STAR*NET V6 - Circe

Appendix C Additional Technical Information
C.3 Datum Schemes for 2D and 3D “Local” Jobs
In the Adjustment Options for the project, when you set the coordinate system to “Local”
(i.e. you are not running a grid job), you must choose a Datum Scheme as discussed on
page 17 in Chapter 4, “Options.” You can choose one of two schemes:
1. Apply an Average Scale Factor: Whatever “Average Scale Factor” you choose is
applied to the horizontal components of all distances and this becomes the basis for
calculating the horizontal coordinates. Elevations of points are not considered in this
scheme. If your project varies greatly in elevation, and you want horizontal
coordinates calculated to a certain elevation (datum), it is up to you to change the
scale factor of your job as needed using the “.SCALE” inline option.
You might use this scheme in various parts for small, fairly flat projects. Entering a
factor of 1.0 causes your coordinates to be calculated right at the elevation of your
observations. Users sometimes choose this scheme to simulate a grid adjustment by
entering a grid factor that is known for the project area.
Even though this datum scheme doesn’t consider elevations, it is still important to
enter a realistic “Avg. Project Elevation” for 2D jobs so that any entered 3D slope
distances will be properly reduced to horizontal using curved-earth geometry!
2. Reduce to a Common Elevation: Using this scheme, elevations of stations in the
project are considered. Using curved-earth geometry (given earth radius with the
divergence of verticals) plus elevations of the instrument and target points,
horizontal distance components are factored up or down to a common elevation. One
often-used reduction to a common datum is the reduction to Sea Level. For this you
would enter 0.0 as your “Reduce to Common Elevation” value. However, you can
enter any elevation, one above, below or right at your project.
You would normally use this scheme for projects containing stations that vary
considerably in elevation. This allows you to choose a consistent elevation datum at
which all horizontal coordinates will be created whether it be sea level, an average
project elevation or some other desired elevation.
Using the “Reduce to a Common Elevation” datum scheme, the factor applied to the
horizontal component of a distance observation is computed by:
Factor = (R + Common Elevation) / (R + (ElevA + ElevB) / 2)
Where: R is the given earth radius value
Common Elevation is the given (desired) elevation datum
ElevA + ElevB are the elevations at the ends of the distance observation
Whichever datum scheme you choose, always enter horizontal “controlling” coordinates
at the datum you selected. For example, if you choose to reduce to a “Sea Level”
elevation during an adjustment, and you enter some fixed controlling coordinates, these
coordinates must be sea level coordinates, not coordinates at your project elevation!
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