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.
• I would define sampling as retrieving data from specific sections in an overall study area. For example, sampling elevation from a plot of land requires taking elevation levels from many spread out points in the study area. 
• List out the various sampling techniques
• Random sampling, stratified sampling, cluster sampling, and systematic random sampling are all types of sampling. 
• What is the lab objective?
• The objective of this lab is to create a landscape in a sandbox and sample elevation of the landscape in a way that is most efficient to us. The landscape must contain a ridge, hill, depression, valley, and plain. 

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?
• Our group decided to use systematic, stratified sampling. This is because we wanted to create a grid across our landscape and take at least one elevation point from each grid section. It is stratified because we took some extra measurements in specific areas where we could see a lot of elevation change. We didn't want to use a purely systematic sample because we wanted to make sure that areas with a lot of elevation change were more accurately sampled. 
• List out the location of your sample plot. Be as specific as possible going from general to specific. 
• Our sample plot represents a costal region with mountains on the coast and the depression and hills on the other side of the mountains. The hills and depressions flatten out into a plain. 
• What are the materials you are using?
• We used a 114cm X 114cm wooden box to contain the sand that was molded into a landscape. A measuring stick was used to create the measurements for an even grid system and to measure the elevation inside the grids. Tacks were put on the box to indicate the x,y boundaries of each grid. String was used to create the grid. Finally, a notebook and pencil were used to record the elevations collected. 
• How did you set up your sampling scheme? Spacing?
• An X,Y plain was used. A 10X10 grid was created and each grid was 11cm X 11cm. This was about as even as it could be made with the box being 114X114. 
• How did you address your zero elevation?
• Sea level was considered the top of the box for us. This means that most of our land will be below sea level. 
• How was the data entered/recorded? Why did you choose this data entry method?
• We recored all of our elevations in a notebook and converted it into a spread sheet with values for X,Y, and Z. This will allow for it to easily be entered into the computer program.

Results

• What was the resulting number of sample points you recorded?
• We recorded a total of 145 sample points.
• Discuss the sample values? What was the minimum value, the maximum, the mean, standard deviation?
• The minimum value was -15, the maximum value was 16, the mean was -3.05, and the standard deviation was 6.13. These values show that the majority of our points were below sea level but the mountains were relatively high above sea level. 
• Did the sampling relate to the method you chose, or could another method ave met your objective better?
• I think that our sampling method was the best choice. we decided to take extra points around our large elevation changes so that those extreme changes could be seen. Our overall mean being below sea level is because the mountain and hill were the only areas that peaked above sea level. 
• 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? 
• We stuck to our original plan throughout the survey. This didn't really affect our resulting data set. It turned out pretty much how it was expected to.
• What problems were encountered during the sampling, and how were those problems overcome?
• The areas where sand was above our sea level were difficult to measure because the string couldn't be placed evenly over them. Our solution was to use the string on the next grid space over and use a ruler to read the measuring stick that was placed in the area being measured. 

Conclusion

• How does your sampling relate to the definition of sampling and the sampling methods out there? 
• I think we did a pretty good job sticking to our systematic sampling method a long with taking extra points where we needed to. 
• Why use sampling in spatial situation?
• Sampling is an efficient way of evenly collecting data in an organized fashion. Sampling helps to collect spatial data that is needed. 
• How does this activity relate to sampling spatial data over larger areas?
• It is the same idea as sampling spatial data over a larger area. The difference is the sampling grid would be a different size and you would actually have to move around to collect the data. 
• Using the numbers you gathered, did you 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?
• I think that our survey did an adequate job to represent the sampled area. It is hard to know quite yet if it truly was adequate because the numbers haven't been put into the program that will create the digital elevation model. If the DEM doesn't come out as hoped, I think the biggest thing that we could have done to be more accurate would just be to take more points. Dividing each grid into fourths would give us four points to every one that we collected which would give us a more accurate DEM.