Activity 7: Topographic Surveys with Dual-Frequency GPS and Total Station

Introduction

The goal of this activity was to learn how to set up and utilize both a Dual-Frequency GPS and a Total Station to run topographic surveys. During the first week of this activity, we used a Dual-Frequency GPS and created a TIN from the resulting topographic data. During the second week, we did the same with a Total Station. 

Our study area was within the campus common areas on the University of Wisconsin- Eau Claire campus. My groups surveyed the outskirts of the commons area along Little Niagara Creek in front of Davies Center. The first week, my group surveyed the eastern portion of the creek near Phillips Hall on the east side of the bridge (Fig. 1) and the second week, my group surveyed the western side of the same bridge. The first week, using the Dual-Frequency GPS unit, we surveyed on November 5 from 9am-11am. It was cloudy and chilly, but no wind. The second week, using the Total Station, we surveyed on November 12 from 12pm-2pm and there was a significant amount of wind.

Fig. 1: Study area for week one using the Dual-Frequency GPS to survey
the eastern side of the bridge crossing Little Niagara nearest to Phillips Hall.

Fig. 2: Study area for week two using the Total Station to survey
the western side of the bridge crossing Little Niagara nearest to Phillips Hall.

Methods

Equipment

Equipment used for surveying with a Dual-Frequency GPS included the TopCon HiPer S4, the TopCon Tesla, a MiFi portable hotspot, and a tripod stand on which to attach both TopCon devices (Fig. 3). The TopCon HiPer S4 served as the GPS receiver unit and it screwed in to the top of our tripod stand to assure the height from the ground was consistent. We used the TopCon Tesla to create our files and record our data. The MiFi portable Hotspot was used as such to assure we always had a connection. For week two, the total station included both the MiFi portable hotspot and Tesla as well as the TopCon Total Station all situated on a tripod stand and a prism (Fig. 4). With these pieces of equipment, we used the Tesla to record points, the Mifi to assure connection, and the Total Station to shoot points towards the Prism to gather the positional data. Because the Total Station did not record its elevation from the ground as a standard, we needed to measure the elevation of the Total station manually.

Fig. 3: TopCon HiPer S4 (left), TopCon Tesla (center), and MiFi Portable Hotspot (right) comprised the Dual-Frequency GPS unit from which we surveyed topography the first week.

Fig. 4: The TopCon Total Station (left) and the Prism (right) in addition to the equipment listed in use for the Dual-Frequency GPS survey were used to survey topography the second week.

Recording Points with the Dual-Frequency GPS

Fig. 5: Collecting points using the Tesla after leveling the tripod.

My partner, Scott, and I set up our tripod stand in our study area with all necessary equipment attached. Because the TopCon HiPer S4 collected elevation data on its own, it was not needed for us to measure this manually. We simply created our job in the Tesla, leveled the tripod with attached equipment, and gathered the point with the Tesla. This was a fairly simple process once we created our job as all that was necessary was for us to level the tripod and press the "collect" button. We did this 100 times to collect a total of 100 points (Fig. 5). In this lab, one person leveled the tripod as the other person collected the points on the Tesla. However, because the Tesla was in demo mode, we were forced to create 4 jobs each collecting a maximum of 25 points, to complete the data set. While collecting points, we tried to stay fairly regular in spacing between each point except for areas the slope of the landscape was more drastic. In these areas, we tried to collect more points to accurately survey the slope.

Fig. 6: Screen shot of a portion of the resulting
combined text file table later imported to ArcMap.

At the end of the collection process, we saved our data and exported each job to a file. We then transferred the files from the Tesla to the computer via USB. The resulting text tables had to be combined then normalized to fit headings transferable from a text file to ArcMap on one single table. To do this, we simply copy pasted data from 3 of the text files from 3 of the jobs onto one text file from one of the jobs then altered the top row of text to include the name of the point, the latitude (N), longitude (E), and the height with appropriate commas separating each column (Fig. 6). To properly import it into ArcMap, we needed to specify our X value as Lat (N), our Y value as Lon (E), and our Z value as Ht (Z) in the Import XY Data Window.






Recording Points with the Total Station


For this portion of the activity, we were broken into groups of 3 as opposed to 2 like we were for the first portion of the activity. This is because it was easier to work the equipment with three people--one to shoot the Total Station at the Prism, one to hold the Prism over the area we were collecting a point for, and one person to collect the points with the Tesla.

