Lab 4: Creation of Digital Elevation Surface

Introduction

The purpose of this exercise was to provide students with experience in designing, creating, surveying and mapping the elevation of a terrain.  This work was completed in groups of three in a square 4 foot by 4 foot sandbox.  For this group, the activity was carried out on Saturday, October 8th from 2:00pm to 6:00pm.  Before carrying out the activity, each team spent time reading about various sampling techniques.  Sampling is way to quickly collect data on a whole population by gathering data from only a portion of the population (Royal Geographical Society).  For field methods, sampling can be a way to save time, money and energy when dealing with large areas.  There are three main varieties of sampling: 

• Random: samples chosen randomly

• Systematic: samples chosen in a regular way

• Stratified: samples chosen in stratified way; used when population is comprised of sub-groups

Methods

The following materials were used in this activity: 

• Sand
• 2 meter sticks
• Thumb tacks
• String
• Straightened metal hanger
• Computer

A systematic sampling approach was used in this lab, as the student team determined it would be the best way to accurately gather data.  A random sampling approach could have been used through utilizing Excel to randomly select squares of the grid to measure, however, the team wanted to ensure all features of the terrain were measured in detail.

After some discussion, it was collectively determined to create the grid in 5 centimeter intervals.  The team also decided to consider the ground (beneath the sand) as zero elevation to avoid dealing with negative numbers and to make measurement easier.

While Andrew placed thumb tacks around the wooden sandbox in 5cm intervals, Jackie and Anneli created the terrain in the sand.  This terrain included a hill, ridge, depression, plain and valley (see Figure 1).  The team then used string to set up the grid (see Figure 2).  Then, the elevation measuring process began, with Andrew using a makeshift straightened hanger to measure elevations at grid intersections (Figure 3).  This measurement was then compared to a meter stick by Anneli and relayed orally to Jackie, who entered the values into Excel on her computer (Figure 4).  This digital data entry method was chosen for speed and to avoid having to transfer manually recorded data into a computer later on.  This process continued for around three hours until all of the terrain was assessed.

Figure 1: Andrew placing thumb tacks in 5cm intervals

Figure 2: Jackie laying the grid with string

Figure 3: Andrew measuring sand elevation with the hanger

Figure 4: Andrew comparing hanger to meter stick

Results

Figure 5: recorded elevations (z) in Excel

Data about 532 points was collected and entered into an Excel spreadsheet (Figure 5).  Here is a look at the data:

• Minimum: 6.6 cm
• Maximum: 23.2 cm
• Mean: 14.6 cm
• Standard Deviation: 2.6

The chosen systematic sampling method worked well for the data at hand.  That being said, the team did change the sampling technique near the end of the data collection process.  Upon encountering the flat "plain" feature of the terrain, it was decided to complete elevation measurement at every other grid intersection since the elevations were very similar.  

Problems encountered during sampling included human error when conducting measurements with the hanger.  Since the sand and ground was a bit wet during the measuring process, it was sometimes hard to tell where the sand ended and soft ground began.  To try to overcome this problem, spots were often measured twice when the first elevation number seemed to be inconsistent with surrounding measurements.  Additionally, the computer used for data entry died partway through data collection.  This problem was solved by simply recording the last two rows of data by hand and later entering them into the computer.

Discussion

This sampling exercise exemplifies the definition of sampling in that data was systematically collected at some points amidst a larger "population" of points.  Sampling is an important tool in spatial situations to gain insight about entire areas without expending huge amounts of time or money.  Though this exercise only dealt with a small area, the same principles can be applied for larger areas.  Being able to gain accurate information about a larger area through sampling can save lots of time and money, while still providing useful information.  

The survey this team completed did perform an adequate job of sampling the terrain at hand.  That being said, it would of course be better to gather more data about the unique areas of the terrain such as the hill and ridge.  If doing this exercise again, multiple measurements could be made in such areas to reflect the relief there. 

Sources

Royal Geographical Society. Retrieved October 5th, 2016 from http://www.rgs.org/OurWork/Schools/Fieldwork+and+local+learning/Fieldwork+techniques/Sampling+techniques.html