STAR*NET V6 - Circe

Chapter 4 Options
The default method used in STAR*NET to compute the actual standard error used
for a distance is to add the constant and the proportional parts. This method, called
the “additive” method, is preferred by most manufacturers.
For a distance of 2,000 feet, a constant of 0.016 feet and a 3 ppm part:
Std Error = 0.016 + (3 ppm x 2,000 feet) / 1,000,000 = 0.022 feet
Another method preferred by some is called the “propagated” method in which the
standard error is computed by taking the square root of the sum of the squares of the
two parts. This method is available by using a special “.EDM” inline option. See the
section “ Inline Options” in Chapter 5 for details on its use.
Note that standard errors for distances explicitly entered on data lines are not
affected by an entered ppm value. Only those distances accepting the default
standard error are affected by the proportional ppm part.
Elev Diff PPM – Since an elevation difference is often derived from measuring a
zenith angle and slope distance, you may want to inflate its actual standard error
based on how large the distance is by applying a PPM. This PPM is based on the
slope distance between the instrument and target, not the elevation difference.
STAR*NET calculates this slope distance using the given or computed approximate
coordinates for the instrument and target stations. Entering a ppm value for elevation
difference observations cause larger total standard errors (less weight) to be applied
when long slope distances are involved than short.
The method used to compute the actual standard error value is to add the constant
and the proportional parts. For example, using a standard error of 0.05 feet and 25
ppm, the total standard error for an elevation difference observation measured along
a slope distance of 1,000 would be:
Std Error = 0.05 + (25 ppm x 1,000 feet) / 1,000,000 = 0.0750 feet
The value you actually use for “ppm” must be some value you feel comfortable with,
and should be based on your experience in the field. A good value to start with might
be the 25 ppm figure used in the example above. To do a rough validity check on this
figure, assume a zenith angle uncertainty of plus or minus 5 seconds. Therefore, the
vertical distance error calculated using a 5 second arc swing over a distance of
1,000,000 feet is about 25 feet, therefore the 25 ppm. Or for example, if you feel
your zenith angle uncertainty is more in the 10 second range, then you might use 50
ppm for your proportional elevation difference standard error.
This method of calculating the total standard error for an elevation difference is an
empirical approach. An alternate but more complex method is to propagate the
uncertainties of both the measured distance and zenith angle. However, based on
normal survey measurements and practices, we believe this method will provide a
proper yet easy to understand way of weighting these derived measurements.
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