Lab 4: Creation of a Digital Elevation Surface

Lab 4: Creation of a Digital Elevation Surface

Introduction
The objective of this lab was to construct an elevation surface, survey the terrain and record the elevation of it using a proper coordinate system and survey technique. Students worked in groups of three to construct a diverse terrain using a 4 square foot sandbox. Group 3 decided to carry out this exercise on Friday, January 27th at approximately 11:00 AM. Group 3 considered a variety of sampling techniques before beginning the outdoor portion of the exercise. Sampling is a method used to gather data on an entire population by collecting data on a smaller, yet representative portion of that population (Royal Geographical Society). This method can be quick and cost-effective when collecting spatial data, especially when a large study area is involved. There are three main types of sampling:

• Random-sample points are chosen at random
• Systematic-samples that are distributed evenly in a spatial context
• Stratified-samples that are taken in sub-sets to more accurately represent different portions of the whole population

Methods

Materials used for lab:

• Sand
• Meter stick
• Thumb tacks
• String
• Notebook
• Spoon (to carve out terrain)
• Measuring tape

Group 3 determined that systematic sampling would be the most appropriate sampling technique for this lab. By using a simple coordinate system and collecting samples at each X,Y intersection it would be simple to record the data in a table. It would be difficult to keep track of and plot data collection points on a table using random and stratified sampling techniques.

After some consideration, Group 3 decided that the coordinate system grid would contain 6 cm intervals in order to collect an effective amount of sample points while being time-effective. Zero elevation was determined to be the top of the wooden frame. A meter stick was used to measure elevation.

The outdoor portion of this activity was conducted under cold and snowy conditions and, while the team was dressed appropriately, the weather conditions played a factor in the speed at which the data was collected. The work was split up evenly. Max and Shannon began to carve out the terrain using their hands and a spoon while Alex placed thumb tacks at 6 cm intervals starting from the inner edge of the wooden sandbox resulting in the use of 19 thumb tacks on each side of the sandbox (Figure 1). The terrain was constructed to contain a hill, ridge, valley, depression and a plain. Next, Shannon and Max worked together to carefully lay the grid by wrapping string around the thumb tacks set up by Alex (Figure 2). Once the grid was set up data was collected at every intersection starting at the determined 0,0 grid point. Alex held the yard stick at every intersection while Max read the measurement to Shannon so she could record the measurement in a notebook. The collected data could not be directly entered by computer because the weather proved harmful for a computer, so the data would later be transfer to an Excel spreadsheet. Setting up the grid and collecting the samples took about 2 hours to complete. 

Figure 1. Max and Shannon created a unique terrain while Alex (taking picture) placed thumb tacks.

Figure 2. Max and Shannon lay out a grid using string.

Results

401 elevation sample points were collected and entered into an Excel spreadsheet (figure 3). Here is some sample values:

Figure 3. Column Z contains elevation data. Values
are negative since samples were collected below "sea level".

• Minimum:-0.8 cm
• Maximum:-11.0 cm
• Mean:-5.2 cm
• Standard Deviation:2.0 cm

The systematic sampling method worked very well. Group 3 stuck with the systematic sampling method for the entire exercise, although there were some concerns of whether the more drastic changes in terrain were captured thoroughly with the amount of samples taken or not. 

Some problems that were encountered during data collection included the shifting of the sand. When the meter stick would contact the sand, the sand would sometimes crumble and change the initial elevation of the model. Human error could have also played a role in collecting data. In order to read the meter stick Max had to get to "sea level", which required him to bend over and some error in reading the meter stick could have occurred after some fatigue.

Conclusion

This exercise appropriately fits the definition of sampling when it comes to collecting data spatially. Unless a mold of the elevation terrain was created, there is no exact way of creating a model of an area. Sampling gives a geographer the ability to create a model that can be as accurate as necessary for the geographer's use without spending a ridiculous amount of time and energy creating an exact model. An advantage of sampling is that it can be used at any scale, from an area as small as a sandbox (Figure 4.) to an area as large as a forest. Sampling can be used to collect data over larger areas as well. 

Figure 4. The unique terrain spells out "I love EC".

This survey did perform an adequate job of sampling the sandbox, however some improvements could be made. Time-permitting, the grid intervals could be decreased to about 4 or 5 cm to create more samples so that the more dynamic terrain could be better represented in the model. Also, stratified sampling could be used to collect more samples of the dynamic terrain that was isolated into sub-sets.

Sources

Royal Geographical Society. Retrieved January 29th, 2016

from http://www.rgs.org/OurWork/Schools/Fieldwork+and+local+learning/Fieldwork+techniques/Sampling+techniques.htm.