Terrain Part I: Creation and Survey

Report by: Jonathan Luczak
Group 4: J. Luczak; C. McLeish; E. Merkel; B. Miracle; N. Schaffer; Smith

Introduction:

  The objective of this lab was to conduct a geospatial survey on the earth surface features (ESFs) created in the garden box, which were molded out of snow. Figure 1 shows a plan view of the terrain that was created during this lab, and includes the following features: hills (blue circles), a valley (black outline), ridges (green rectangles), a plateau (yellow outline), a depression (purple, translucent circle), and flat-bottomed depressions (red outlines). Figure 1 also illustrates the direction of north, which from now on will coincides specifically to this garden box, e.g. box-north, -south, -east, -west.

Figure 1

Figure 1 shows a plan-view of the terrain. Green rectangles represent ridgelines; hollow blue ellipses represent the apex of hills; the black outline represents a valley depression; red outlines represent lowland plains; and the yellow outline shows a plateau. The direction of north for this specific box (box-north) is indicated by the red arrow in the lower left-hand corner of figure 1 and points towards Davies Center on the UW-Eau Claire campus. Also, note that the grey gridlines that transverse the box horizontally and vertically are each 5 cm² for scale.

 

Methods:

Creation of the landscape:

  ESFs were carved out of snow within the garden box. Figure 2 shows group 4 shaping the ESFs in the garden box using their hands and shovels.

   It was decided that ESFs were not to extend above the wooden edges of the box, as this is where a grid system was to be laid out to obtain x, y, and z coordinates; details of the grid system and data collection methods will be discussed later in this report.

Figure 2

 Group 4 shapes the terrain in the garden box using their hands and shovels. Camera is facing approximately north and east( box-directions).

Setting up the survey:

           A Cartesian coordinate system was set up over the garden box in order to systematically survey the box in terms of x, y, and z coordinates. An example of the completed grid pattern that resulted is shown in figure 3a. Thumbtacks and string were used to create the 5 cm²grid pattern across the box, as shown in figure 3b.

Figure 3

a

 b

 Figure 3a shows the grid system that was created to obtain x-, y-, and z-coordinates needed to model the box landscape using computer programs; the camera is approximately facing north and west. Figure 3b details how thumbtacks and string were used to create the 5 cm² grid-cells.

Survey:

 

           Shown in figure 4, the dimensions of the inside wooden edges were determined to be 235 on the east-west axis and 110 cm on the north-south axis. This meant that a total of 1104 data-points would need to be collected on a 5 cm² grid system in order to establish the elevation of the ESFs.

            Elevation data were collected from the southwest corner of each 5 cm² intersection, an example of which is shown in detail in figure 5.

Figure 4

Figure 4 shows how the working measurements for the garden box were taken inside the wooden perimeter of the box.

Figure 5

Figure 5 shows how measurements were taken from the south-west corner of each 5cm² grid (yellow intersection).

 

Data Input:

Data collected during the survey were entered into a Microsoft excel spreadsheet shown in part in figure 6a. The example excel equation illustrated in figure 6b was used to convert the negative elevation values obtained during the survey into positive ones based off a uniform sea-level of 20 cm below the gridline. The “B2” portion of the equation in figure 6b is a variable and corresponds to different excel cells while the value of 20 is constant.

Figure 6

a b

Figure 6a shows the excel spreadsheet used to record elevation data that were collected from the box; the north-south axis runs along the top of the spreadsheet. Figure 6b gives an example of the excel equation used to convert the negative values in the field to positive values with a uniform sea-level of 20 cm below the gridlines on the box.

 

 

Discussion:

  Prior to the creation of ESFs out of snow, the garden box was filled with snow which had to be shoveled out in order to reach the sand frozen beneath. The frozen sand was initially to serve as the foundation from which the ESFs would rise from or sink into, i.e. sea-level. However, the base-layer of sand was unequally distributed throughout the box, and thus unreliable as a point of uniform sea-level. Figure 1 shows the various ESFs created during this project.

   Shown in figure 2, group 4 sculpts the ESFs in the garden boxes out of snow. These features were then compacted to help ensure that elevation measurements, to be obtained later on, would be taken off a hard surface, as opposed to having the ruler sink into the snow. Once the terrain was complete, it was covered for the night with a make-shift tarp to prevent snow from filling the garden box and ruining the project.

  A Cartesian coordinate system was set up over the box using string and thumbtacks (figures 3a and b, respectively) so that x, y, and z data could be easily and systematically collected. The reasoning behind the methodical collection of data was that 1) it would be collected uniformly throughout the box and 2) that it could be easily transferred onto a Microsoft excel spreadsheet for conversion into a computer-generated model in part II of this project.

   Shown in detail in figure 4, the grid covered the inside area of the box and was 235 cm on the east-west axis and 110 cm on the north-south. Based on recommendations made by Joel Albrecht (2013) in his blog posting for a similar lab, 5 cm was chosen as a grid interval because it would allow for more elevation points to be collected compared to a 10 cm grid pattern. It was hoped that this greater volume of data points would allow for more accurate portrayals of elevation when they were converted to 3D models in part II.

   Once the grid was laid over the garden box, elevation data were collected by dropping a ruler down into the box until it reached whatever ESF was under it. Demonstrated in figure 5, this measurement was always taken off the southwest corner (box-direction) of each intersection point of the grid for consistency. The resulting values were then rounded to the nearest 0.5 cm before being recorded. For instance, using “X.x” as an arbitrary example, a reading of X.0, X.1, or X.2 cm would all be recorded as X.0. In contrast, a reading of X.3, X.4 or X.5 cm would all be recorded as X.5. Similarly, this rounding method would be repeated for numbers greater-than/equal-to X.5 as follows: measurements of X.5, X.6, and X.7 were all recorded as X.5; while measurements of X.8, X.9, and Y.0 were recorded as Y.0; where “Y” is symbolic of the next greater number relative to “X”.

   The reason for rounding in the manner described above is so that any slack in the string would not result in skewed elevation values. Also, since the thumbtacks were not driven into the wood completely, this rounding would account for any height differences among them as well. An example of the height variations among the thumbtacks is shown in figure 7 below.

Figure 7

Since thumbtacks were not driven completely into the wood edges of the box, measurements were rounded to 0.5 cm to account the resulting variance in elevation.

  Values resulting from the collection of elevation data were entered into a spreadsheet created in Microsoft excel. Elevation data were entered as negative values into the spreadsheet, shown figure 6a.  A second, identical excel spreadsheet was created that used an excel formula to add 20 units (figure 6b) to each corresponding value in the first spreadsheet. The decision to make 20 cm sea-level in the box was based on the fact that the lowest data-point collected in the box was -20 cm, and a positive range of elevation data was desired for this project. This excel data was used to model the ESFs in 3D-raster programs for part II of this project.

 

 Conclusions:

   The terrain survey was interesting because it forced one to think in spatial terms. For instance, planning and using a Cartesian coordinate system in order to obtain accurate data points was challenging because one had to ensure that all points in the real-world matched those on paper, in addition to those in excel.

   While it seemed that the data was fairly accurate, some simple measures to ensure a greater degree accuracy could be undertaken in the future projects. Some of these measures include, making sure that thumbtacks were inserted completely and that the lines were held tight during the survey. Taking these precautions may have allowed for more accurate measurements to include rounding to the nearest tenth of a centimeter rather than the nearest half.

   One interesting exercise for future labs may be to take a final survey of the terrain using some sort of laser scanner. Elevation data using this method could then be modeled and compared to the data obtained manually at the beginning of the lab.