Get Absolute Shift/Scales And Apply

Get Absolute Shift/Scales And Apply calculates the best absolute shift (magnetics) or shift+scale (radiometrics) for each grid using the information in the Overlap Grids File. The "absolute" shift/scale is the shift/scale that will best level each grid such that all the grids in the collection are best levelled together. Grids are read from the input directory, adjusted, and written to the output directory. If one or more grids are not connected to the base grid/Control Line "island", then it will not be written to the output directory. In this case a warning message is displayed in the Messages list box. This means that you will have to create a new Priority Order for use with Merge Grids/Feather And Merge Grids, as there must be a one-to-one relationship between the Priority Order and grids in a directory.

The aliases and parameters that must be set for this process (via the Set Aliases and Parameters menu) are shown in the table below.

Control File Keyword	Type	Description
inputGridDirectoryAbs	string	The input grid directory. This is a sub-directory of the Working Directory.
outputGridDirectoryAbs	string	The output grid directory. This will be created in the Working Directory.
overlapGridsFileAbs	string	The Overlap Grids File.
minOverlapPointsAbs	integer	The minimum number of overlap points for a grid pair required to calculate an adjustment.
minDataSpreadAbs	float	The minimum data spread of the data points in the overlap (km). If the spread is less than this, the overlap is not used.
scaleWeight	float	Preferentially weighting of scale (relative to shift).
controlLineWeightAbs	float	Preferential weighting of Control Lines relative to grid-to-grid overlaps.
baseGridWeightsAbs	float	Preferential weighting of Base Grids relative to other grid-to-grid overlaps.
baseGridAbsName	string	Base grid name. Can specify multiple base grids. If control lines are not being used, then at least one base grid must be specified.

Get Absolute Shift/Scales is trying to solve an inverse problem for both scaling factors (small numbers) and shift factors (large numbers) – so it is necessary to give a greater weight to the scales. If you are having trouble getting the radiometric grids level using both a shift and scale adjustment, then try different values for the scaleWeight parameter.

Check the output to see what adjustment has been made to the control lines/base grid. These should be close to 0 for shift, and close to 1 for scale. If significantly different, raise the controlLineWeightAbs/baseGridWeightAbs parameters.

Note that you can manually edit the Overlap Grids File, prior to running this process, in order to increase or decrease the weight for a particular overlap. This can be used to force any inconsistency in the data into areas of the final merge where it is less visible.