Minimum Curvature Advanced Options

Use the Minimum Curvature Advanced Options dialog to define advanced settings for minimum curvature gridding.

Minimum Curvature Advanced Options

Xmin, Ymin,

Xmax, Ymax

The minimum and maximum X and Y coordinates of the grid.

Leave this field blank to create a grid that covers the entire dataset.
To grid a subset of the data, specify the bounds of the desired rectangular area.
To default any value while preserving field order, use commas as placeholders.
Example: ,#,#,# sets only Ymin, Xmax, and Ymax, with Xmin defaulting to its maximum extent plus the blanking distance.

If a blanking distance is not specified, the grid will extend to the data's maximum limits plus eight times the grid cell size.

This field is unavailable when the dialog is launched from Grid Sections.

Script Parameter: RANGRID.XY

Log option

Choose whether to grid the original data or its logarithmic (base 10) form. Gridding in log space can help reduce distortion in highly skewed data, such as geochemical measurements.

Available options:

linear: Grid the data as-is.
log, save as linear: Grid the base-10 log of the data (in logarithmic space). Any values below Log minimum are clipped at that threshold. The resulting grid is converted back to linear space before being saved to a grid file.
log-linear, save as linear:

Values above the positive Log minimum and below the negative Log minimum are gridded in logarithmic space.
Negative values are temporarily sign-flipped to compute logs, then flipped back.
Values outside the ± Log minimum range are converted back to linear space post-gridding and scaled to ensure boundary continuity.
Values within the ± Log minimum range remain intact and are gridded linearly.
Finally, the linear representation of the data (in both ranges) is saved to the grid file.

log, save as log: Same as log, save as linear, but grid values are kept in log space - the gridded data is saved to the grid file in its logarithmic representation.
log-linear, save as log: Same as log-linear, save as linear, but the grid values are not converted back from log space. The output grid remains in logarithmic space with large amplitude anomalies being somewhat moderated.

Script Parameter: RANGRID.LOGOPT
linear: 0
log, save as linear: -1
log-linear, save as linear: -2
log, save as log: 1
log-linear, save as log: 2

Log minimum

When gridding in logarithmic space (see Log option above), this parameter sets the minimum threshold value. The default is 1.

Script Parameter: RANGRID.LOGMIN

Blanking distance

Grid cells located farther than the specified blanking distance from any valid data point will be assigned dummy values in the output grid. The default is calculated as:

Ideally, set this parameter to slightly greater than the maximum distance over which interpolation is required.

Script Parameter: RANGRID.BKD

Tolerance

Specify the allowable absolute error per grid cell (in grid data units). The default is 0.1% of the observed data's maximum range.

Lowering the tolerance increases grid accuracy.

Script Parameter: RANGRID.TOL

% pass tolerance

The minimum percentage of points that must meet the specified tolerance. The default is 99%.

For a higher-quality grid, consider increasing this threshold.

Script Parameter: RANGRID.PASTOL

Maximum iterations

At the finest coarseness level, iterations will stop once this limit is reached. (Default: 100 iterations). At each coarser level, the maximum allowed iterations decrease by 2.

Increase this value if more iterations are required to produce a more accurate grid.

Script Parameter: RANGRID.ITRMAX

Starting coarse grid

Select a value of 16, 8, 4, 2, or 1 to set the desired coarseness of the starting iteration level relative to the final grid cell size.

By starting with a coarser grid, the algorithm is able to produce a reasonable result in under-sampled areas.
As the coarseness decreases, the algorithm becomes more sensitive to local features.
Only reduce the starting coarseness from 16 if faster processing time is required.

Script Parameter: RANGRID.ICGR

Starting search radius

Set the maximum search radius used to establish starting grid values for the coarse grid.

Default: four times the coarse grid size set by the Starting coarse grid parameter.
If no data is found within this radius, the mean of all data is used as the starting value.
If the search radius is too small, the starting grid can be a poor approximation of the desired grid, resulting in excessive processing time.
A radius that is too large may unnecessarily delay coarse grid generation.

Script Parameter: RANGRID.SRD

Internal tension (0-1)

The degree of internal tension (between 0 and 1).

Default: 0, resulting in a true minimum curvature grid with no tension.
Increasing tension can be used to prevent overshooting of valid data in sparse areas; however, curvature in the vicinity of real data will increase.

In general, the sparser the areas in the data—with localized highs and lows— the higher the tension should be set.

Script Parameter: RANGRID.TENS

Cells to extend beyond data

This is the number of grid cells to extend beyond the outside limits of the data. The default is the integer value of blanking distance/grid cell size.

If the X and Y range values aren’t explicitly defined, the grid size defaults to the actual data range plus this value (the specified number of cells).

This parameter is typically used in conjunction with the blanking distance parameter. For best results, use the blanking distance to control coverage around the data, and use 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 to fill interior regions of the grid. It yields inferior results because blanking distance measures the true radial distance between a given location and the nearest data point, whereas cells to extend beyond data is measured only in the X or Y. Since the blanking distance is a true distance and this parameter is a count of grid cells, you must multiply the number of cells by the actual cell size to compare the two properly.

Script Parameter: RANGRID.EDGCLP

Weighting power

Weighting power is used to generate the coarse starting grid. Within each coarse cell, values are weighted by the inverse of their distance from the coarse grid nodes, raised to this power.

The two weighting settings, weighting power and weighting slope, can be used to reduce high-frequency aliasing that may occur when gridding at overly coarse intervals.

Script Parameter: RANGRID.IWT

Weighting slope

Weighting can be further refined using the slope parameter. The overall weighting is determined by the following expression:

Where:

distance is in grid distance units.
power is applied to the distance.
slope is in cell units; the default is 0.0, meaning only the nearest data points influence the weight at each grid node.

To grid anisotropic data, set the weighting slope greater than 1. For instance, in aeromagnetic survey data with 200-metre line spacing and readings taken every 5 metres along the lines, a 50-metre cell-size block will include a large number of real data points to be combined into a single weighted location and z value. In such cases, setting the weighting slope to 5.0 will yield a smoother, non-aliased outcome relative to using a slope of 0.0.

Script Parameter: RANGRID.WTSLP

Application Notes

This GX will create a control file named _rangrid.con in the working directory. The file is then used by the gridding process. Alternatively, you can build a RANGRID control file manually using a text editor to take advantage of additional specialized settings not available through the Minimum Curvature Advanced Options dialog. For this, refer to the Gridding > From Control File option in the Grid and Image menu.