Lab 1: Creation of a Digital Elevation Surface using Critical Thinking Skills and Improvised Survey Techniques

Lab 1: Creation of a Digital Elevation Surface using Critical Thinking Skills and Improvised Survey Techniques

Introduction

            The process of sampling is a simplification process that allows one to describe a whole population or area. One takes a sample of data from just a portion of the population or area and then applies those data points to the whole population or area.

Major Sampling Techniques:

•                  Random: This method takes data in a random fashion with no designated order. This technique works well within the surveying of a human population.

•                  Regular: This technique provides a designated order for collecting data points. Regular sampling technique works well within a survey of a geographic area. 

   

            The objective of this lab was to help students utilize their critical thinking skills to develop an efficient way to survey an area. The survey area was a sandbox with the dimensions 114 cm x 114 cm. Students were divided into groups and directed to make a terrain of their choice in the sandbox. This group (group 4) completed the field exercise on Saturday, February 3^rd, 2018 between 10:00 am and 1:15 pm.

Methods

This group chose to use a regular sampling technique for the surveying of the sandbox. Using this method ensured data would be evenly distributed across the survey area. In order to enact a regular sampling technique string and tacks were used to make a grid across the sandbox. This allowed for an accurate and efficient way to survey the terrain. A random sampling technique was discussed but quickly dismissed for this project because a regular sampling technique was deemed more efficient.

The sandbox used for this lab was on the University of Wisconsin – Eau Claire campus. It was located approximately 50 meters to the east of the science building, Phillips Hall. Sandbox number 4 or the sandbox the furthest to the east was used to complete this exercise.

The materials used for this lab included:

•                     Sandbox

•                     Sand

•                     Meter Stick

•                     String

•                     Tacks

•                     Hammer

•                     Pencil

•                     Paper

•                     Computer

•                     Smart Phone    

            A terrain needed to be created before a sampling scheme was set up. Group 4 decided to form a U-shaped ridge along the north side of the sandbox with two prominent hills in the south side to create a smiley face (Figure 1). The sampling area was 114 cm x 114 cm. This group set up a grid with 6 cm x 6 cm squares. This was achieved by placing tacks 6 cm apart along the perimeter of the sandbox and then using string to make a grid (Figure 2 and Figure 3).

Figure 1. The terrain created by group 4. The ridge on the bottom creating
a mouth and two hills on the top creating eyes. This task was a little
difficult as the cold temperature froze a lot of the sand together. 

Figure 2. Group 4 in the process of creating 6 cm x 6 cm boxes around the sandbox.

Figure 3. The finished grid of 6 cm x 6 cm squares within the sandbox. The origin that was
 designated for data collection is located in the bottom, left corner of this image. 

            A sea level needed to be established for the sandbox in order to obtain accurate z values for our survey. Group 4 decided that sea level would be the height of the string grid. Meaning that all terrain below the string grid would be below sea level, or a negative z value.

            Data was then collected with a meter stick at any point where two strings met and where a string met the wooden edge of the sandbox. It was determined the lower left corner (north-east) would be the origin or (0,0). Due to the high volume of points that were going to be collected, it was decided that the data would be entered right into the excel app on a group members phone (Figure 4). A phone was used instead of a laptop due to the weather. A phone is much easier to hold and handle in snowy conditions.

Figure 4. This image is the excel app that was used to collect data in the field. 

Results/Discussion

            Exactly 400 sample points were recorded during this survey project. The sample z values ranged from -19.5 to -6 millimeters below sea level. The mean of the data points was -14.844 mm, while the median was -15 mm. The standard deviation of this data set is 2.217. All of the data points taken for elevation were negative, meaning that none of the terrain created went above the string grid or sea level.

            The use of a regular sampling method worked well for the surveying of the sandbox. Group 4 started off with one person holding the meter stick and reading off the z values and another person recording the data into excel. Shortly into the data collection process (3-4 data points in) the group switch into having one person holding the meter stick in the correct location on the grid, another reading off the z value from the meter stick and the final person recording the data onto excel. This was found to be most efficient. This change in collection method may have affect the first few data points since a different person was reading the measurements but only slightly and not any major error was created. The largest issue that was encountered for this group during sampling was the weather. The cold temperature causes a lot of the sand to be frozen together, which made creating a new terrain difficult, but using some persuasion a smiley face was able to be created. It was snowing the entire collection period; therefore, the sand became covered with snow during the collection time (see Figure 3). This group faced this problem by just entering the meter stick into the snow until sand was hit; therefore, attempting to get the elevation of just the sand and not the snow as well. The snow and cold temperatures also make it hard to be type in the data points as quickly as they could be read; therefore, the data recorder needed good communication with the data collectors to ensure all the data was entered properly.

Conclusion

            This method is classified as a regular sampling technique because the data points were collected in a grid fashion; therefore, a regular pattern. The use of sampling in a spatial situation is very helpful to gain an accurate estimate of the terrain in an area without sampling every single section of the area. Collecting data from every point in a survey area is not possible, so sampling is very necessary. This applies to an area that is larger than a 114 cm x 114 cm box, a survey area of 1 square mile will need a sampling method applied to it as well. Just looking at the raw numbers, the data collected looks like it will accurately represent the terrain design. For future surveys, a larger scale could be used in terms of the grid. This terrain had a lot of flat region and therefore not as many data points were needed. If given a flat terrain in the future with few hills and ridges, it would be better to have a higher density of points near these points of relief and less dense on areas of little to no relief. Overall, this group had an efficient and accurate collection process for this sandbox survey.