Improvised Survey Techniques to Create a Digital Elevation Surface

INTRODUCTION

This lab demonstrates the ability to think spatially to measure and produce a digital terrain model. This was done by creating a custom topography in a sandbox and devising a grid-system and sampling method to best represent this topography digitally. Sampling can be defined as part or single item that is representative of a larger group. In a spatial perspective, it needs to be able to allow for enough data to create an accurate representation of an area or object. Common sampling techniques include:
  • Simple random sampling
  • Stratified sampling
  • Cluster sampling
  • Systematic sampling
  • Multistage sampling
  • Multiphase sampling
  • Convenience sampling
  • Purposive sampling

METHODOLOGY

A systematic sampling method was used to record data because it allowed for a controlled method capturing all surface elevations of features while not sacrificing time. A random sampling method was not used because although faster, would not capture all features to the same degree of accuracy. A stratified sampling method is related but not used because although it would be more accurate than random sampling, it still wouldn't capture the same amount of elevations of the features created.

The sample plot of the sandbox was located on the campus of the University of Wisconsin-Eau Claire 500 feet east of Phillip's Hall at 44°47'48.7"N 91°29'54.5"W.

 To build the custom grid, thumb tacks lined the sandbox walls at 5 cm intervals as Figure 1 demonstrates.
Figure 1: Grid system spaced at 5 cm between coordinates as shown by thumb tacks
String was then laced around these tacks as tightly as possible as shown by Figure 2 to form an x, y coordinate grid at 5 cm intervals. For the two sections above and below the distinct topographic features that form our beloved professor's name, less sampling points were needed. For these areas, we took a systematic approach with a sample point for each 25 cm square area.

Figure 2: Intersecting strings woven around thumbtacks to form a custom survey grid
Once the grid was in place, measurements were taken at each coordinate of the grid as demonstrated by Figure 3. Each centimeter depth was recorded for each coordinate in a field journal.
Figure 3: Taking and recording elevations for each point on the custom x.y grid
The measurements were taken as the distance from the string grid to the surface of the sand below. As a standard, the string grid was used as zero elevation to our datum. This allowed for an easy transfer of data from measurement to records. As Figure 4 shows, the depth would be 19 cm, but would be recorded as -19 cm in the field journal.
Figure 4: A measurement taken at a specific coordinate on the custom grid
The methods used and strategy for recording provided the fastest collection while preserving the most accuracy to properly represent the features created. 

RESULTS

Using the methodology described above, a resulting 340 data points were collected. The maximum depth was at -20 cm below datum, minimum depth was at -7 cm below datum, the mean was -12.78 cm below datum, and a standard deviation of seven. In all, the topology depth varied by 13 cm at its most extreme locations. It averaged in at about -13 cm below datum, but had a very distributed concentration of depths in all with features constantly rising and falling with a large standard deviation of seven.
The systematic sampling method succeeded in providing an accurate way to represent the features collected while minimizing time spent collecting data. This was the original plan and was executed as such. The 5 cm interval grid was decided after creating the features and when there was a 25 cm flat surface north and south of the distinguished topography, an adjusted 25 cm interval collection method was decided upon. This maximized efficiency and accuracy given the project goals.
It was difficult to accurately take a measurement at precisely the given coordinate. When holding the ruler, it was difficult to hold perpendicular to the rope grid to not record depth at a diagonal angle. It was also difficult because as the ruler was placed, the strings had some give because it was impossible to secure them without any slack. One person took the measurement standing directly above to better compensate for as accurate of a perpendicular angle from ruler to grid while another person crouched from the side of the sandbox to give an accurate reading. Additionally, the ruler was gently placed in the corner of each intersecting rope coordinate so as to just barely stretch the rope to record in a consistent manner.  

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