Wetland Bathymetry and Mapping - Wetland Techniques - page 58

Fig. 2.6 National Geodetic Survey Benchmark ( left )( http://www.ngs.noaa.gov/ ) and local county
benchmark ( right ) used for the Molera Wetland, California, USA
analyzed once. If the object of surveying is to map physical change through time,
it is most advantageous to establish at least one long-term, stable benchmark near
the survey site. The USGS usually uses brass monuments set in rock, a concrete
pylon, or a pipe driven into the ground (Fig. 2.6 ). Ideally, there will be an NGS or
USGS benchmark near your study area and we recommend that you use it. If not,
you will need to establish a new local benchmark. This local benchmark can be a
wide range of objects such as a chisel mark in exposed bedrock, nails and tin
washers driven into a road, concrete pads used for street signs, fencing, or other
public infrastructure. You must be sure to select or construct a benchmark that is
vertically and horizontally stable, preferably for many years. All bathymetric
surveys will follow these basic steps for data collection: (1) establish vertical
datum, (2) establish horizontal datum, (3) record wetland water stage, and (4) mea-
sure the relative position of wetland features in the X, Y, and Z space. Developing
the vertical and horizontal datums allow every survey shot to be referenced to a
stationary and stable point—thus, allowing reproducibility and the capacity to
measure precision.
The vertical benchmark is the starting point of any topographic and bathymetric
survey. We will assume that the ultimate goal of your survey is to develop a
“topographic” wetland model, which has elevation values rising in the uplands.
“Bathymetric” surveying is analogous, but values rise as you descend into the
wetland. Regardless of technique, all surveys will start from a benchmark. There
are published benchmarks and local benchmarks that can be used (Fig. 2.6 ). Local
(arbitrary) vertical datums are assigned elevations that are not based on published
benchmarks. Assumed elevations can be based on water surfaces, elevations of
fixed structures (such as outfalls or crossings), pool points, or staff plate elevations.
We recommend you visit the NGS website ( http://www.ngs.noaa.gov/ ) to access
published benchmarks. The elevation datum on published benchmarks will either
be referenced to the National Geodetic Vertical Datum (NGVD) 1929 or North
American Vertical Datum (NAVD) 1988, which are merely vertical scales with a
0 m mark that corresponds to an estimate of mean sea level. You must recognize