Wetland Map Methodology
J. Marquisee
Introduction:
It is often important in scientific study to be able to locate phenomena in
the three dimensions of space. This allows phenomena to be easily described
and monitored as their size, or the extent of their effects changes. There are
currently no maps of the wetland that are up to date. The only map currently
available is the original construction plan of the wetland that lacks values for
elevation (Fig. 1). A computer-based map would be a great tool for organizing
and locating data spatially and the creation of one a very important step in the
ongoing wetland projects.
Fig. 1
Topographic maps show elevations in the form of contour lines. These
lines are at even intervals of elevation such as one meter. These lines then make
it easy to see how elevation changes over space. A topographic map would be
the best way to show the wetland, as it would show the various features’ height
and their location on the horizontal plane. This map would be a very useful tool
were it in a modifiable digital form.
Digital maps are very versatile in their many uses. The data they contain
can be displayed in many different ways, and software such as ArcGIS can be
used to add overlaying layers of information to create custom maps of almost any
subject imaginable. With the addition of Digital Elevation Models (DEM’s) the
maps can even be shown in three dimensions for more interesting presentation
options.
Methods:
Vertical Data.
Jen Bowman, Heather Combs, Ashley Reynolds and Jonathan Marquisee
collected vertical data in September 2006. As described in their paper, the
process by which this was done was:
“Vertical Data (depth) will be obtained by tying strings around
several series of pipes located along the individual berms in the
wetland approximately ten meters apart. The series of pipes were
already labeled (A1, A2, A3… etc.) and this improved our
organization and the pipes were used to support a series of
strings from which we were able to measure. A laser level was
used to create a level plane between the white pipes protruding
from the ground. Marks were made at the level on each pipe. This
gave us a level field from which we could make vertical
measurements. Strings were used to connect the pipes over the
wetland’s canals from one berm’s pipe to the next. They were
pulled as tightly as possible to minimize sagging. The depth of the
wetland was measured at one meter intervals from the string to
the ground and recorded in centimeters.” (Bowman, et al. 2006)
Horizontal Data
Horizontal data was collected by gathering the distances and angles of
features from a defined starting point using a compass and a 100-meter tape
measure. This was done by first setting an arbitrary starting point. From this
point, a reading was taken to a second arbitrary point. The resulting angle was
used as a bearing of zero degrees. From the starting point, lines were then
sighted across the wetland using a compass and their angles recorded. The tape
measure was then stretched along these angles from the starting point.
Measurements were taken and recorded in meters from the tape as it crossed
points of note, such as the beginning and ends of slopes, pipes, corners, etc.
This gave data on the wetland’s features in measurements that could be spatially
located in the x and y dimension. The same process was used when a new
starting point for the tape was needed, only at the desired new point, a nail with
labeled ribbon was inserted into the ground. Doing this made sure the
measurements from the new point would relate to the already taken
measurements. The same process of recording angles by compass and distance
with tape was used to record data for different sections of the wetland.
The data recorded in the field was then drawn onto a piece of paper using
a drafting compass and ruler (Fig. 2). This image was then scanned and, using
the software program ArcGIS, the points were connected to create an image of
the wetland’s features. The resulting image was a map giving the likeness of the
wetland from a bird’s eye view. This map was then superimposed on an aerial
photo of the area to show how the wetland fits into its surrounding area.
Fig.2
Results:
The methods described resulted in the creation of the following two maps (Figs. 3
and 4):
Fig. 3
Fig. 4
Discussion/Conclusion:
Unfortunately, the maps that resulted form the survey are not topographic
maps as intended. They are approximately the same as the map we currently
have, only they are more up to date with the wetland’s finished dimensions and
they include the locations of the plastic poles. While creating the map in ArcGIS,
the data concerning the features’ elevations was incorrectly entered and did not
register in the final map. Also, because of the way the data was organized and
the maps were created, they were impossible to turn into topographic maps.
There are several sources of error in the maps created. Because of my
inability to use the Total Station and GPS system, respectively the most accurate
and easy ways to perform the desired task, I used a hand-held compass and
tape measure. Because it was hand-held the compass was unstable and the
resulting angles may reflect this. Due to the long distances being measured, the
tape was prone to sagging, which may have caused error in the distance
measurements. In the process of drawing out the points on paper, I completed
several redundant measurements of the same points and I estimate the total
error to be between one and two meters. The amount of data transfer in the
processing of the data: from numbers in the field to paper to computer may also
have been cause for error. I am unable to estimate how much distortion of the
final map this may have caused.
It is my recommendation that someone map the wetland once more with
knowledge of surveying, the use of a total station, and the manipulation of the
data acquired in the field. The accurate measurements a total station is capable
or recording would provide the best map possible, which would be a proper
topographic map.