Exercise 9 & 10 - Topographic Survey (Dual Frequency GPS & Total Station)

Introduction

For this exercise we were creating a elevation map of the campus mall using two different techniques to capture the points we needed. We were to use both a Dual Frequency GPS and a more accurate Total Station technique. For exercise 9 we used the Dual Frequency GPS and the following we in exercise 10 we used the Total Station. After we have collected points with both of these techniques we will be able to compare and contrast the benefits of each technique. Below is the study area (fig. 1) of the campus mall here at UWEC. The photo is about two years old and shows the campus mall during initial construction in 2013.

Study Area of the campus mall at UW-Eau Claire (shown during construction in 2013) (fig. 1)

Methods

The first technique is the Dual Frequency GPS which uses both a TopCon HiPer (fig. 2) and a TopCon Tesla (fig. 3). These devices work together to give us very accurate GPS data points which are produced by the HiPer and recorded wirelessly with the Tesla using the Magnet application. The HiPer sits on top of a rod supported by two legs and the Tesla is attached directly to the rod using a clamp. To collect a point you just make sure the rod is perpendicular to the ground using the built-in level and save the point in the Magnet application on the Tesla. The Tesla takes multiple points and averages them out to provide a more reliable reading. This method is very easy to do and quick, however the accuracy is not as good as a total station.

A TopCon HiPer SR unit used in the Dual Frequency method (fig. 2)

A TopCon Tesla unit used in the Dual Frequency method (fig. 3)

The other technique is a Total Station GPS which uses a TopCon Total Station (fig. 4) as well as a TopCon Tesla from the previous technique. This technique is different that the Dual Frequency method in that it stays stationary while another person moves a prism target (fig. 5) on top of a rod around to collect points. First we need to collect a occupy point and a back-site point using the TopCon HiPer and Tesla. The occupy point is determined by the height of the total station and the exact coordinates of where it sits. The back-site is collected using the HiPer at a point away from the occupy point in order to set azimuth. This will allow us to collect points with the Total Station by calculating the direction and elevation of the total station unit from the collected points. Next, the other person goes to each point we are to collect and holds the prism target while the total station operator collects a point by shooting a laser at the target which gives us the distance and direction. It collects the point by knowing how high the target is above the ground and the laser gives the distance and direction from the occupy point that the total station is standing on.

A TopCon Total Station unit (fig. 4)

A prism target used with the TopCon Total Station as the point collection indicator (fig. 5)

Once we had collected all of the data we were able to export the data from the TopCon Tesla to ArcGIS. The data came in a text file (fig. 6) as seen below which can be imported into ArcGIS by importing the x and y data. It is also important to make sure to set the projection before converting the text file into a point feature class, otherwise the data is distorted and doesn't show up in the correct locations.

Text file exported from the TopCon Tesla giving the corrdinates of the points collected for each technique. (fig. 6)

Once the data is in ArcGIS, we can put the satellite image behind the data and the maps below are created. The first (fig. 7) was created using the Dual Frequency method while the second (fig. 8) was created using the Total Station method.

Data points collected by the Dual Frequency GPS unit. (fig. 7)

The Dual Frequency method shows very accurate points and many more data points due to the ease at which it was to collect these points. It was very easy to collect points with the method and we were able to collect many points very quickly.

Data points collected by the Total Station unit. (fig. 8)

The Total Station method shows also very accurate points although due to the long setup time it takes to get the TopCon ready to collect points and we ran out of time. We still had enough points to create a descent interpolation seen below.

Once we had the points I created Spline interpolation rasters for both techniques. The first (fig. 9) is of the points gathered using the Dual Frequency method while the second (fig. 10) is from the points gathered using the Total Station method.

Spline interpolation of the points gathered using the Dual Frequency technique. (fig. 9)

This interpolation shows very detailed terrain due to the large number of points we collected. There are however, large variations in the elevation that are not represented correctly such as the same hill and dip in the left side of the map which is not there in reality. 

A Spline interpolation from the the points gathered using the Total Station technique. (fig. 10)

The interpolation from the Total Station is accurate in terms of what the actual landscape is like, but it lacks in terms of detail due to the low number of points collected by us.

Discussion

In the Dual Frequency method there was a hill and dip in the left side of the map that should not be there and this could be due inaccuracy in the elevation data collected by the Dual Frequency method. This lead me to believe that the Dual Frequency method isn't the most accurate method even though it is very easy and quick. This could also just be explained by user error, which is possible.

During the setup of the Total Station we had trouble figuring out how to get the back-site point set and then it took a little while practicing shooting the laser before we were able to start collecting points. If we had finished the setup more quickly we would of been able to collect more points and have a more detailed terrain model for the Total Station method.

In terms of which technique is better, I would definitely say that the the Total Station is the superior technique. Even though it takes a while to setup, it is very easy to collect points once it is ready to go. Also, now that I have had experience in setting it up, it would go much more quickly the second time.

Conclusion

If I were to re-do the data collection for this exercise I would have collected more points in the Total Station because it would have given me the most accurate and detailed terrain model of the study area. I believe the Total Station to be the best of the two techniques. If I needed to map out a terrain for a future project or for my future job I would definitely go with the Total Station technique because it gives you a more accurate model. Once it is setup it can collect a large amount of points very quickly which would provide a more detailed model.