Lab Ten: Total Station Topographic Survey

Introduction

The purpose of this lab is to set up a micro topographic profile of the area with the use of a Topcon Total Station. Measurements taken by this survey method were then used to create a digital elevation model (DEM) of the study area.

Study area

Map one. Study area for total station demo.

As this exercise was designed to introduce students to the concepts of total station data recording it was unnecessary to go far from campus. The total station was set up just a few meters west of Phillips Science Hall on the University of Wisconsin Eau Claire campus (Map one). 

Methods

The first step of this project was the initial set up of the Topcon total station. Students did not participate in this task as course instructor Dr. Hupy completed it before their arrival. When setting up a total station there are several expectations that need to be met in order for data to be recorded properly. The operators need to define an occupied point, this is place which the total station occupies; essentially the central hub of the study area. The occupied point is recorded by a high accuracy GPS reading in the same fashion as those done in the previous lab, the total station is then placed directly above it. The height of the Topcon unit above ground level is then measured and defined in the software. This is a very important step in setup as the height measurement is necessary to be able to accurately measure elevation of the stadia rod via relative comparison. The final step for getting the total station ready is to define where due north is in relation to the occupied point. This direction is entered into the machine allowing it to record accurate azimuth measurements. Once the total station is set up it is very important not to touch the sensor, or the tripod on which it stands. The slightest disturbance will throw survey points off by a very large amount.

Figure one. Students collecting survey points with total station. Note the stadia rod being held by the student in the foreground and total station in the background. 

After the station was set up students used the equipment in teams of two to collect several points. It is important to note that cooperation and solid communication was needed amongst team members to efficiently and accurately record points. The survey style was essentially random but attempts were made to fill in some of gaps toward the end of the survey.

Once all points were collected they were exported from the total station as a comma delaminated text file. This file was then converted to an Excel spreadsheet and uploaded as a new feature class to ESRI ArcMap with the create feature from XY coordinates tool. This new feature class was then ready to go through the processes of 3d interpolation to create a DEM of the area. A Kriging interpolation method was used to display the results in three dimensions through ESRI’s ArcScene program.

Results/discussion

 

Map two. Total station survey results shown in two dimensions with Kriging interpolation raster. Contour intervals generated from this raster at half meter intervals. 

Figure two. Total station survey results shown in three dimensions with Kriging interpolation, elevation data exaggerated five times. 

Despite the setup being longer than with a survey GPS unit there are several advantages of a total station in comparison. A total station doesn’t have any interference problems relating to picking up satellite signals. So, once an occupied point is defined other points can be recorded in areas where satellite signals would be impossible to detect with a GPS device. The operation of the stadia rod is also a lot faster than using a survey GPS for individual points. The elevation measurements are also far more accurate than those taken by a survey grade GPS device.

Total stations have several disadvantages as well, chief amongst them being the small area which can be recorded without moving the station to a different occupied point. The stadia rod will not receive any laser signal from the total station if there is an object obstructing the two devices. The stadia rod may have to be extended if the area being measured is at a significantly low elevation. This leaves a lot of room for errors in measurements as it introduces a human component to the survey process. As with other point based surveys, a total statin survey is still subject to the limitations of a survey which doesn’t equally represent the area of interest well. In this survey the lack of representation is most apparent on the edges of the Kriging raster, as one would expect the river banks to be of similar elevation for the entire length of the stream. In the survey it appears as if it is a depression and not a stream. In order to correct this issue, which is partially due to the interpolation technique, one would either have to accept a smaller focus area from the existing data or collect more points to correct fill in the gaps.

Conclusion

Students were introduced to a total station survey device built by Topcon by collecting points in pairs. As a class a survey of the area West of UWEC’s Phillip’s Science hall was completed. This survey data was then exported from the total station to a comma delimited text file. This file was then imported to ArcMap and used to visualize the survey area in both two and three dimensions by converting the point feature class to an interpolated raster file. From this analysis it was easy to see changes in elevation across the survey area and places where additional survey points could be used were identified.