This exercise asked the class in groups of three to create a terrain surface in a sandbox and thoroughly survey the area by means of sampling, for eventual creation of an digital elevation model in Arc Map after transferring data into an excel spreadsheet. Sampling is the act of collecting data points for later creation of a model of the sampled area. The collected points model the area where they were collected, sometimes more accurately than others. Where and how these data points are collected can influence the final accuracy of the model in representing the specific area. This is why it is important to carefully consider all sampling options and procedures before execution of surveying.
Sampling techniques to consider include random, stratified, and systematic. Random sampling is simply the choosing of random points out of the total collection of points to consider in a predefined grid. Each point has the same chance of being chosen. Functions in excel can generate random numbers for the user to ease this process. For purposes of time, efficiency, and evenly thorough surveying in all areas of terrain, this option did not seem adequate for us. The second option, stratified, is characterized by a population of data points divided into subcategories, of which each need to be represented. This means that these need to be proportionally sampled, which can happen in a random or systematic fashion. The last sampling method is systematic, which, when done spatially, means that a point is collected every certain number of spatial units. This was our method of sampling in our surveying.
Methods
We surveyed on Monday, January 30th from 3:00 pm until 4:30 pm. The sandbox we used was located on the UWEC campus, next to the river just east of the Phillips science building across the road. Coordinates adequate for finding the sandbox are 44° 47' 48.80" N, 91° 29' 54.56" W. We chose to sample systematically because we didn't have any subsets of data points for stratified sampling and because we wanted to cover every area in an equally thorough manner, something we thought random sampling might jeopardize. The sandbox, one square meter, worked perfectly for this. It had already been divided by tacks into a 20 by 20 grid and string only needed to be tightened around these tacks slightly to make points accurate. With one person recording on a piece of paper with a 20 by 20 grid we measured heights at each intersection of the string. Points under the imaginary plane flush with the top of the wood edging the sandbox and indicated by the string were considered negative and points greater were considered positive. Starting with a corner deemed 0, 0, and with designated x and y axes, we measured each point's height in centimeters.
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| Point Height Measurement |
Results and Discussion
Our survey resulted in 400 individual points whose resolution adequately represented all features created in the sandbox, yet did not burden us in terms of time spent in the cold collecting points. Data points were collected about 5 cm apart. We decided on our sampling method and technique and all went smoothly and efficiently, our group never straying from this planned method. Our maximum value was 4 and minimum -8.
Although everything went smoothly, minor problems did occur. The sand was frozen and we had no shovels so we needed to use ice shards to dig. Tacks came out at times which needed to be put back in, proving that nails would have been a better option for holding the string forming the grid. Finally, it would have been much more efficient to type or enter the point heights into excel directly instead of writing and then later making a spread sheet. This process seemed poorly planned out in hindsight.
Conclusion
This exercise had the group thinking through options for surveying to find the best for our application and required the group to think through a systematic and efficient workflow balancing time and effort and also accuracy of the resulting model. This is exactly what someone surveying on a larger scale would be thinking through, only they would need a different system for measuring distance between points, for measuring elevation, and for recording on the go. I imagine this process could be streamlined with the use of a larger team, UAS technology programmed to do the surveying for someone, and GPS technology. In conclusion the group completely adequately surveyed the area of interest, but could use a slightly larger grid for a higher resolution and more confident data with less distance between points.
Sources
http://www.rgs.org/OurWork/Schools/Fieldwork+and+local+learning/Fieldwork+techniques/Sampling+techniques.htm
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