Total Station Topographical Field Survey

Introduction:

Previously we conducted a survey using the Distance/Azimuth Survey method , in which we used a laser measure to find the distance of objects relative to one central point on the UWEC campus mall.

During this survey, we are going to build upon the previous distance/azimuth method by using a Topcon Total Station and a Tesla GPS Unit. The differences between the two survey methods are as follows; first, with the Topcon Total Station, Due North has to be known in order to calibrate the Total Station, second, the topographical survey generated by using a total station will also assign a height measurement to each point collected. The height measurement may not seem like a lot, but it gives us another dimension to work with, as we then have measurements on the X,Y, and Z axis. As you may have seen in previous blog posts, the Z measurement will give us the capability to model the surface of any area surveyed in 3D using ArcScene. 

Fig 1. Dr Hupy explaining the setup of the Total Station, again this week it was raining and again we used the campus mall as our study area. It is important to note that the pink flag under the total station designated the occupied point, over which the total station was calibrated. The equipment listed from left to right is the Topcon Survey Grade GPS, the Total Station, and the Prism Pole

Study Area:

Similarly to most of our other surveys, the UWEC campus mall with its diverse topography, would serve as the study area for this survey. Specifically we set the total station on the lawn between the Davies Center and Phillips Hall, before the bridge cross Little Niagara Creek. 

Fig 2. The UWEC Lower Campus, and the location of the campus mall.

The survey area is a small section of lawn bisected by sidewalks that extend through the campus mall, and which lead up to the entrances of Phillips Hall. The area is on the south side of Little Niagara Creek, and is obstructed with young trees that have been planted as the campus mall has been rebuilt. The ground has an incline leading up to the West side of Phillips hall and a also features a decline as you near Little Niagara Creek (Fig 3).

Fig 3. The location of the Total Station Topographical Field Survey is the area of lawn in the lower right hand corner of the picture, near where the small tree is planted, and stretch to Little Niagara Creek. 

Methods:



The Total Station has a few additional setup and technology requirements in order to function. Rather than having a central point that may be slightly variable (due to human/procedural error) the total station has to be set on known benchmark, known as an occupied point. In addition to this occupied point, 2 or 3 back sites are required which are also known points and used to calibrate the total station. The back sites are calibrated using a secondary unit known as a Prism Pole, which reflects a laser shot by the total station and using triangulation with the Prism Pole, measures the height the height of the object being measured. It is VERY important to note that at no time can the Total Station be moved, adjusted, or bumped and that the Prism Pole's height must not be changed without recording the adjustment. Failing to note the adjustment or a disturbance of the Total Station will result in inaccuracy and error which will propagate as more points are surveyed.

Here is another resource from the UW-Madison as to how to set up a total station, and what individual components make up the whole unit. 

For written step by step directions to set op the Total Station please see Appendix A at the bottom of this post.  

After the total station is set up, every time a points is surveyed three different measurements are taken. 

1. Angle measurement: taken in arc-seconds (theta, Fig 3)
2. Distance measurement: via the reflection of the prism pole (D, Fig 3)
3. Coordinate Measurement: the coordinate measurement of any point is determinedly using retaliative position of the distance measurement to the known point that the total station is over. Height measurements are then calculated using trigonometry and triangulation from the angle of the total station to the reflection of the prism pole.  

How these three measurements come together is pictured below (Fig 3). 

Fig 3. A representative picture of how Total Stations capture all two of the three measurements, angle (theta), distance (D). Not shown specifically is coordinate measurements, which is captured by linking the total station to the survey grade GPS unit, at which point the coordinates of surveyed points are estimated using the distance measurement relative to the known location of the total station, The level road is also not used in our analysis, but rather the prism rod which reflects the total stations beam back to the total station for digital measurements and calculations.

With the knowledge of how the total station is set up and the unit captures data points, one is now ready to proceed with a survey. In this case we will be doing a topographic survey, so data points of the local landscape will be to capture the numerical attributes of various points in the area. 

Fig 4. The total station set up and working, ready to capture survey points of the landscape. 

Fig 5. The view of the total station from the perspective of the prism rod.

Fig 6. A picture of the total station capturing a point via the prism rod. 

When using the Total Station all you have to do is look through the lens piece and line the cross hairs up on the prism, once the cross hairs are set on the prism, you hit the capture button on the GPS to capture the point. As the prism is moved to various points, this process is repeated until enough survey points are captured or the desired number of points are surveyed.

Results/discussion:

For our survey we collected 97 points on the UWEC campus mall, with both the known control point and the two back sites, a total of 100 points were collected (Fig 7).

Fig 7. The attribute table of all 100 survey points opened in ArcMap with X,Y and Z data recorded.

The Map below reiterates the specific survey location, showing both banks of Niagara Creek captured, as well as the various points on the campus mall surrounding Phillips Hall.  The known control point is shown on the map below (Fig 8) in yellow and labeled "OCC", this same point is depicted in Figure 1, as the pink flag under the Total Station. 

Fig 8. The survey points of the Topographical Survey are depicted by the purple circles while the known control point and the two back sites are depicted by the varying colored circles with different symbolism than the survey points. The Total Station was set up of the point OCC , with the survey grade GPS directly behind the site.

After the data was validated by point all of the survey points into ArcMap, and making sure that the points could be read by GIS software, the data was put into ArcScene for 3D rendering. In order to make the data points show more drastic height differences between the data points, a floating surface was created for the maps base height and a factor of 4 was used to differentiate the heights of the individual data points. With out these manipulations, the 3D image would have had very little variation in height and would have looked remarkable flat due how close the data was, in terms of elevation. In Fig 9, below, the green represents the lowest portion of the data, and would have been the data that was captured closest to Little Niagara Creek. While the white portions of the map, would be the highest points captured, on the left hand side, these points are in a garden near Phillips Hall.

