Lab 9: Comparing Surveying Methods: Total Station vs. Dual Frequency GPS

Introduction:

The class was introduced to two labs over a two week period. They both had the same goal in that points were to be collected using survey grade GPS units on the University of Wisconsin-Eau Claire's campus mall (see figure 1). The last lab using the distance/azimuth survey of the Davies Student Center parking lot was very useful, but there were many errors found in the results once imported into ArcMap. With this method of surveying, there was no way in which to calculate elevation, however, with the two GPS survey grade devices used in this lab, distance, azimuth, and elevation could be calculated.

Figure 1. Image of the campus mall. Take note of the Little Niagara Creek in the bottom portion of the image.

The means in which the points were to be collected were by using a Topcon HiPer GPS (see figure 2) and Topcon Total Station (see figure 3). Our professor, Joe Hupy, gave the class hands on tutorials for using the equipment before the class was split into groups of two to collect the data. He stressed the importance of taking the time to make sure the GPS's were level on the ground, because without it, the data collected would basically be obsolete.

Figure 2. Topcon HiPer (device on the end of the stand) and Tesla (held in man's hand) used for the dual frequency GPS survey.

Figure 3. Topcon Total Station device. This was used in the second survey. Unlike the HiPer, the Total Station was stationary which required a second person to hold the reflector pole in order to capture a point (as seen by the man in the distance). 

Methods:

There were two parts to this lab, the first portion of the lab focused on the use of the dual frequency GPS, also known as the Topcon HiPer. For this survey system, a free standing post was used in which the HiPer GPS was connected to the top. A circle level is located on the device which is to be used to level the GPS before taking a point. The Tesla, for both surveying methods, was used as the field controller which regulated how the points were stored (see figure 4). The particular program that was used for this surveying was called Magnet. Once the HiPer was level, the Tesla was activated to collect the GPS point wirelessly from the HiPer via Bluetooth (see figure 5).

Figure 4. Tesla field controller. This device was used in both surveys to control the point collection via wireless Bluetooth.

Figure 5. Taking points using the Topcon HiPer. Note that the legs of the post are not being used.

The second method of surveying involved the Topcon Total Station. This required a number of tools to be brought into the field including the Tesla, HiPer with the pole, Total Station with a tripod, flags (for marking the occupied point and back site point), and a reflector with a pole (see figure 6). In order to use the Total Station, the HiPer was used to take two points, the occupied point (where the Total Station would be set up for point collection) and the backsite point which was used a spatial reference in relation to the occupied point.

After the two points were collected, the Total Station was centered and leveled on the occupied point flag. This was done by using the 'laser plummet' from the bottom of the Total Station and moving the tripod legs up and down. The elevation points could then be collected. Unlike the HiPer, the Total Station remained stationary and the person holding the reflector moved around the survey area (see figure 7). The Total Station has a viewer in which the person controlling the Total Station matches the middle of the reflector with the center of the Total Station viewer in order to collect accurate distance/azimuth information. In addition to those two jobs, a third person was used in this survey to control the Tesla (see figure 8).



Figure 6. Reflector head used with the Total Station laser to gauge distance and elevation. The reflector that was used for the class was set on top of a 2 meter pole.



 

Figure 7. Using the Topcon Total Station with the reflector pole (seen in the distant right).

Figure 8. Galen is using the Topcon Total Station to find the reflector point while I am using the Tesla to collect the points wirelessly from the Total Station.

The data for both surveys was imported as a text file (see figure 9) and then brought into ArcMap to be turned into x, y coordinates to give the points a geographic location.

Figure 9. The text file that was imported from the Tesla unit for one of the surveys. Note that the text is coded by name, longitude, latitude, and height, thus giving the x, y coordinates of the points.

To be able to visualize the campus mall's elevation, an interpolation tool was used in ArcMap to develop the static GPS points into a continuous visualization. The interpolation type that was chosen was the triangulated irregular network or TIN interpolation. Below are the two dimensional interpolations created in ArcMap. These figures indicate a gradual decline in elevation toward the Little Niagara Creek, seen in the bottom of the images (see figures 10 and 11).

Figure 10. TIN interpolation of the points collected via HiPer GPS.

Figure 11. TIN interpolation of the points collected via Total Station GPS.

Once the interpolation was created in ArcMap, the interpolation was opened in ArcScene to be able to create 3D imaging of the study area, thus providing an x, y, and z coordinate visualization (see figures 12 and 13).

Figure 12. TIN of HiPer survey points. The left tip of the image shows the area surveyed closest to the Little Niagara Creek, where as the right tip is closest to Schofield Hall.

Figure 13. TIN of Total Station survey points. The left tip of the image shows the area surveyed closest to the Little Niagara Creek, where as the right tip is closest to Schofield Hall.

Discussion:

While there is a generally downward sloping trend toward the creek, it is difficult to compare our two survey methods. Our group thought that the points collected in during the HiPer survey would be collated as a class, which would have given a better sampling area. In the analysis of the TIN topographic map using the Total Station, there appeared to be an area in which the elevation seemed much higher than it actually was in the field. It would be interesting to take a more in depth look at why that portion of the interpolation seems 'off'. It could be that not enough points were taken so the elevation seems more exaggerated than it actually is.

Technology, no matter how good one is at using it, can be an annoyance. Using the Tesla GPS and connecting it wirelessly to the HiPer and Total Station was quite tricky at times. There was a particular order in which the devices needed to be connected and disconnected via Bluetooth. My partner and I attempted to connect to the Total Station for a couple of hours and then had to go back out the next day to try again. We basically did the same steps for both days, but we must have changed one small thing the next day to make the Total Station connect to the Tesla.

An annoyance of using ArcMap in this particular situation was that the interpolation layer and basemap were not visible on the same layer. This is potentially due to my lack of in depth use in ArcMap, but I know that other people ran into the same issue. In addition, others had trouble lining up the points with the campus mall basemap.

The biggest difference between the two GPS survey systems was the ease and set-up time. The total station required a long process to set up the equipment, including using four different leveling processes and matching a laser to the precise point where the occupied point was taken. The Topcon HiPer was much quicker, but probably slightly less accurate due to the device needing to be moved around to every GPS point, which required the user to level the device before taking a reading.

Another difference between the two surveying methods is the number of people required during the process. The HiPer could definitely be done by one person, but when doing the total station, you need at least two people, preferably three. This is important to take note of when choosing a surveying technique in the future.

Conclusion:

Using survey grade GPS systems requires much more time and effort than a Trimble or less accurate GPS does. I have found that throughout this course, I have learned many ways in which to collect and export data into ArcMap. In future projects, I will have a much greater grasp on what the different surveying methods entail. This will allow me to appropriately choose my method of collection depending on the job required.

In addition, these labs made me realize the importance of apprenticeship. When working with the Total Station, it was apparent that asking others for help through their experience (and failures) was vital in understanding how to use the Tesla in combination with the total station.