Creating a digital elevation surface created from a
surface terrain model.
Introduction:
This activity included using the XYZ data obtained in
our first scale model exercise to build a digital elevation surface using Arc map and Arc scene We tested several methods within Arc map to create a three-dimensional
surface and then used Arc scene to display the results. We studied our outputs
and determined what changes we could make to improve different steps of our
methods. Finally, we used the new data to create a second set of XYZ data to
map an updated version of our terrain surface model. Data for this exercise
were gathered from a planter box located in the courtyard of the Phillips Science Hall at the University of
Wisconsin Eau Claire; in Eau Claire, Wisconsin.
Methods:
 |
| Fig.2. Digital elevation model using Kriging method of transformation. |
 |
| Fig.1. Digital elevation model using IDW method of transformation. |
We used Arc map to import the xyz data from our first model
as a shapefile into a file geodatabase. We converted the data using several
methods of 3-d analyst; including, IDW (figure 1), Spline, Natural neighbor, TIN and Kriging (figure 2).
We imported several of these models into Arc scene to view them in 3-d. While analyzing
the model data and 3-d conversions from the first model we decided the Kriging
method best represented our data.
 |
| Fig.3. New snow covers the planter box containing the terrain model. |
The planter box we
used to build our model was the second box to the east of the north/south
sidewalk which runs through the courtyard. The box measured approximately 1.2
meters wide by 2.4 meters long. The depth from the top edge of the box to the
soil level was approximately 10 centimeters, but did vary somewhat throughout
the box. Due to the frozen nature of the soil the model was constructed with
snow which was readily available. For the creation of the original model the
snow was scraped from the southern portion of the box to reveal a large flat
area. That snow along with snow gathered from around the courtyard was used in
creating a sizable formation in the center of the box. This formation was
shaped into hills, a valley and mountainous ridges. Near the other end of the
box we simulated a plateau with a gorge cutting through it. Since the
construction of the first model we received about ¾ of an inch of new snow (figure 3) and
there may have been some melting of the model. We used the new snow to reshape
the existing model.
 |
| Fig.5. The terrain model with north/south parallels at 5 cm increments. |
 |
| Fig.4. Pins were placed every 5 cm across the north and south ends of the box. |
We used the south/west corner of the box as our point of
origin for gathering data. We measured five centimeter (cm) points across the north
and south ends of the box (figure 4), beginning at the side of origin, inserting a pin at
every point. With these points we were able to run a series of parallels on the
north/south (x) axis at five cm intervals. The parallels ran up over the high
points in our terrain (figure 5). Many of them touched the terrain so we measured the
parallels at the points of contact to ensure accuracy. We then measured the
east and west sides of the box, beginning at the side of origin, using a pencil
to mark off five cm increments. To overcome the model being higher than the
sides of the box we used two pieces of available 2x6 lumber to increase the
height of the east and west sides of the box by fifteen cm.
 |
| Fig.6. Measuring the distance from the cross stick to the terrain model. By adding 2x6 sides to the box we gained enough height to clear the terrain model. |
 |
| Fig.7. One of the corner blocks which stopped us from measuring the height in the corners. |
We placed a meter measuring stick on top of the 2x6 at one
end and moved the boards down the length of the box by our premeasured five cm
increments. As we moved the stick and boards down the length of the box,
beginning at the point of origin, we measured the distance from the stick to
the terrain at every point the stick crossed a parallel on the x-axis (figure 6). The data
were recorded by hand in a notebook in row, column format as follows: Y0,X1,
X2, X3…, Y2,X1, X2, X3…,
all vertical measurements were made to the nearest cm. Data collection we were
unable to get the data points immediately in the corners of the box due to an
obstruction used in building the box (figure 7). After collecting all of the data we
removed all tools used in the process. The data were entered into an excel
spreadsheet in the same form as by hand. The single missing data points at each
of the four corners of the model were obtained by averaging the three points
surrounding the missing point and rounding that value to the nearest whole cm. We
used the highest value in the spreadsheet to determine the lowest point in the model;
we subtracted each value in the table from the lowest point value giving us positive
elevation values. These data were then converted to a simple XYZ table of
coordinates. We used the previously described process to build the digital
model of the new data set.
Discussion:
 |
| Fig.8. Digital elevation model (10 cm grid), shown using Jenks classification set at 20, and an adjusted color scheme. |
As in the first model we ran into trouble with our grid
system. The parallels that ran north/south could run up and over the terrain
but not the east/west. When we tried the string would slide down the hill
created by the terrain and the parallels. By using the available materials we
increased the height of the sides of the box enough to clear the terrain and accurately
gather the data. Using the unattached boards presented another small problem,
while moving them between measurements one might pull the other off of the box
because they were connected with a string. For our second attempt at the model
we used a meter stick so all of the parts remained free floating, if one part
failed it did not pull the rest with it. After viewing the results from the
first model (figure 8), we decided they were somewhat coarse so we increased the resolution
of the new model by decreasing the grid size used for measuring from 10 cm
squares to 5 cm squares (figure 9). This increased the number of data points by four times,
increasing the time to gather the data but also increasing the quality of the
final project (figure 10). The time involved in setup and tear down of this project was approximately
45 minutes and the time to gather the data was about 1 hour.
 |
| Fig.9. Digital elevation model (5 cm grid), shown using Jenks classification set at 20, and an adjusted color scheme. |
 |
| Fig.10. Digital elevation model (5 cm grid), shown using stretched color palette. |
Conclusion:
The overall idea of this activity is good; we built our own
model and measure it just as if it were a real world scenario. In the real
world we may encounter difficulties which impede the success of our project. The
use of this exercise to develop and understand critical thinking, methods and
possible changes to our project that may have been unforeseen but allow us to
be successful.
Our team, which included Mitchell Collins, Kory Dercks and
myself worked well together as a team. Each was prepared to work, both with the
tools needed to do the exercise and in the cold weather. We were easily able to
navigate each other’s schedules and find the time needed complete the project. I
enjoy being outdoors especially in the winter; however, the weather was an
minor inconvenience, the temperatures during the first part of the exercise
were below zero with a north wind.
No comments:
Post a Comment