Datum observations are shown in the Datum table on the network adjustment tab:
Datum observations are the "known" values from which delta-g's are observed. At least one datum observation must be present for each survey to perform an adjustment. A datum observation can be an arbitrary value, such as 0 or 50,000, or it can be a measured absolute-gravity value. There are three options for adding datum observations:
When datum observations are added, they are associated with the currently selected survey. Datum observations can be excluded from the network adjustment by unchecking their respective checkbox.
Datum observations, as shown in the Datum table, include the following fields:
Station name
Gravity (µGal)
Standard deviation (µGal)
Date
Measurement height (centimeters)
Gradient (µGal/m)
Residual (µGal)
The first three (name, gravity, standard deviation) are required. The station name must match exactly at least one station with a delta-g observation (names are case sensitive). The date is not used (datums are associated with the selected survey when they are added). The measurement height and gradient are used to transfer an absolute-gravity measurement from the measaured height (about 71 cm or 131 cm for the A-10 and FG-5 absolute-gravity meters, respectively) to the land surface (i.e., the height of the relative-gravity meter). The gravity value for the respective datum measurement is increased by the amount [measurement height] * abs([gradient]) (the gradient shown in the Datum table is negative to indicate gravity decreases with elevation). Finally, the residual column in the Datum table shows the difference between the specified absolute-gravity value and the network adjusted value. If only one Datum observation is specified, the residual will be zero.
The date, g, standard deviation, measurement height, and gradient fields can be changed by double-clicking on the respective cell.
