The vertical gravity gradient, expressed in µGal/cm, has two functions in combined absolute/relative gravity surveys. First, the gradient is used in the absolute gravity data-collection software ('g', by Micro-g LaCoste) to calculate g, because gravity varies over the height of the free-fall. Especially for the A-10 meter, having a relatively short free-fall, the vertical gradient has only a minor effect on g and a standard value (-3 or -3.086 µGal/cm) is often used. Second, and more importantly, the vertical gradient is used to calculate g at the transfer height at which it is reported. Typically the transfer height is 100 cm, whereas the A-10 measures at about 71 cm, and the FG-5 at about 131 cm.
The latter application of the vertical gradient is important when combining relative and absolute measurements, which are typically made at different measurement heights. Generally the absolute-gravity measurement is transferred from the transfer height to the land surface (or, the measurement height of the relative-gravity meter). GSadjust uses the transfer height and gradients shown for each datum in the Datum Table to transfer the absolute-gravity value to the land surfacer (these are imported from the *.project.txt files output by 'g' if the "Import abs. g - complete" menu option is selected). An offset equal to [transfer height] * [gradient] is added to each datum. If the imported/specified absolute-gravity value (or datum) should be used as-is, the transfer height should be set to zero. If the gradient is the same for every datum, it will have no effect; a constant offset will be added to each datum.
The adjusted gravity value at each station in the Results Table has the transfer offset applied (that is, it is the gravity value at the land surface). When comparing adjusted to measured values at datum stations, the offset is taken into account.
Vertical gradients are typically measured over a discrete interval using a relative-gravity meter and a tripod or similar stand. A mini loop is measured between the ground and the measurement height of the A-10 or FG-5 meter; typically the ground is occupied 3 or 4 times, and the tripod 2 or 3 times (additional occupations may improve accuracy). If this type of survey, with only two stations, is processed with GSadjust, the menu option "Write vertical gradient file..." can be used to calculate a vertical gradient, in microGal/cm, and write it to file. The gradient is calculated over the interval specified in the dialog opened by the "Vertical gradient interval..." menu command.
