Creation of a Digital Elevation Surface

Introduction

This was the first project we had in our class and was designed to use critical thinking skills and improvised survey techniques. We were to use snow to make our own terrain surface models that included a: ridge, hill, depression, valley and plain. The landscape we created then had to be surveyed, so an appropriate surveying technique had to be thought of and implemented. Emphasis was placed on the ability to create a co-ordinate system in order to take measurements of our land model. Planning before going out in to the field was key, so as to avoid spending unnecessarily large amounts of time in the -40⁰ wind chill weather we were experiencing.

Methods

The first step in our project was to examine the field site we had been given. The size of the area, the depth of the boxes we would be modeling in, as well as the amount of snow that was covering our site were all important things to take note of. 

So we cleared a path in order to be able to access our box more easily, as well as taking the snow away from the box which exceeded the height of the box walls, this is shown in figure . In this figure where a group member is clearing a path around the box with a shovel, two other members are clearing the snow from inside the box with a smaller shovel and by hand, and another member is preparing to level of the snow with a wooden beam. . This was in order to have a somewhat flat starting ground, and stop what we created from falling over the sides of the box, so it could be preserved for the several days we would need it to carry out fieldwork. 

Figure 1: Our group clearing the surplus snow from our field box in order to be able to construct a terrain model that would not exceed the height of the box.

We then set about sculpting our land masses, using the geographical knowledge we had on how these formations are created, as can be seen in figure 2, someone is constructing a ridge by gathering snow together in a straight lie, another is hollowing out a mass of snow for a corrie, and another is flattening a valley floor. The grid for our co-ordinate system was then constructed, we decided that based on past classes work that we would create 5cm² grid squares. A meter stick was used to measure these squares out and push pins were used to mark the axes on the top of the side walls of our box, this is portrayed in figure 3. We then wove coloured string around these pins to create our grid.

    Figure 2: Modelling the snow in to the five specified land forms of; ridge, hill, depression, valley and plain. In order to produce a terrain model.

Figure 3: Measuring and marking 5cm intervals on the sides of our box in order to create a co-ordinate grid over or model with string and pins. 

Before heading out to the field we had put together a spreadsheet in Microsoft Excel with our grid co-ordinates on it, in order to make taking measurements slightly quicker, it contained columns for the x,y and z (our elevation measurement) values. The meter stick was then placed at the bottom left corner of every grid square and the elevation at that point was recorded, as can be seen in figure 4. 

Figure 4: Using a meter stick in order to measure the distance from the grid squares down to the top of the relief of the constructed land.

Due to the extreme temperatures and trying to find times throughout the week when people were free at the same time, we carried out our fieldwork over several days. So, we had to ensure that we covered our box with a heavy duty plastic sheet every time we left the site. As, it was likely that it would snow and completely alter the topography of our land.

The results from our data collection were then entered back in to our digital spreadsheet, so that it was ready to be placed in to ArcMap. Also, as we made sure that all of our formations were below the top of our box, all of our measurements were negative values. So, we put an equation in to our excel spreadsheet, in order to use positive values, this took the recorded value and added 20cm to it, as we measured that this was the biggest depth of the box. 

Discussions

I would have to say that definitely the most challenging part of this assignment was having to deal with such extreme weather, when we had so much work to do outside. We had to do our work outside in little sections over a period of time. This meant that perhaps it was a bit slower as we had to set up and tidy away after each data collection. It also made it quite difficult to even get the box or be able to maneuver around it easily. 

Our original model of the landscape was slightly altered, as we tried to remember what made, for example a valley, e.g. steep sides, flat floor, and a nearby hill for the glacier that formed it to flow from.

The boxes were made of fairly thick wood, and push pins are fairly small, and the freezing temperatures made people’s hands a little bit less functioning than normal, so it was at times quite challenging to push the pins in all the way. So some of them ended up sitting slightly higher than others which could impact some of our z values.

Over time the string that formed our grid became slightly slack, so at some points may not have been exactly 5cm², so perhaps with the use of a more solid structure placed over the bx for a grid system this may nothave been a problem.  I think it is also important to take in to account that string and snow are maneuverable so perhaps at some points when the meter stick was leaning against them it may have sunk in to the snow, or lent back against the string, altering the results slightly from what was there.

As we had different people recording the data at various areas of the box, we forgot that different people would have different ways of rounding the values. As, some were rounded to the nearest half centimeter, and others to the one decimal point.  So when the data was entered in to the final spread sheet, we ended up having to round them all to the nearest half centimeter. So this was a problem of not necessarily a lack of communication, but a lack of realisation that there would be different methods.

Many of these are issues we did not realise would arise until we were out in the field or after the data had been collected. So it was surprising to see how much room for error there was in our method. But where possible we tried to eliminate this by standardising exactly where we took the measurements from, and measuring out the grid as accurately as we could. Also, by switching roles in the group and checking other peoples work (especially when it came to the data entry) there was less room for human error.

Conclusion

Throughout this project it was discovered that something which at first may have seemed fairly small scale and trivial, could actually have a lot of different thought processes that have to be completed. Dealing with adverse weather conditions when there is nothing you can do to stop that was challenging. Making sure that the small scale model was an accurate portrayal of a real life situation was important, and I think is the key to many geographical models, whether manual or digital. The elimination of error where possible must be thought about more abstractly in order to account for things that you would at first not think would have an effect on results. Consistency in the method, and organisation before entering the field and a clear understanding of the objectives of the assignment are all vital.