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| Group member measuring one of the many grid points |
Charlie Krueger
Creation of a Digital Elevation Surface-
Field Activity 4
Introduction
Sampling
is the method of investigating and gathering data on a population whether this
is people or a plot of land. Instead of gathering information on the entire
area sampling takes a small frame of the information that represents what the
whole area is like. When sampling in a spatial perspective this would consist
of gathering information on points around a land area that would best describe
the layout of the land. If the area was flat with a hill in the center a
geographer would make sure to include that hill with all of the elevation
change it has when sampling that area.
There are three
different techniques in sampling and these are random, systematic, and
stratified. Random sampling involves each section of the area you are surveying
having the possibility of being selected, there no subjectivity in random
sampling. Random sampling places a grid over the area that is being sampled and
randomly choosing point from where the data will then be gathered from.
Systematic sampling is taking a grid pattern that is evenly spaced out for
every interval and placed over the area being sampled. The intervals can be
whatever the surveyor wants as long as they are kept consistence throughout the
gathering of the sample. When the grid is finished there will be intersections
of the grid lines and this is where the points will be that the data will be
retrieved from. Last is stratified sampling which takes similar areas of the
land and divides them into separate groups to then gather data from. The groups
that are created must be represented by the proportion that take up in the area
to get a sample that represents the land area. So if a land area had a really
rocky area but it was small than that of the wooded area around it the sample
data should show that the rocky area represents a smaller area than that of the
wooded area.
The lab objective
that was given to the class was to create a terrain of sorts in a sandbox and
create features that could be represented by the change in elevation. These
were features like a ridge, hill, depression, valley, and a plain. Once the
terrain was created by the groups they would then have to come up with their
own sampling method that they would like to use to gather the data of the
terrain. The data that would be gathered would then be taken and placed into a
data frame that would be easily transferable into mapping programs. The final
outcome of the lab would be for each of the groups to have a topographic map
that was created from the data collected and then run through a computer software
program.
Methods
The
technique that the group used was that of systematic sampling. This sampling
technique would best represent the terrain because there would be a
representation of multiple points of the area that was sampled. Systematic
would allow the group to create a grid pattern that would then be able to
easily obtain points for the data set. This method may miss smaller details in
the terrain being studied but with all the points gathered it would be sure
that almost all of the major elevation changes would be captured and these are
necessary for the final map to be created. The random sampling technique also
uses a grid when gathering data from points but unlike gathering data from
every point like systematic, random sampling chooses random points on the grid
to select points. This method would not have produced a quality map of the
terrain because if one of the features was missed then no one would know that
it was even there in the first place. Also if a feature did get randomly
selected to have a point there and then not another point for a distance around
the feature it would result in the feature being distorted and not looking like
it did when the group created it.
The location of the sample
plot was a sand box that was located on University property off of Roosevelt
Ave. The sand boxes were created by the class instructor Professor Hupy and
then moved there and set up by all of the students of the class.
The material that was
being used in the sampling was sand that was given to the class by Professor
Hupy. Along with the sand the grid pattern was made up of string that was connected to pushpins that were spaced the length apart.
The group first
determined the diameter of the area that was being surveyed. The box that the
sand was placed in was 114 cm by 114 cm, so it was a perfect square. The group
then determined that a 19 by 19 grid would fit perfectly inside the square and
give us spacing of 6 cm between each of the grid lines. Using a grid that had
more grid lines would let more points be gathered which would then lead to a
higher detailed map coming from the data. The group also determined that
setting the 0,0 point in the bottom left hand corner would help because this
would allow all of the X and Y values to be positively recorded and not easy to
mess up with the Z values.
When thinking about the
zero elevation or sea level that would be used the group wanted to make the
elevation change as easy as possible. There were two options when selecting a
sand box and choosing the one with more sand would mean that using the top of
the wooden frame would work perfectly as zero elevation. This did mean that the
majority of our Z values were in the negative but, this was good because it meant
that the features that were above zero elevation would show clearly in the map.
The data was collected by
the members of the group by measuring all of the points of the grid and then
recording them in a field notebook. This was done by two of the group members
measuring with a fold-able meter stick while one was documenting the points.
This process did take quite a while and was difficult because the measurements
needed to be as accurate as possible. The data was then transferred over to an
Xcel spread sheet so that it could easily be moved around into different
computer programs.
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| This was the depression area of our terrain |
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| This was the 0,0 of the grid pattern |
Results/Discussion
By
recording data from every point on the grid pattern the group ended up with 400
points. This was by measuring a zero on the grid pattern on both the X and Y
axis.
The
sample values were not that varying except for the features that were created
to show up on the map. The minimum value that was located when collecting data
was that of -6cm and was found in the depression that was created. The maximum
value that was found in the data sample was 3cm. The average or mean of the
data sample was -2cm. The standard deviation of the data set was plus or minus
1.38cm.
Yes,
the sampling absolutely related to the method that was chosen as how to sample
the area. No, no other sampling method would have provided as many data points
as the systematic method did and the other two methods would have not captured
all the details of the features that were created on the terrain.
No
the group stuck with the original plan to measure every data point that was
created by the grid system. The data set turned out perfectly with every point
having a measurement for the Z value which is what will make up the topographic
map. Although the group did not know how big the data set was going to be it
was handled easily when working together.
The
only problem which was a small one was that it was difficult to tell the exact
measurement of some of the points because of having to close to a slanted
surface yet get a good measurement. One issue that went along with that was that
the rule would sometimes sink into the sand because it is not a solid surface
which may have slanted some data.
Conclusion
The
sampling that was given as a task for the assignment was sampling but on a very
small scale. Systematic sampling worked well for the group because we did not
have that large of an area to survey and in the field the grid pattern layout
could be much more difficult. The group did gather many data point and only
took data were the grid pattern laid so sampling was accomplished in this
activity.
The
use of sampling in spatial situation is a huge time saver and when you have a
huge land mass that needs surveying the only option may be sampling. This not
only gives geographers a good sense of what the land is like but allows them to
analyze and maybe find something that was not looked at before. Any new
information that can be provided on land areas that have none is a good thing,
sampling may not get all of the information but can provide a good overview of
it.
The
approach that the group would have made on a larger land mass would have been
very different. With a larger diverse land area, the stratified method could have
been used to determine groups which would help create a layout of the land. This
activity defiantly relates to sampling a larger area but the methods that were
used here would not apply that well on a large scale site. With this activity a
better understanding of sampling did come so that would obviously be useful
when trying a larger area.
Yes,
the survey that was conducted did an adequate job of sampling the area that was
given. The only way to accommodate the sampling density would be to expand the
grid pattern to include more lines so that way more data points would be
collected resulting in a more accurate map.
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