To begin this survey, we first collected a back point in order to collect the location of the Total Station. We did this by collecting one point using the same method we used during the Dual-Frequency GPS survey and the same equipment. This back point was logged in the same job as the other collected points. We then began to set up the Total Station--the most time-consuming portion of the activity. We first positioned the Total Station atop the tripod and began to level it in such a manner that the laser from the Total Station facing the ground was over our desired point--the occupancy point (Fig. 7).

Fig. 7: The laser from the Total Station line
 up over the occupancy point.

Once the Total Station was leveled on the tripod stand over the
occupancy point, we leveled the Total Station itself. We swiveled the Total station in all three directions it allowed in ordert to point the laser on one of the faces of the Total Station with which we shot the Prism to record the data. We leveled the Total Station when it faced each of these directions by twisting the circular knobs on its base which we positioned at "neutral" to begin leveling properly (Fig. 8). We made sure to only twist one knob each time we re-directed the Total Station so as to not interfere with previous levelings.

Fig. 8: My group mate, Peter, leveling the Total Station by twisting the knobs at its base.

When we finished leveling the Total Station, we began shooting our points with the laser from the Total Station to the Prism. We made sure we were not too far from the Total Station and not too close so the laser could be received and sent back to the Total Station relatively quickly and without trouble. We sampled both sides of the rive and the edge of the river itself. This time, we only recorded 25 points on one job. The resulting able had to be normalized in the same manner as we normalized the text file table with the Dual-Frequency GPS (Fig. 8). We then imported the data as XY data and specified the Y, X, and Z values. From the resulting point data, I created a TIN and a break line to characterize the river edge.

Fig. 8: Total Station normalized text file table.

Results

I created a TIN for the Dual-Frequency GPS and a TIN for the Total Station survey points and displayed them on a map (Fig. 9). Metadata was added to each TIN (Fig. 10).

Fig. 9: TINs of the Dual Frequency GPS and the Total Station survey points.

Fig. 10: Metadata created for Dual Frequency GPS TIN (top) and Total Station TIN (bottom).

Discussion

Being that both the Dual Frequency GPS and the Total Station are both survey-grade instruments, I was interested in comparing the accuracy between the two. However, because we operated only on demo versions with the Tesla and the amount of time needed to do these tasks, I was not able to collect the same amount of points for both TINs. As we also had different groups from the Dual Frequency GPS portion of this lab and the Total Station portion, I also did not get the opportunity to survey the same area. However, I was able to survey an area in very close proximity and similar elevation characteristics for both portions of the lab. With this, I can at least say that both pieces of equipment detected very similar elevations around the bank of Little Niagara.

Because the back point was included in the TIN, however, I am inclined to say our result for the Total Station may be less accurate as there were not enough points to characterize the space between our study area and the back point included. This may be why the elevation near the side walk nearest the TINs in the North are so different.

The Total Station was a lot more time consuming to set up than the Dual-Frequency GPS was. Set up time for the Total Station was approximately 1 hour, while the set up for the Dual-Frequency GPS was almost instant.However, because the Dual-Frequency GPS unit needed to be physically moved to each survey point and re-leveled for every point, the Total Station  may be a better choice if many points needed to be collected. Once set-up for the Total Station was complete, collecting the points only took approximately 1-1.5 minutes/point.

A draw back to the time-efficiency of the Total Station may be that it is fairly weather-dependent for its accuracy and efficiency. The Prism is held on a monopole and faced towards the laser on  the Total Station. The prism must not move otherwise the point will either not be collected or will be collected improperly. While we were out collecting points with the Total Station, my group had some difficulty at the beginning with the wind twisting the Prism face away from the Total Station resulting in its inability to collected. It is also rather difficult to keep the monopole steadied in such winds and occasionally, the pole would sway too much and result, again, in the Total Station's inability to collect the point. 

Conclusion

Though this exercise, I gained experience using the Dual-Frequency GPS unit and the Total Station unit to collect elevation data. Overall, both instruments have relatively accurate data collecting capabilities as both are survey-grade. However, they vary slightly in set up time and data collection time--the Total Station taking more time to set up but less time to collect individual points than the Dual-Frequency GPS. The Total Station is more touchy, though, and may not collect the data points if it is too windy or the person holding the monopole with the Prism is not steady enough. There exist advantages and disadvantages of both pieces of equipment and ultimately it is the data collector's choice on what they prefer to use. Knowledge of these differences, however, are important to be aware of before beginning a project.