Creation of a Digital Elevation Surface using Critical Thinking Skills and Improvised Survey Techniques

Introduction: Digital surface models are a new, extremely useful piece of information a geographer has available to help study the Earth. The ability to generate a digital version away from the field allows researchers to review and test hypotheses on a study area without having to travel in and out of the site. Taking a real world feature or landscape and transforming it to a model on the computer screen can be achieved several ways one which will be described in the following sections. The goal of this exercise was to create and survey a terrain field of interest.

Methods: For an introduction to surface modeling, a snow-filled planter box measuring 112cm X 232cm served as the study area. The snow in the box was leveled off by sliding a board along the top edges of the box to create an even starting point for the terrain. From this point, the terrain was shaped by hand resulting in the formation of a depression, ridge, two peaks, valley, and plain.

The snow of the planter box was first leveled with the help of a board. The terrain was then sculpted by hand and featured two peaks, valley, plain, ridge, and depression.

The use of string allowed the grid to follow the contours of the terrain without compromising the line of the grid itself.

Each grid line was placed a 8cm intervals from one end of the box to the other. By the string being draped over the terrain rather than taught from end to end, allowed the grid lines to mold to the surface without damaging the soft snow structures. 

The completed coordinate grid of 14x29 8cm cells. 

  Once the formations were completed, a string grid was placed across the formations. Each string was placed 8cm apart and stretched from one end of the box to the other and from one side to the other. The 8cm spacing was marked on the edges of the box using meter sticks and black pens. Since the peaks and ridge formations reached above the extent of the top of the box, the strings were draped across the surface to prevent damage to the formations.

Using Microsoft Excel, a spread sheet was created to help record the elevation values with the horizontal and vertical positions of cells in the string grid as well as their corresponding elevations. Surface elevation was then measured using a meter stick and the string grid as a zero reference. The grid formed by the string allowed the most extreme value (highest point if above zero or lowest point if below) of each grid square to be recorded with the coordinate position of the cell. To help keep the elevation values true, each string was pulled tight, if possible, to keep the zero constant with the top of the box.

After all grid cells were assigned elevation values, a new zero was assigned to the lowest elevation value. The difference or value of the lowest elevation was added to all cell elevations to prevent negative values. For example, the lowest recorded elevation was 12cm below the top of the box (-12cm), therefore, using that point as the new zero, an addition of 12 was added to the recorded elevations preventing those negative values. This addition of 12 cm was added to all cell values including the X and Y cells to keep the scale true to the real features.         

Discussion: Several challenges arose throughout the duration of this exercise; the first being where was zero.  The group decided that using the sturdy frame of the planter box would work well as a starting zero but wouldn't prevent negative values for elevations. It was then discussed and decided that the later relative adjustment based on the lowest recorded value would augment the elevations relative to the top edge of the box but not distort the shape and location of the formations. The second challenge was how to grid the survey area. If the strings were taught and secured on the box edges, they would cut into the soft snow formations that rose above the box. To remove the risk of damage or alterations, the strings were pulled tight then gently laid over the formations so that the line stayed true but didn’t alter the snow. The third challenge was the surface elevation measurements. The survey method chosen was a single point maximum where the point measured in the cell was either the highest (above 0) or lowest (below 0). Average point measurements for each cell may have been a more accurate survey technique but with the meter stick to measure with and the resolution of the grid, there wouldn’t be much variation in values. The measurements of those formations that exceeded the height of the box were tricky to record also. Because the surface was uneven in the cells, the meter stick often stuck in the snow about a centimeter. This variation may also skew the results slightly, leading to a model that isn’t as accurate.

Conclusion: The method of using a grid system is common when surveying terrains. Different elements that can be present in models and in real world settings pose different challenges when trying to replicate. Modeling and field work in general requires on to think on their feet and problem solve with limited resources available. Not every idea’s going to work, so the process of trial and error continues until the best option is found. All field skills are extremely important and can be applied to any instance in a person’s life.