Environmental Engineering Reference
In-Depth Information
autolevel is mounted to the bracket, without over-tightening the mounting screw.
Precise instrument leveling is accomplished with reference to a bubble level as you
adjust three leveling wheels on the autolevel base. When the instrument is leveled
in this way, yet more precise leveling occurs within the instrument, which is the
basis for the instrument name. Once the instrument is level, the horizontal cross
hairs in the scope are focused for an individual's eyesight. The cross hairs delineate
a horizontal plane as the instrument is rotated about a vertical axis. In other words,
everything that the horizontal cross hair hits is at the same elevation. This elevation
is called “instrument height,” and the value is denoted “HI” in survey notes. The
value of the instrument height is determined by the first shot of the survey when
the rod is placed on a benchmark (BM) of known, or arbitrarily assigned, elevation.
The shot used to determine the HI is called a “backshot” (BS). The instrument height
is determined by summing the BM elevation and the BS reading. The HI value is
assumed to remain constant, unless the instrument is moved during the survey.
Once the HI is determined, you can survey the ground elevations of any rod
positions where the rod can be seen by the scope person. The shots used to determine
unknown ground elevations are called “foreshots” (FS). The ground elevation for
each FS is determined by subtracting the FS from HI. All that remains is to place the
rod on the ground at the grid points determined by the tape grid described above,
record the FS, and calculate the elevation. Then, each X and Y position will have a
corresponding elevation Z. As with the water depth method, and any other method,
additional shots should be taken at many places along the current shoreline and at any
points required to capture the details dictated by your survey goals. The last shot of
the survey is the “closing” shot. It is a FS taken with the rod on the BM on which you
opened the survey. The resulting ground elevation (HI-FS) should match the real
(or assigned) BM elevation. The mathematical difference between the elevation from
the closing and the real elevation is the “closing error.” Closing error is a measure of
the precision of the survey. The source of survey error is typically due to physical
changes in the tripod or tripod feet positions that change the HI or scope leveling.
Given your calculated closing error, you must decide if the precision is acceptable.
Based on our experience, autolevel surveys that last 1-2 h typically may have a
closing error of less than 1 cm. Errors greater than 1.5 cm are uncommon.
Resurveying may be necessary if higher precision is required. If low precision is
acceptable, then a larger closing error is allowable.
A standard autolevel practice is to perform a “two-peg” test of instrument
calibration prior to a survey or sporadically throughout the survey season. The
two peg test involves firmly driving two pegs (A and B) in approximately level
ground separated by approximately 30 m. The tripod and scope should be set in the
middle of the two pegs and a rod reading should be recorded on pegs A and B. In
your notes, the rod readings should be recorded as “a” and “b,” respectively. The
next step is to move the tripod and scope as close as possible to one of the pegs, but
not so close that the rod cannot be read. From this location, another set of readings
will be recorded with the rods back on pegs A and B, but record the shots as “c” and
“d” in your notes. The difference between “a” and “b” is a measure of the difference
in elevations of the tops of pegs A and B. An independent estimate of the difference
