Corrections

(This functionality is available with the Adjustment module)

Sets the corrections to be applied when reducing observations.

The corrections applied when reducing observations are governed by the settings in this dialog box. It is therefore imperative that these settings are correct prior to reducing a field file, adjusting a traverse or performing a least squares adjustment.

Corrections dialog box

Item	Used to
Projection	Display the projection of the currently open database
Direction	Set the type of directions to be corrected. For a plane projection, the direction type is automatically set to plane and may not be changed. As such, no corrections are applied to directions for a plane projection. For any other projection, the direction type may be one of the following:
Azimuth	Azimuths are oriented directions (bearings) based on true north. They are typically bearings based on astronomical observations. They have not been corrected for grid convergence or arc to chord correction. If you select this option: - any bearings will have grid convergence and arc to chord corrections applied; - any angles and directions will have arc to chord correction applied.
Grid	Grid bearings are oriented directions based on grid north. They have been corrected for grid convergence but not for arc to chord correction. If you select this option: - any bearings will have arc to chord correction applied; - any angles and directions will have arc to chord correction applied.
Plane	Plane bearings are oriented directions based on grid north but which have had arc to chord correction applied. If you select this option, no geodetic corrections will be applied to bearings, directions or angles.
Distance	Set the type of distances to be corrected. For a plane projection, the distance type may be ground or grid but not ellipsoid. For any other projection, all distance type options are available.
Ground	Ground distances are the distances actually measured on the ground, reduced only for slope. They have not been corrected for sea level, geoidal separation or scale factor. If you select this option, the distances will be corrected for sea level, geoidal separation and scale factor when adjusted.
Ellipsoidal	Ellipsoidal distances are distances on the ellipsoid. They have been corrected for sea level and geoidal separation, but not for scale factor. If you select this option, the distances will be corrected for scale factor when adjusted.
Grid	Grid distances are distances on the projection. They have been corrected for sea level, geoidal separation and scale factor. If you select this option, no corrections will be applied to the distances.
Corrections to be Applied	Display the corrections that will be applied. This is entirely dependent on the type of directions and distances selected.
Earth Constants	Display the Earth Constants dialog box and enter the required values. It is imperative that these constants are correct, as they govern the relationship between ground distances and grid distances.
OK	Save any changes to the settings and close the dialog box. Any changes are also reflected in the Utilities/Configure/Earth Constants settings.
Cancel	Close the dialog box without saving any changes.

Typical settings for a Plane projection.

1. Select Distance/Ground if you wish to:
   
   • -apply sea level, geoidal separation and scale factor, or
   • -apply combined scale factor.
   Alternatively, select Distance/Grid if:
   • -the distances are already considered to be on the required grid and no corrections are needed for height or scale factor.
2. If you selected Distance/Ground and wish to use a combined scale factor, select "Use Combined scale factor only" and then enter the required value in the Combined Scale Factor field. (Earth constants dialog)
   If you selected Distance/Ground and do not wish to use a combined scale factor, deselect "Use Combined scale factor only", and then enter the required values for Average Elevation, Geoidal Separation and Point Scale factor. (Earth Constants dialog)

Typical settings for a projection.

1. Select Direction/Azimuth if you wish to apply grid convergence and arc to chord correction.
   Alternatively, select Direction/Grid if you wish to apply only arc to chord correction.
   Alternatively, select Direction/Plane if you do not wish to apply grid convergence nor arc to chord correction.
2. Select Distance/Ground if you wish to apply sea level, geoidal separation and scale factor corrections.
   Alternatively, select Distance/Ellipsoid if you wish to apply only scale factor correction.
   Alternatively, select Distance/Grid if you do not wish to apply any distance corrections.
3. If you selected Distance/Ground, enter the required values for Average Elevation and Geoidal Separation. (Earth Constants dialog).

Notes:

When computing a Horizontal Least Squares Adjustment, all data is taken from the spread sheet at the time of computation. If points have no elevation, the average height set for the project is used. A warning indicating some points are without elevations appear when the adjustment is performed.

To ensure that correct Seal Level Corrections are computed, always:

1. Set the average elevation to a realistic value.
2. Set the geoidal separation correctly.
3. Perform the level adjustment FIRST. This will then give any floating points an elevation which can then be used by the horizontal adjustment in computing its Sea Level Correction. This gives the best solution. However if not possible, if the average elevation is set to a realistic value, any errors in the SLC calculation are generally insignificant.