology issues.'

In our own survey Nik, Scott and I got our station leveled fairly quickly and needed to employ the help of Prof. Hupy to get us started.(Figure 12.) The next step was to establish backsight points. These points were taken by Scott with the Hiper using the same technique as the Dual-Frequency GPS survey. (Figure 13.) As we learned in the distance azimuth survey exercise, once an accurate location of a point is known, we can then use them to orient another survey method, in this case our total station.

Figure 12. Scott and Nik setting up the total station.

Figure 13. Scott obtaining the backsight points.

Data points were collected by aiming the laser within the Total Station at a reflector within the prism mounted at a known height of two meters. The angle of the reflection and height of both the prism and the total station can then be used to generate highly accurate data points that can be used in a microtopographic analysis.

After connecting all of our equipment we were ready to take the survey points. Blessed with a steady hand and generally poor eyesight, I elected to hold to prism at various locations and elevations within the study area while Nik and Scott took turns aiming the laser housed within the total station and collected the data points. (Figure 14.)We collected points at a variety of elevations along Little Niagara because that area offered the steepest elevation gradient within the campus mall area and was where Scott had initially taken his dual-frequency data. The demo mode only allowed us to collect 25 points and with a heavy and unrelenting downpour adding to the geospatial fun, we collectively decided that enough data had been collected to achieve the purpose of the exercise. Data extraction to XY data followed the same process as above and resulted in the following map.(Figure 15.)


 

Figure 14. Nik getting ready to collect the datapoints using the total station.

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Figure 15. Elevation map created from collected data.

Results/Discussion
Both of the survey methods produced highly accurate elevation data. Unfortunately due both to weather, time constraints, and equipment issues, I was not able to use both survey methods to survey the same area.

I found the dual-frequency GPS survey method quick and easy to setup. There was less equipment to worry about and this method could realistically be completed with only one person. The disadvantages of this method is that I questioned the accuracy because I don't believe that we were ensuring that each data point was perfectly level; especially when we started going a little faster in order to return the equipment at the required time. The tripod was a little cumbersome especially with a broken leg and needing to adjust the legs at every point became a bother. If you are by yourself and need to collect a limited number of points (0-500) I would recommend this survey method.

I found the total station setup complicated and confusing. At first I had a hard time understanding what the backsights were for but I eventually connected it to the distance azimuth survey concepts which made the concept more palatable. Leveling the total station was not as difficult as I had anticipated and once we got the total station working I found that data collection went especially quick, even more than the dual-frequency method. Although set up took longer and there was more equipment to account for, the accuracy for this method was better because it was not necessarily relying on human adjustment, and instead relied on the established parameters. A disadvantage of this method is that it can take a novice a long time to set up and it requires at least two people; one to hold the prism and the other to take the data measurement.

Generally, I found these survey methods initially complicated, but ultimately accurate. I am sure that an expert can produce great quantities of data at an incredibly fast pace but I am unfortunately not presently at that level. Perhaps with practice I can increase my setup efficiency. I am thankful that I could rely both on my group members, Martin, and Prof. Hupy when we got stuck.

Conclusion
This field exercise was a good introduction to the realm of 'real world survey methods.' Data setup is often overlooked in the bigger picture, but it is nonetheless vital to achieve proper accuracy. Knowing how to troubleshoot is also an ever improving skill that I personally am working on for all fieldwork. As always I am thankful that I can add this method to my field survey repertoire.

Sources
Google Earth, UWEC Campus
Hiper image from Bunce Industries LLC website
Mifi image from Softpedia SoftNews website
Topcon Tesla, Prism, and Total Station image from Topcon website