Terrain Surveying with a TOPCON Total Station

Introduction: In more critical and precise applications, a total station can be used to survey the surface of an area. The total station uses both distance and azimuth as well as an elevation to record and plot the points selected by the operators. The base stations location is plotted using a GPS receiver. Using that location as a reference, points in the survey area are captured using a laser and reflective prism by the base station. The base station emits a laser that hits the prism at the point location, then relays the information via Bluetooth to the GPS receiver. The distance to that point is recorded and automatically calculated into an x, y, z position based again on the base station’s original GPS location and the use of a backsight. The backsight provides the base station a reference to north. Without this reference, the points would only be relative to that single base station location and not real world location. In this exercise, a 1 hectare section of the University of Wisconsin-Eau Claire campus mall was to be surveyed to capture the micro-topography. A TOPCON GTS-250 series total station with GMS-2 receiver were used to collect the positions and elevation data from the hectare plot.

Figure 1: The new UW-Eau Claire campus mall was formerly the home of the old Davies Student Center. Now the space is used as a seating and performance area for campus events. 

Study Area: The campus mall of the University of Wisconsin-Eau Claire has undergone major construction in the last couple years. The area was previously occupied by the old Davies student center but now is a new green space with seating and performance areas. This opened up the center of campus for different events provided by the University’s numerous organizations (Figure 1). The seating area is a miniature amphitheater with a gentle slope down to the new brick staging area.

Methods:

Figure 2: When setting up the total station, one has to step with care around the legs of the tripod. This prevents the legs from settling more after the station is leveled. By course then fine adjusting the level using the legs, fine adjustment is quicker and easier with the adjustment knobs at the base of the total station. 

        Setting up: Before starting to collect data, the survey plot and base station need to be properly setup. The survey hectare was measured using a pace count. Since a hectare is 100 x 100 meters, the pace counts measured in the previous navigation exercise was applied. The pacer placed a flag at each corner of the survey area. This provided a rough measurement for what area needed to be captured using the total station. With the hectare now set, a location for the base station was selected. The tripod was then set and secured over that location. When setting up the tripod and total station, it is important to make sure that the legs of the tripod are wide enough to provide stability and the tripod is at a comfortable height to work with (Figure 2). Using the level on the tripod, the station was “rough” leveled then more precisely leveled using the fine adjustment tools on the TOPCON station. Once the station was level, the GPS location and backsight information can be set. It is important to check to make sure the Bluetooth is enabled on both the station and the receiver. The settings – parameters menu on the total station allows you to turn the Bluetooth on. The “B” icon will the visible on the home screen once the Bluetooth is connected to the receiver. Now using the GMS-2 receiver, the base station GPS location was collected. The averaged location named “OCC1” for occupied point 1 was then able to be selected from a list to be assigned as the base station’s location in the TOPSURV software on the GMS-2. Next was to set the backsight in the observation – total station menu. Using a land mark and flag, the azimuth from the base station to the point was measured. A TruPulse laser range finder provided a precise azimuth which was then set as the default backsight on the station. Since the goal of the exercise was to capture the micro-topography, it is also important to set a base height for the station and prism rod. Using a measuring tape and the designated mark on the TOPCON station, the height was measured then set in the menu. The height of the prism rod is marked on the rod itself which was kept at 2 meters.

        Data Collection: Before evening thinking about collecting points, make sure that the black locking knobs on the total station are facing away from the viewing lens. If they are on the same side you are working on, the station will not allow you to collect any data points. Now that is cleared up, the surface data can be collected. The rod operator will walk out to the corner of the hectare plot and hold the prism level facing the total station. The total station operator will then align the view finder cross hairs with the prism rod and collects the point with the GMS-2. Once the GMS-2 notifies that the point has been collected, the rod operator will move and the collection process with repeat. The area was surveyed at 6-7 pace intervals between collection points.

Figure 3: The data exported from the GMS-2 to a text file result in this form. The point name, y location, x location, and z locations are listed from left to right. When opening the text file in Excel, make sure to select the comma as the deliminator and not the default tab setting. This will keep each set of information values in their own columns, x in one, y in another and so on. 

