Field Activity 4: Digital Elevation Surveying

Introduction

Sampling is a scientific collection method that involves precise measurement of small portions of a large study. It is very important to sample, especially when studying areas of the earth. Sampling allows a study to be completed faster, more efficiently, and saves many resources in the process of creating a valid sample of the entire study area. There are three types of sampling used; random, systematic, and stratified. Random sampling use completely random location collecting. It is completely unbiased, where everything within the study has an equal chance to be selected. It can also be a bad choice to sample a study randomly if large areas within the study are not being sampled. Systematic sampling uses a regular, evenly distributed interval of the study to collect samples. Things such as a grid, or lines of study can be used to achieve a systematic sampling system. It is good to do very straight forward data collection, but can be biased in collection. Stratified sampling is the third sampling method. It uses known groups of a study, and collects data points according to the size of the groups in the study. It can be done in concurrence with the other sampling techniques, and can generate good data that represents the entire study. It can be bad when the groups of the data are not known, as this generates skewed data.

The lab's objective is to accurately construct an elevation surface of a terrain. The study will be completed using a 1 m x 1 m sandbox that students will construct a landscape within using sand to create a unique terrain. This terrain will then be sampled using the sampling method of choice by the students and entered into a spreadsheet to be used for creating a DEM in ArcGIS.

Methods

The sampling method chosen by group 1 was to create a sample using a stratified sampling technique, with a systematic sampling grid to create an easier sample collection. This became effective when the group ran into time constraints, and needed to collect data in a timely matter. This is similar to a systematic sampling method, but involves the shape of the terrain within the study area mapped out as "groups" within the larger area. The sample plot is located near the University of Wisconsin -Eau Claire's Phillips Building. It is directly across the East side of Roosevelt St in a yard of a local resident. The sample plot studied by the group was the western box of the two (Figure 1).

Figure 1: Completed sandbox set-up that includes a forest to the east, prairie lands to the west, and various terrains in between.

Materials Used

• Meter Sticks
• String
• Thumb Tacks
• Data Collection Notebook
• Pencil
• Samsung Galaxy S6 Edge

The sampling scheme was set up using a grid with roughly 10 cm spaces between each reference point on the x and y axis. The sea level was decided to be at the top of the box. Strings were then set up as a spatial reference point at each x and y point to draw over the grid the rough drawing of the elevation model. This allowed group 1 to create a stratified system of equal groups within the model that streamlined the data collection (Figure 2).

Figure 2: Creating a Sampling technique.

Once this was set up, elevation points were collected using a meter stick to collect to data of elevation compared to the sea level (0 cm point). It was entered directly onto the drawn sampling grid (Figure 3). This created a good system for the group to complete the task at hand.

Figure 3: Drawn out sampling method.

Results/ Discussion

In total, there were 178 data points collected using the stratified method. They ranged from very low in the deepest part of the depression to very high on the highest peak. The prairie lands have similar values throughout much of the western portion of the sample.

Minimum: -16 cm
Maximum: 12 cm
Mean Elevation: -4.71 cm
Standard Deviation: 4.64 cm

Figure 4: An example of the collected data.

The sampling method was effective for the group. It became more effective when the group decided to use the string to print lines across the sand box to easily view the cells. The biggest problem for group 1 was the fact that the group only included 2 students, so moving the data points, collecting the data, and writing it was difficult to do between 2 people.

Conclusion

The sampling method deployed by the group utilized a good stratified system that allowed the students to involve a systematic sampling approach to the data collection. One must always consider sampling spatial situations as the time and resources saved become very valuable when a sample can be as efficient as possible. This same sampling technique could be deployed in a much large area to collect data, when the time used to collect the data is amplified many times over. This survey did a very good job of collecting data and the DEM created on ArcGIS will show if the methods were successful. One could add the strings across the whole plot to view the data much more clearly when collecting.