Fig 9.  The 3D Rendering of all 100 data points, the lowest portion of the image (green), are areas that were recorded near Little Niagara Creek. The white area represent the highest points captured, which would have been near Phillips Hall (left side) and on the other side of the Little Niagara Creek bank (right side)

Fig 10.  Another 3D rendering of the data points, the 3 green pins in this image represent the known control point (standing) and the 2 back stations (flat). 

While the quality of the data that was collected with the total station was precise, some of the data was inaccurately represented in the map (Fig 8).  Which brings us back to the idea of accuracy and precision, and what those terms truly mean (Fig 11). The points that were recorded in using the Total Station were low in accuracy and high in precision, at least according to the points when overlaid with a base map of campus. What this means is that relative to each other, each point was placed correctly in terms of relative position, however some points were low in accuracy, appearing in the Little Niagara Creek, and in Phillips Hall.

Fig. 11 Accuracy vs. Precision. 

These errors may be attributed to potentially a few things

1.  Human Error; either in the set up or in the import of the data or the calibration of the units.
2. Error in the base map; which reports all points to be off by some measure across the board.
3. Actual Error in the tools. 

More than likely, because we are dealing with a survey grade GPS that can be accurate down to the sub-meter level, the errors are not coming from the GPS and as the GPS is connected to the total station which uses quite a bit of technology to accurately measure distance, the error probably does not lie in the equipment.

Due to the fact that the points that are inside Phillips Hall are just barley on the outside edge of where the building is drawn, it may be that the scale at which the data points are represented and a combination of that data being projected or converted and/or an inaccurate base map, it would seem that these errors are procedural or a result of human error rather than error in the equipment. 

Conclusion:

While the Total Station survey technique may seem straightforward, it's really the culmination of multiple survey techniques that we have been building on over the course of the semester. From the Sandbox Survey we have taken the aspect of 3 dimensional measurements (X,Y, and Z). From the second survey, Data Interpolation in ArcScene, we included the aspects of proper survey procedure with have data normalization, data interpolation, and also included dimensional analysis and arc scene.  Then from the Distance/Azimuth Survey, we have core idea of all of the survey points being taken in reference to one standard base point. And finally we have the accuracy of the survey grade GPS from the Topographical Survey.  While the total station adds a new element of technology and a more systematic way of referencing survey points than we had previously explored in the Distance/Azimuth Survey, the methodology of the topographical survey contains elements we have seen before.  What the Total Station Topographical Field Survey adds is the combination of the survey grade GPS to enhanced accuracy and precision of the survey points in relation to the known control point and to capture many survey points in three dimensions.

This is why it is not a surprising that this survey technique is used in industry where accuracy and precision are required.  Potentially another way to include increased data normalization, is also something that we have worked with before, which would be to include capturing attribute data in addition to X, Y, and Z data through the use of domains to capture more information about each survey point, while constraining the data entry to relevant information.

Practical applications of adding attribute information via domains would include surveying points for road construction that have objects or obstructions which would need to be removed or built around in order to complete the project.

Appendix A:

Steps to collecting data with the TSS

1. Set up Survey
1. Outside set pin flags where you will set up the total station (Occupied Point and the BS point
2. Set up Magnet Job
1. Set up a job within Magnet using the RTK option. This is the same procedure as setting up the job for surveying with RTK GPS.
2. Gather your backsight(s). Name them BS1, BS2, etc.
3. Gather your OCC1 (This is your occupied point where the Total station will
3. Set up the TSS
1. Tripod Setup
• Wipe off head to ensure that surface is clean and free of dirt
• Extend all three legs equally prior to spreading the legs.  Secure locking Mechanism
• Spread the legs sufficiently to ensure a stable base for the tripod
• Center tripod head over the point while maintaining a fairly level tripod  head
• Check centering by dropping pebble from the center of the tripod head. (within 2" of Pt)
• Step down firmly on the footpads to set the legs
2.  Instrument Setup
• Secure instrument to tripod and center over tripod head
• Bring all leveling screws to a neutral position, just below the line on the leveling screw post
• While looking through optical plummet (if needed adjust o.p. for parallax and focus on the ground) or
• with Laser plummet on, position instrument directly over point by using Leveling screws only.
• Observe what two legs need to be adjusted to bring Bullseye bubble into the middle
• Be careful not to move the third Leg
• Release Horizontal tangent lock and rotate instrument until tubular level vial is parallel (in Line)  to 2 of the leveling screws.(this position 1)
• Rotate both leveling screws equal amounts in opposing directions until tubular level vial is centered
• Rotate instrument until tubular level vial is perpendicular to Position 1
• Rotate leveling screw equal until tubular level vial is centered
• Re-observe the point with o.p or laser plummet an adjust instrument over the point by loosening the center screw and shifting instrument over the point and re-tighten screw.
• Re-check the fine(tubular) bubble vial in position 1 and 2 and adjust as needed
4. Set up Blue Tooth
1. Turn on the total station.
2. Turn on the station Bluetooth. This is done within the menu area, and within the parameters portion.
3. Make sure your Bluetooth is on for the Tesla to recognize it.
4. Disconnect from the Hiper in your job, and now connect to the Total Station
5. To begin the OCC/BS setup
1. On the Home Screen for Magnet, select the Setup icon
2. Click on the backsight icon
3. Enter in all needed information for the TS and for the Prism rod.
4. Place your prism rod over the backsight point and gather the backsight. This is needed to zero out the total station for north
6. Collect GPS points with the Tesla in Magnet, using the Total Station and prism*

1. From the Magnet Home screen, open the Survey icon.
2. Begin your toposurvey, but now use the prism and total station to do the survey.