Configure Earth Constants

Enables you to set the constants required for the calculation of:

• Curvature and Refraction Correction
• Sea Level and Geoidal Separation Correction
• Scale Factor Correction

The Configure/Earth Constants command is selected from the Utilities task.

Earth Constants dialog box

Item	Used to
Refraction	Method 1 Select method 1 for calculation of curvature and refraction correction. A typical value of K when using this method is 0.07. If this method is selected, the formula used to calculate the height correction for earth curvature and refraction is as follows: Height Correction = (distance² x (1 - (2 x K))) / (2 x Radius of Earth) Method 2 Select method 2 for calculation of curvature and refraction correction. A typical value of K when using this method is 0.14. If this method is selected, the formula used to calculate the height correction for earth curvature and refraction is as follows: Height Correction = (distance² x (1 - K)) / (2 x Radius of Earth)
Constant (K)	Key in the K factor to be used in the application of curvature and refraction correction. A typical value would be 0.07 for method 1, or 0.14 for method 2. Ensure that you set an appropriate value according to the method selected above.
Earth's Radius	Key in a value for the radius of the earth to be used in the application of curvature and refraction correction and mean sea level correction. This field is irrelevant for a projection as radius is then automatically calculated by the system.
Use Combined scale Factor only	Select whether only a combined scale factor is to be used. If selected, then: -Average Elevation, Geoidal Separation and Point Scale Factor are irrelevant and not used. -Any Sea Level and Scale Factor corrections to be applied are replaced with Combined Scale Factor correction. This field is irrelevant for a projection as scale factor is then automatically calculated by the system.
Combined Scale Factor	Enter a combined scale factor to be used instead of sea level and scale factor corrections. This field is only available if "Use Combined scale Factor only" is selected. This field is irrelevant for a projection as scale factor is then automatically calculated by the system.

Transfer/Resolve/Reduce Field File command, or when extracting data to a control file or traverse file. Refer to Resolve/Least Squares Adjustment/File/Extract and Resolve/Traverse Adjustment/File/Extract.

Mean Sea Level correction may be applied when reducing a field file using the Field Transfer/Resolve/Reduce Field File command or when adjusting control files or traverse files. Refer to the Corrections command which governs whether this correction is applied in adjustments.

If mean sea level is applied, then an additional correction to the distance for geoidal separation is also applied at the time of applying mean sea level correction. This is to correct the distance from a distance on the geoid to a distance on the spheroid.

Average Elevation	Enter an average elevation for the points. This average elevation is used to calculate the sea level correction if points do not have elevations.
Geoidal Separation	Enter the Geoidal Separation. This value is used when applying sea level correction to the distances to allow for separation between the geoid and ellipsoid. The default is zero. The Geoidal Separation may also be entered at the time of creating a new data base. The value entered is stored with the data base and reflected in this dialog box until changed by the user. Enter the geoidal separation as negative if the geoid is below the spheroid.
Point Scale factor	Enter the point scale factor to be used in plane projections as the scale factor correction. This field is irrelevant for a projection as scale factor is then automatically calculated by the system.
System	Save any changes to the system settings to be used as the default settings when a new database is created.
OK	Save any changes to the settings and close the dialog box.
Cancel	Close the dialog box without saving any changes.

Curvature and refraction correction may be applied when reducing a field file using the Field

Point Scale Factor is only relevant for a plane projection.

Summary of Corrections for a Plane

{Ground Dist - Sea Level Corn (inc geoidal corn)} x Point Scale Factor = Plane distance

or

Ground Dist x Combined Scale Factor = Plane distance

Summary of Corrections for a Projection

Ground Dist - Sea Level Corn (inc geoidal corn) = Ellipsoidal Distance

Ellipsoidal Distance X Line Scale Factor = Plane Distance

Note: line scale factor is computed by the system.