In these two conjoined labs we were introduced to the procedures and units involved in survey grade GPS analysis. The four units we used were the Tesla (on the fly sub-centimeter accuracy gps capable handheld unit with touch screen interface), HIPER (high accuracy GPS receiver), MIFI (4G modible hotspot device) and Topcon Total Station (survey grade optical laser distance and bearing measurement deice). Using all four of the units, my lab partner and I were to survey out a roughly 25 x 25 meter study area and plot out 100 points (first lab) and 25 points (second lab) over its surface area. The resulting data would be akin to our first and second labs where we used rudimentary surveying methods in our topographic sandbox's, but in this lab's case the x,y,z points would be highly accurate survey grade data. After all our classmates had collected their data, our data set would be capable of being combined with other group's data sets if they were in the same study area.
Study area/Methods
Dual frequency GPS
The study area for this lab was (for my group) the campus mall extent inside the surrounding sidewalk at UWEC. Once we had collected the neccessary equipment from out department storage room (HIPER, TESLA, MIFI, and Mounting Tripod) we set out to the mall and began setup. Once the Tesla handheld was powered on we entered the Magnet Field Program used for surveying and created a new Job with our groups identification information. Then we configured the GPS to use the HIPER SR RTK NET OC to ensure we were getting the highest possible accuracy for our location and elevation data. The Projection were kept as UTM North Zone 15 90w, as well as the same Datum NAD83(2011) and GeoID. After continuing through the rest of the options set to default and okaying the selection we hit the home button to return to the main menu screen. Then we entered the Connect sub menu and chose to connect to the HIPER, beforehand ensuring the MIFI device was turned on and connecting to the HIPER device which would further enhance localization accuracy by having a pinpointed 4G hotspot. After returning to the home menu we then began the process of collecting data points by entering the Survey menu, and then the Topo menu. We were group 1 so our data points began at 100 and would go to 200. The height of the unit was denoted as 2m and the code for the data points was set to elev (elevation). To ensure the accuracy of the data points we set the number of averaged points collected per single end result data point to be 10. Once these prerequisite fields were filled out we began the process of collecting a points, and relocating to other parts of the campus mall, making sure we covered the whole extent of the mall until we reached out 100 point cap. Due to the fact that the Tesla unit was stuck in demo mode and would cap our data points collected per job to 25 we actually ended up having to have four separate projects, which we would later merge into a master data set for use in ArcMap geoprocessing of topological elevation maps. The point data we acquired over the four data sets was then exported onto a thumb drive in a txt. file format which after slight alteration of the field names was easily compatible with ArcMap's create feature layer from x,y coordinates option.The feature layer was then used to create a topgraphic map of the campus mall using Spline interpolation and also exported as a tif for use in 3D analysis in ArcScene.
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| Figure 1: Campus Mall microtopography and x,y points in Arc Map from Tesla/HIPER/MIFI using spline interpolation |
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| Figure 2: Campus mall microtopography in Arc Map from Tesla/HIPER/MIFI using spline interpolation |
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| Figure 3: Campus mall 3D microtopography in Arc Scene from Tesla/HIPER/MIFI |
Topcon Total Station
On our second outing to the campus mall we were instructed to use the Topcon Total Station, Prism Rod, Tesla handheld unit, and the MIFI portable 4G hotspot to gather location and elevation data points. This time the major difference was that instead of using the Tesla as a GPS to gather the points, it instead would be bluetoothed to the Total Station which would be collecting the data from its distance and bearing data relative to the Occupied point and backsights. To gather the occupied point (exactly where the center of the Total Station would be positioned) and the backsights (denoted points which are used as a reference for true north in the Total Stations inner computer) we followed the same workflow used in the first lab outing by using the Tesla, HIPER, and MIFI. Once those points were collected the Total Station was constructed atop a sturdy tripod with adjustable legs. Care had to be taken in order to have the unit exactly level to the ground in order for the Total Station to function properly, and this process entailed adjusting two of the legs, and then the leveling screws, and then adjusting the legs again. Once the station was set up the Tesla had to be restarted and the Total Station turned on and bluetooth activated for the Tesla to sync to it. Before connecting the HIPER had to be disconnected, and once the Tesla and Total Station were connected we were then able to designate the backsight we would be using by doing the following workflow we used for the normal points. Afterwards we would begin collecting data points using the Total Stations Optical distancing laser. One of our group members (myself) had to move to the desired point with the Prism Rod while the second group member (Ally) would re-position the optical lens to aim directly at the Prism mirrors. The third group member (Matt) would then hit the record button on the Tesla and so long as the Optical laser hit directly within the Prism Rod's center the data point would be collected. This continued for 21 points (due to the demo mode cap of 25 and the already recorded Occupied point and three backsights) all across the campus mall and once we had finished and disassambled the Total Station we transferred the data points to a thumb drive in the form of a txt. file and renamed the attribute fields to better suit integration into ArcMap create feature class from x,y. This feature class was then also used to create a topographic map using Spline interpolation and also exported as a tif for use in 3D analysis in ArcScene.
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| Figure 4: Campus mall microtopography and x,y points in Arc Map from Total Station |
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| Figure 5: Campus mall microtopography in Arc Map from Total Station |
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| Figure 6: Campus mall 3D microtopography in Arc Scene using Total Station |
Results/Discussion
Based on the results of spline interpolation in both data sets, it is hard to determine any reliable difference in accuracy that can be used for comparison. This is due to the fact that the first data set using the Tesla/HIPER/MIFI was a collaboration of several groups x,y data sets and has much higher accuracy due to a higher number of elevation points used during interpolation. The higher point density makes for a much more accurate representation of the campus mall as opposed to the low point density of the Total Station data set which produced an inaccurate and generalized topography due to only 16 points being taken (3 backsights and 1 occupied point took up 4 of the total 20 allowed in demo mode). The scale of the study area is also marginally smaller (data points do not extend to left 1/4 of the campus mall study area) due to the difficulty sighting the Total Station's optics to the prism during lowlight hours of the evening which caused us to have to take elevation points closer to the total station's occupied point. These elevation points could not be collaborated with other groups due to the lack of groups sharing their data sets in our departments communal temp folder.
Conclusion
The two methods of collecting microtopography data using the "Dual Frequency GPS" and the "Total Station" each have differing pros and cons. The dual frequency gps method has much more mobility and is not impeded by line of sight from the Total station to the prism rod due to the ability of taking the Tesla/HIPER/MIFI to any location and setting up for point collection. This can also be a drawback though in cases of uneven or easily shifted terrain such as sand or loose gravel/dirt. The Total Station also suffers from lack of mobility but in this case from a single occupied point, but with practice the set up can and take down be greatly accelerated. From several occupied points it is capable of gathering more points at great distances in a shorter period of time which are not quite as impeded by uneven terrain so long as the occupied point is on sturdy ground. Each method had it's own technical difficulties such as connections between devices and data point collection (with the total station needing exact line of sight to the prism rod or else the point would not collect). Overall, each method of microtopography surveying has its preferred uses based on strengths and weaknesses but neither can render the other obsolete due to the vastly differing scope and goals of surveying projects which make use of both methods.
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