Grid Total Field Magnetic Data

Use the Grid Magnetic Data option (UXMGRGRD GX) to create a total field magnetic grid (.GRD) file using the minimum curvature gridding method.

Grid Total Field Magnetic Data dialog options

Mag channel to grid

Magnetic data channel to grid

Script Parameter: UXMGRGRD.CHAN

Output mag grid

Output grid file name (*.grd)

Script Parameter: UXMGRGRD.GRID

Grid cell size

Specify the grid cell size. This should normally be ¼ to ½ the nominal data sample interval. If not specified, the data points are assumed to be evenly distributed and the default cell size will be. ¼* (sqrt (grid area / #data points)). Note that, a smaller cell size enables contouring to better honour the original data, but at a cost in processing time.

Script Parameter: UXMGRGRD.CS

Xmin,Ymin,
Xmax,Ymax

X and Y minimum and maximum coordinates. This defines the area on which to define the grid. You MUST enter all 3 commas if you enter ANY of these four values. The default produces a grid to the limits of the data. Entering a range of ,0,0,0 will cause the grid to cover the maximum limits of the data plus the blanking distance. For example, to produce a grid between 1000 and 10000 in X, and 500 and 5000 in Y, enter: 1000,500,10000,5000.

Script Parameter: UXMGRGRD.XY

Low-pass
desampling factor

Low-pass filtering can be applied to a multiple of the grid cell size. Filtering should be considered if the data has uneven sampling and the cell size is less than about half the largest sample interval.

For example, if the cell size is 50 metres and the largest nominal sampling is 200 metres, specify a desampling factor of 4. The default is 1.

Script Parameter: UXMGRGRD.DSF

Blanking distance

Areas of the grid beyond the blanking distance of valid data points will be dummied out. The blanking distance should be greater than the nominal sample interval. The default is 8 times the desampling factor times the the cell size.

Script Parameter: UXMGRGRD.BKD

Tolerance

The tolerance to which the minimum curvature surface must fit the data points before stopping (also see % pass tolerance). By default a tolerance of 0.1% of the data range is used.

Script Parameter: UXMGRGRD.TOL

% pass tolerance

When this percentage of data points has been fit within the tolerance, iterations cease. The default is 99%.

Script Parameter: UXMGRGRD.PASTOL

Max. iterations

The maximum number of iterations to enable in any iteration step. The default is 100 and should be increased only if the maximum iterations are exceeded and an acceptable solution has not yet been reached.

Script Parameter: UXMGRGRD.ITRMAX

Internal tension
(0-1)

Overshoot of the valid data into voids can be reduced by increasing the internal tension of the fitted surface. The default is no tension (0). The tension can be increased to a maximum of 1.0.

Script Parameter: UXMGRGRD.TENS

Cells to extend beyond data

This is the number of grid cells to extend beyond the outside limits of the data. If the X and Y range values are not defined, the grid size will be set to the actual data range plus this value. At the present time, this operation also extends cells into interior regions without data, much like the blanking distance. For best results, use the blanking distance parameter to control the amount of coverage around data, and this parameter to set the default grid size, with this distance generally being smaller than the blanking distance to provide clipping around the grid.

  • Do not use this parameter in place of the blanking distance in order to fill interior regions of the grid, as results are generally poorer than using the blanking distance. This is because the blanking distance measures the true distance between a given location and the nearest data point, while the "cells to extend" is measured only in X or in Y. Remember that the blanking distance is a true distance, while this parameter is a number of cell sizes, so you must multiply by the actual cell size to compare the two.

    • Script Parameters: UXMGRGRD.EDGCLP

    Display mag grid

    Display in a single grid (1), colour shaded grid (2) or don't display (0)

    Script Parameter: UXMGRGRD.DISPL

    See Also: