Fill Grid Dummies

Use the Grid and Image > Utilities > Fill Grid Dummies menu option (GRIDFILL GX) to interpolate a grid to fill all dummy areas.

Fill Grid Dummies dialog options

Input grid file	File Name of the Input Grid. (.grd file) Script Parameter: GRIDFILL.GRDIN
Output filled grid file	File Name of the Filled Output Grid. (.grd file) Script Parameter: GRIDFILL.GRDOUT
Roll Off method	Select "LINEAR" or "SQUARE" for the roll-off method. (Default is SQUARE). Script Parameter: GRIDFILL.ROLLOPT
Distance at which to roll to 0	Distance in grid cells at which to roll to 0. Default is twice the width of the narrowest dummy border of the grid. Script Parameter: GRIDFILL.DIST
Maximum prediction filter length	Maximum prediction filter length (Default is 0) Script Parameter: GRIDFILL.MXF
Maximum area to define filter	Maximum area used to define filter (Default is 0). Script Parameter: GRIDFILL.MXA
Roll Off Base	Roll-off base value (if not given, the statistical mean of the grid is used) Script Parameter: GRIDFILL.RBASE
Maximum grid amplitude allowed	Maximum amplitude to allow in grid. Default is NO limit. Script Parameter: GRIDFILL.ALIMIT
Maximum edge amplitude allowed	Maximum amplitude to allow on the EDGE of the grid. Default is NO limit. Script Parameter: GRIDFILL.ELIMIT
Width from edge to start limiting	Width from edge to start limiting from. Default = 10 if ELIMIT is set. Script Parameter: GRIDFILL.EWIDTH
Number of Convolution passes	Number of convolution passes to make over the filled grid (Default is 1). Script Parameter: GRIDFILL.PASS

Application Notes

If the GRIDFILL.MXF or GRIDFILL.MXA parameters are set, then a maximum entropy prediction is used. Otherwise inverse distance is used as the predictor.

If GRIDFILL.ALIMIT is <= 0, the maximum amplitude is unlimited If GRIDFILL.ELIMIT is < 0, the maximum edge amplitude is unlimited.

GRIDFILL replaces all dummies in a grid with valid values determined by interpolation from the valid parts of the grid. The interpolation may either use maximum entropy prediction (the predicted area will have the same spectral look as the valid area), or linear interpolation. The filled area may also be forced to zero at a specified distance from the valid area. The interpolation is always done in such a way that the grid is periodic on its edges.

A completely filled and periodic grid is required by the FFT programs in MAGMAP. Filled grids are also useful to prevent edge effects when applying convolution filters. Amplitude limiting and edge limiting are sometimes useful to prevent ringing problems in FFT processes.

Maximum entropy prediction can be quite slow on large grids, particularly if the prediction filter is increased significantly from the default. This should only be done when necessary. If you are applying high-frequency filters with MAGMAP, and the grid edges produce narrow anomalies in the processed map, then maximum entropy prediction should be used.

Amplitude limiting, edge limiting and rolling to zero should only be applied to trend removed grids.

If the predicted area appears too noisy, it may be smoothed with a Hanning filter using GRIDFILT. Then the GRIDBOOL program can be used to place the original valid area into the smoothed filled grid.