Control Lines And Base Grids

In order to bring the grids to a common level, a "baseline" must be specified. Typically, the baseline could comprise: (a) a set of control traverses that intersect one or more of the grids; or (b) a grid designated as a "base grid".  GammaGrid will accept any number of base grids, or indeed a combination of control lines and base grids. But this would be unusual, as the user would have to be certain that the control lines and base grids are all registered to the same datum – i.e. they are all at the same level. 

The control lines and base grids are not adjusted. If control lines are used to generate the overlaps in the Overlap Points File, then there is no need to specify a base grid (or grids) for the Get Absolute ShiftScale And Apply or Get Grid Tilts And Apply processes. However, if control lines have not been used to generate overlaps, then the specification of at least one base grid is mandatory – otherwise there is no solution to the inverse problem.

 Choose the base grid carefully – it represents the baseline to which all the other grids are levelled. 

Using Conditioned Base Grids

The final merged grids are at the same level (datum) as the control lines or base grid – whichever was used. If the base grid was conditioned prior to levelling, then the final merge will be level to this conditioned grid's level. In this case, there are two options for getting the merged grid to the correct level:

    1. Don't condition the base grid. Or after conditioning all grids, overwrite (using Windows File Manager to copy/paste) the base grid with the corresponding unconditioned grid from the input directory; or
    2. Run statistics on all the grids in the input directory prior to conditioning (Utilities|Grid Statistics). Then, after merging, scale the merged grid by the base grid's standard deviation (radiometrics only) and add the base grid's mean (Utilities|Shift/Scale A Grid). For magnetic grids, use a scaling value of 1, as the conditioning does not scale the data. This will bring the merged grid to the same level as the (un-conditioned) base grid.