Environmental Engineering Reference
In-Depth Information
that even those scales are somewhat arbitrary, given the dynamic nature of sea level
at all time scales. Nevertheless, the vertical datum places the wetland site in a
vertical framework so that vertical positions can be compared to one another
through time, and in the case of an NGS benchmark, referenced to sea level.
Using a published benchmark to determine wetland elevations on an established
datum is useful for relating the bathymetric data to other data sources such as
Federal Emergency Management Agency flood data, tidal ranges, USGS stream
gage records, and USGS topographic information.
Local benchmarks can be established in the field for a specific wetland, and
referenced to an arbitrary datum. It is common practice to drive a 1 m long, 1.5 cm
diameter rebar vertically into the ground within 1.5 cm of the ground surface for use
as a local benchmark. Other local benchmarks can be established with spikes,
chiseled “x” on concrete or boulders, and nails in pavement. If a high order of
vertical accuracy is desired, the survey should use at least two benchmarks on a
common datum and check elevations between the benchmarks regularly. Using
more than one benchmark is good practice when establishing control for long-term
bathymetric monitoring because it allows recovery of the benchmark even if one is
lost. A rebar benchmark can be found (recovered) in future surveys even if it is
buried by sediment in intervening years. Carefully sketched maps, GPS locations, a
shovel, and a metal detector are standard tools for locating benchmarks.
A “staff plate” (sometimes referred to as a staff gauge) is an acceptable supple-
mental vertical benchmark (Fig.
2.7
). A staff plate can be installed using a
graduated meter stick extending vertically from the bottom of the wetland that
allows for determination of the water surface elevation. The staff plate can be
mounted to a piece of lumber or a metal stake that is driven into the substrate for
stability. The 0 m mark on the staff plate is another arbitrary vertical reference for
surveys and recording data. The water surface elevation can be converted to NAVD
88, or other external references if the 0 m mark (or any other mark) is related to
the external reference by surveying to a nearby-published benchmark or via GPS
survey.
If a local benchmark will not be referenced to NAVD 88, you can assign a
convenient starting elevation to the benchmark. Standard practice is to select a
round elevation value (e.g., 10 m), with the constraint being that it is high enough to
keep all the elevations in your survey positive. This practice reduces the common
math errors stemming from the use of negative numbers. Otherwise, it is strictly a
matter of convenience. Any other arbitrary value will do, as long as you record it in
your survey topic as reference for future surveyors.
The next step will be to establish the horizontal datum and axial framework.
The horizontal position of each elevation point in the survey must be recorded.
The position can be considered a point in a Cartesian coordinate system (X, Y). The
coordinate system requires defining the physical position of at least one reference
point, and the direction of one of the axes (X-axis or Y-axis). The direction of
the other axis is taken to be orthogonal from the first. If you are working in latitude
and longitude (or UTM coordinates), the Y-axis is defined as the direction to true
north.