        Exporting the Data: From the job menu on the GMS-2, select export – to file. Choose points and export as a text file. The text file keeps the x, y, and z information (Figure 3). If the file is exported as a shapefile, just the locations will be kept without the assigned x, y, and z coordinates. Without the coordinates, just the points would be able to be mapped not the surface model. Once the text file is created, transfer the file from the GMS-2 to the computer with Active Sync.

        Creating the Surface model: The new text file created from the GMS-2 was opened in Microsoft Excel. In the dialog box, make sure that the comma is used as the deliminator and NOT tab. The column headings were checked to ensure that the correct data values were listed. In the text file, the X and Y values were switched and needed to be labeled properly before the excel table was saved. Next the excel table with the point values was imported into ArcScene. This process is the same as described in the previous “Terrain Survey” exercise. Right click “Layers” under the table of contents. From the menu select “add data” and browse to find the excel table. Once the table has been added to the layer, right click the table and select “display xy data” from the menu. The default values will be set in the dialog box so verify that the x, y, and z values are all correctly assigned and selected to be displayed. Once the points are displayed in the map area, the feature class can be exported to save the display values with each point position. Without exporting the feature class, the individual point values are lost. Now using the point feature class, interpolation to create the surface model was completed. Based on the results from the interpolation methods explored in the previous “terrain survey” exercise, the Kriging interpolation was chosen to create the surface model. The Kriging interpolation was performed by running the “Kriging” tool from the ArcGIS toolbox. To run the tool, open ArcToolbox and expand the “3D analysis tools” toolset. From the “Raster Interpolation” toolset, select “Kriging”. The tool window will open. Use the point feature class as the “input features”. Make sure to select the Z field as the “Z value field” to make sure the tool uses the elevation values in the interpolation. Name the output raster, accept the all the default values, and select OK to run the interpolation.

Figure 4: The collected survey points were used to create a surface via Kriging interpolation. The occupy point, or location of the total station, is represented by the blue dot towards the center of the surface. Areas with larger changes in elevation were surveyed at higher point densities. This can be seen in the low lying area around what is Little Niagara Creek. 

Results/Discussion: The surface model generated by the Kriging interpolation did represent the real world surface of the UW-Eau Claire campus mall (Figure 4). Because the grade or slope of the hectare was gradual, there isn’t a drastic change in elevation which is mimicked in the model (Figure 5). The areas where the slope was steeper, more survey points were collected. This is demonstrated by the area surrounding the Little Niagara Creek. The banks of the creek were in the low-lying area of the mall, therefore the surrounding approach to the creek displayed most of the elevation change over the hectare.

Figure 5: The profile of the interpolated surface is relatively accurate to the real world area surveyed. The campus green space gradually slopes to the creek with seating on the slopes. The slope levels out towards the base of the creek due to a staging area for campus events. 

As noted before, we first had a problem with collecting survey points due to the locking knobs on the total station being on the same side we were viewing from. Once we rotated the view finder around to the other side, the points were able to be collected successfully. When importing the text file, the x and y coordinates were listed in y, x order. The table needed to be labeled properly before added to the map document in ArcScene. If the columns weren’t labeled it is quite possible to assign the wrong values to the x, y and z fields when displaying the data. This would result in a misrepresentation of the surveyed hectare on the mall.

Conclusion: The use of a total station when surveying a terrain is a very powerful tool, but the accuracy of the survey is only that of the GPS used to record the data. Because the location of the total station is used to reference all other measured points, the accuracy of that first point determines the accuracy of all the later collected data. The degree and precision at which the station collects data is extremely high regardless. The laser and prism allows the station to report high accuracy data points even if the prism rod is hundreds of feet or meters away from the station. This was not the case with TruPulse range finder which was used in the previous distance azimuth survey. The ability for the station to collect and automatically correct the data for x, y, and z positioning is a great feature to save computing and data prepping time.