Environmental Engineering Reference
In-Depth Information
Georeferencing places the survey shots into a geographic framework, such as
latitude and longitude. Georeferencing can be achieved by surveying from a
published benchmark, or by placing a GPS antenna above the local benchmark.
Published benchmark datasheets will list horizontal coordinates for NGS
benchmarks. These published coordinates are either scaled from a USGS topo-
graphic map or referenced to State Plane Coordinates based on a horizontal datum
such as North American Datum (NAD) of 1983 (NAD83) or of 1927 (NAD27).
Although many features are still referenced to NAD27, NAD83 is the official North
American datum. It is important to note which datum is used if you plan to make
maps that are spatially referenced. The horizontal datum places the wetland site
in X, Y space and serves as the initial point from which all measured features will
be referenced. Similar to the vertical datum, using a published horizontal datum
allows the bathymetric data to be related to other sets of spatial data available.
Once the datum have been established, measurement of wetland features that
define the wetland topography can be initiated. The process of measuring wetland
feature locations will vary based on the technique employed. The basic bathymetric
survey establishes the horizontal and vertical location of points throughout the
basin relative to the benchmark. To measure the wetland features, one can establish
points along a number of transects traversing the wetland or establish a grid of
points. In general, if the wetland is a simple depression, a few transects or a simple
grid might be enough to capture the bathymetric variation. But, if the wetland is
geometrically complex and large, it might require more numerous measurements.
For example, for such a wetland, you may need to establish survey points at a closer
spacing where topographic variation is high compared to other parts of the wetland.
Determining the density or number of points comes with surveying experience. One
strategy is to shoot survey points at major breaks in the slope, but never farther apart
than some predetermined value (e.g., 1 or 2 m). Additionally, more points should be
used to define key hydrologic features such as the wetland boundary and outlets.
If your subsequent analysis shows that more detail is needed, an additional field day
can be used to fill in the missing information.
Depending on your objective, you may also be interested in water storage of the
wetland. As such, it will also be important to record the wetland water stage while
conducting the bathymetry survey because it will be the basis for determining a
volume-stage relationship. Stage can be measured by either reading a staff plate
(Fig.
2.7
) if one is installed or surveying the elevation of the water with respect to
your vertical benchmark, which can easily be accomplished at the water's edge.
2.3.2.1 Surveys Using a Taped Grid
The simplest technology for obtaining bathymetric data involves the use of a grid of
points determined by long metric tapes placed at set intervals (e.g., every meter) or
an equivalent method using a grid pattern (Fig.
2.8
). Nodes (points) are created
where the tapes cross. The nodes are sampled for elevation data. If the tape
positions are referenced with rebar or by some other means, the site can be
