Wetland Map Methodology



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.

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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.

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Fig.2

Results:

The methods described resulted in the creation of the following two maps (Figs. 3 and 4):

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Fig. 3

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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.

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