StarNet v6 manual - Engineering Surveyor

Chapter 4 Options
The remaining settings in the conventional instrument settings group are either centering
error values, or proportional errors (PPM) for distances or elevation differences.
Horizontal and Vertical Centering Errors
� Horizontal Instrument and Target Centering - This is an estimate of the horizontal
centering error, the inability to center over a mark. These values are used to inflate
the standard errors of horizontal angles, directions, distances, and to some extent,
observed zenith angles. Horizontal centering error inflates standard error more for
angle readings observed over short sights than those over long sights. Therefore,
angles observed over short sights are given less influence (weight) in the adjustment.
What values are reasonable? As a rough guide, for instrument centering error we find
values around 0.005 feet (0.0015 meters) commonly used for optical plumbs, and
0.010 feet (0.003 meters) or more for string plumbs. Target centering depends on the
kind of target and care used in setting up. What values you use for centering error
must be based on equipment, field conditions, your crew and experience.
� Vertical Centering – This is an estimate of the vertical centering error, the inability
to measure the exact instrument or target height from the ground. The value is
assumed to be the same at both instrument and target. Vertical centering is used to
inflate the standard errors of zenith angles, elevation differences, and to some extent,
slope distances. Vertical centering error inflates the standard error more for zenith
angles observed over short sights than those observed over long sights. Therefore
zenith angles observed over short sights are given less influence in the adjustment.
As with horizontal centering, a vertical centering value of 0.005 feet (0.0015 meters)
might be considered reasonable, but what value you actually use must be based on
equipment, field conditions, your crew and experience.
Note that “fixed” observations are not affected by horizontal or vertical centering
errors. Also by default, standard errors explicitly entered on a line of input data are
not affected by centering unless the “.ADDCENTERING” inline option has been
used in the data. See “Inline Options” in Chapter 5 for details of its use.
PPM (Parts per Million) Settings for Distances and Elevation Differences
� Distance PPM - The actual standard error used for the distance observation is made
up of two parts, the constant and the proportional (distance times ppm). A value
entered here represents the proportional distance measuring error of an EDM in parts
per million. Say for example your EDM is rated as plus or minus 5 millimeters, and
plus or minus 3 ppm, you would enter the value 0.005 meters (assuming your data is
in meters) in the distance standard error field previously described, and the value 3 in
this ppm field. Remember, if your project units are set to feet, you have to convert
any millimeter rating to feet. In the example above you would enter 0.016 feet for
standard error, and the same 3 ppm.
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