Exercise 1: Creation of a Digital Elevation Surface

Introduction

·         Define what sampling means, with a strong focus/emphasis on what it means to sample in a spatial perspective.

o   Sample can be defined as part or single item that is representative of a larger group. Spatially a sampling method needs to be able to allow for enough data to create a stronger representation of the area or object that is being sampled. 

·         List out the various sampling techniques

o   Simple Random Sampling (SRS)

o   Stratified Sampling

o   Cluster Sampling

o   Systematic Sampling

o   Multistage Sampling (in which some of the methods above are combined in stages)

o   Multiphase Sampling

o   Convenience Sample

o   Purposive Sample

·         What is the lab objective(s)

o   The objective of this lab is to use geospatial thinking to construct an elevation surface of a terrain that is located in a square meter sandbox. The profile had to include a ridge, hill, depression, valley, and a plain (Figure 1). Finally, the group will be able to map out the elevated surface using original survey technique.

Figure 1. Photo of an top view of what the profile our group created with all the requirements.

Methods

·         What is the sampling technique you chose to use? Why? What other methods is this similar to and why did you not use them?

o   We used systematic sampling so we could create a method that allows for the most data points that will target the majority of the grid.

·         List out the location of your sample plot. Be as specific as possible going from general to specific.

o   The sandbox was located on the University’s campus.

o   It was to the East of Phillips Hall, across the road of Roosevelt Ave.

o   About 100 ft. from the loading dock of Phillips to the sandbox located next to a shed.

·         What are the materials you are using?

o   Materials that were used were a meter stick, string and tacks, a flag, and a square meter sandbox.

·         How did you set up your sampling scheme? Spacing?

o   For this lab, our group chose to create a grid that allowed for 20 points on the X-axis and 20 points on the Y-axis as shown in Figure 2.

o   The grid consisted of 400 squares, where we measured a point in the top-right corner to achieve our 400 data samples.

Figure 2. Grid of our profile with x and y-axis defined. 

·         How did you address your zero elevation (sea level)?

o   We measured the height of the sandbox to get a defined sea level of 15 cm.

·         How was the data entered/recorded? Why did you choose this data entry method?

o   We created a 3 column table with X-Y-Z as the headers for each column. The x-axis and the y-axis was defined in the row, before the elevation of Z was recorded. The group then took the difference between the string and where the sand ends in order to figure out the elevation or Z (Figure 3).

o   With only a notebook as a form of recording data, creating columns was the most efficient way of entering data before it was entered on a spreadsheet into a table.

Figure 3. Meter stick measuring at one of our sample points by measuring the space between the sand and the string. 

Results/Discussion

·         What was the resulting number of sample points you recorded?

o   400 sample points

·         Discuss the sample values? What was the minimum value, the maximum, the mean, standard deviation, etc.

o   The range of the sample points was 14, with the minimum value being 7 cm and the maximum value being 21 cm (Figure 4).

o   The mean was 12.48 cm, with a standard deviation of 2.14 cm.

o   The mode was 12.

o   Our sea level was also established at 15 cm.

Figure 4. Photo of our hill and maximum Z-value. 

·         Did the sampling relate to the method you chose, or could have another method met your objective better?

o   For this particular activity, we could not have used any of the random samples. Therefore, systematic was the best choice.

·         Did your sampling technique change over the survey, or did your group stick to the original plan? How does this relate to your resulting data set?

o   The sample that we chose was the one that we stuck with throughout our data collection.

·         What problems were encountered during the sampling, and how were those problems overcome.

o   During the sampling, often times the spot that we were going to sample next was often lost. But we were able to follow along with the grid system of our data collection in the notes.

o   There were often times that the meter stick was to big more measuring elevation and some of the data points may not be as accurate as they should.

Conclusion

·         How does your sampling relate to the definition of sampling and the sampling methods out there?

o   While often times, there needs to be a random sample in order for much of the data collection to be considered unbiased and representative of the larger sample. In this case in order to gather enough data to accurately represent the elevations of different points in our sandbox, the sampling method needed to be systematic.

·         Why use sampling in spatial situation?

o   Sampling eliminates the bias and persuasion of how one would collect the data.

·         How does this activity relate to sampling spatial data over larger areas?

o   By using this small scale model of the sandbox, we could conduct another sample of a larger area with the same grid technique as well as use the sampling method we chose for this exercise for a larger area as well.

·         Using the numbers, you gathered, did your survey perform an adequate job of sampling the area you were tasked to sample? How might you refine your survey to accommodate the sampling density desired.

o   It is already noticeable in that sample shows variation in the landscape based on the higher numbers in some areas, lower in other, and a consistency of numbers in the depression. To refine the measuring aspect of the survey, I would find a more efficient and accurate use of measuring. The metering stick may not have been the best way to get accurate measurements of the elevation.