Field Activity 9: Topographical Survey

Introduction

In this lab exercise we focused on topographical surveying techniques which involved the use of a Dual Frequency GPS and a Total Station. While both of these devices are survey grade units we gathered points using both in order to determine which was more accurate as well as become comfortable with how to use each device. 

Dual Frequency GPS

The device we used for our dual frequency survey GPS in this lab exercise was the TopCon HiPer SR unit (Fig. 1). This GPS unit has a remarkably high accuracy of less than 15mm. The HiPer SR unit can be mounted to a tripod and used with the TopCon Tesla controller unit (Fig. 2) to collect data points utilizing bluetooth to send the GPS data collected by the HiPer SR unit to the Tesla controller. 

(Fig. 1) TopCon HiPer SR Unit utilized in this lab.

(Fig. 2) TopCon Tesla handheld unit used to collect data throughout the lab.

In order to record data the HiPer must be angeled perfectly vertical thus it is important to use a tripod to make sure this is the case. Throughout this particular exercise we used the dual frequency GPS to collect elevation data so it was imperative to make sure that the tripod was balanced so that the process of actually collecting the data would only take a matter of seconds. This unit is very efficient because it is very lightweight depending on the tripod used and can be operated by just one person in the field (Fig. 3).

(Fig. 3) A surveyor using the TopCon HiPer unit mounted to a tripod in the field.

Total Station

Like the HiPer SR unit, the total station can be used in conjunction with the Tesla controller to record the data collected by the total station. The total station however utilizes a very different method in order to collect data. To collect data using the HiPer unit the surveyor moves thte entire unit however, when using the total station the unit does not move but stays in the same position. In most cases the total station is set above a US geologic survey marker and a series of points are set up prior to data collection knwon as back-site points.  These points are then used to mark the location of the total station unit. It is imperative to know the location of the unit prior to any data points being taken to ensure accuracy of the data collected. Once this is done the unit will send out a laster which is sent to another item, a prism attached to a rod. It is the prism and rod which are moved to collect data points rather than the GPS unit as in the case of the HiPer unit. The prism recieve the laser signal and then reflects back the data that corresponds to it. Like the tripod and HiPer unit, the prism and rod are light weight and very portable however the total station requires two people to operate it. One is needed to shoot the laser and the other to hold and move the prism and rod (Fig. 5). 

(Fig. 4) TopCon total station unit used in this lab. 

(Fig. 5) A TopCon total station being used in the field to collect survey data.

Methods

Our study area for this lab was the mall, or green space, on the UW- Eau Claire Campus in Eau Claire, Wisconsin. This was area of to focus on because of its size and easy access for students. After a great deal of data was collected using both the dual frequency GPS unit as well as the total station the data was used to determine elevation. The results of this lab can be seen in Figures 6 and 7. 

(Fig. 6) The map above shows the elevation data collected using the dual frequency GPS unit, the TopCon HiPer.

(Fig. 7) The map above shows the elevation data gathered using the total station unit, the TopCon total station.

Discussion

Since both of the instruments we used were of survey grade quality, the results from both units should have been very accurate and therefore it is likely that the errors seen in figures 6 and 7 were caused when we were collecting data. These results were not expected because when we collected the data we made sure to collect data points throughout the entirety of campus mall. However, in the mapped data above the points are not distributed correctly within the study area. Some of the points even intersect with the buildings. Looking at the very basic patterning of the elevation data is fairly correct in the general slope of the area in that closer to the main circle you get the lower the slope.

Setting up the units proved to be quite the challenge for both units because it was new technology my partner and I were not familiar with. There could have been issues setting up the back site points using the total station because that was something we struggled with prior to collecting any of the data points. If the back site data was incorrect then the location of the total station would be incorrectly recorded and then all other data would be collected in error as well. Since the set up process for the total station required so many more steps than the dual frequency GPS there was much more room for error. 

I am not quite sure where we went wrong when conducting these surveys but the data certainly did not turn out correctly as it didn't even expand to the full region of our study area. Also, a lot of the elevation data itself is not correct either and since the units themselves are so accurate it is most likely that the errors are caused by 

Conclusion

This lab proved to be extremely useful in that it helped us to become more familiar with various surveying equipment and now have a better idea of which unit is proper to use for certain situations. For surveying purposes its important to have a wide range of knowledge of various techniques to ensure that errors like those that occurred in this lab do not occur when the data being collected is for practical application.