Discharge measurement data is irrelevant and inaccurate if
correct station levels are not set and maintained. A USGS standard
gaging station has a minimum of three elevation references and a
typical station would have four reference marks and six elevations
referenced to gage datum for redundancy and convenience to perform
stage elevation checks. The USGS standards require a yearly check
of gage datum levels so that any reference movement, settling,
etc. can be noted and corrected. All reference marks are surveyed
in at .001 feet while gage datums are read to .01 feet.
Automatic levels or total stations are used to set accurate
elevations for permanent gaging stations. Hand levels are
typically used for initial field site selection and stream habitat
surveys. Also stream hydraulic gradient measurements can be
obtained with a hand level or clinometer. Level rods, tripods and
other surveying accessories are offered to complete the necessary
measurement and various markers and nails are used for setting
reference marks.
