The limits of the area to be gridded.

The position coordinates of the bottom left corner of a rectangular area of the user's standard reference system which defines the area to be gridded. By default, the minimum limits of the input XYZ data are used.

The position coordinates of the right corner of a rectangular area of the user's standard reference system which defines the maximum limit of the area to be gridded. By default, the maximum limits of the input XYZ data are used.

Entering a range of 0,0,0,0 will cause the grid to cover the maximum limits of the data plus the blanking distance.

Controls use of all data or boundary clipped data from a *.gdb file. The options are:

• 0 - use all the data (default)
• 1 - only use data within boundary clip region

Data selection and output grid values

Selects the channel to be gridded. By default the first Z channel (the first two columns are assumed to be the X and Y position coordinates) is gridded.

The output grid node values by default and for minimizing space usage are stored as 2-byte integers. The 2-byte dynamic range spans from -32766 to 32767 (-32767 is reserved as the grid dummy value). In order to take maximum advantage of this range, a base level is removed from the actual real values and a multiplier is applied to expand or compress the data optimally in this range.

Stored_grid_value = (real_Z- zb) * zm

You actually can control what the base and multiplier will be.

The base level removed from the gridded data before conversion to the output grid value in integers. By default, an appropriate value is determined from the data.

A multiplication factor that is applied to the gridded data after removal of the base level (zb) in order to convert the data to output grid values in integer format. By default an appropriate value is determined.

To set either zb or zm, values must be entered for both. If an improper base or multiplier is chosen, RANGRID will issue an error message stating that grid values are out of range. The out of range values will be replaced by dummy values in the grid.

The base level and multiplier that are used to convert the final grid values to integer grid values. The default values will be acceptable unless sharp peaks are present in the data, in which case the multiplier should be decreased by a factor of 2.

This warning can happen even when RANGRID has chosen the values by default, in which case the interpolated minimum curvature surface extends significantly beyond the data range. This is usually not a problem since only a small number of points are blanked out in the grid. If not acceptable, reduce the multiplier.

The multiplier determines the precision of the data in the grid. If the data is to be contoured, the multiplier should be at least 10 times the lowest contour interval. The minimum contour interval must always be a multiple of the grid multiplier.

The logarithm (base 10) of the data can be gridded rather than the original data. Once gridded, RANGRID can store either the logarithmic data in the output grid, or rescale the data to the original units before storing in the final grid. Gridding the log of the data can be a very effective way to reduce distortion due to highly skewed data such as geochemical data.

Logarithmic Gridding option (log base 10):

grid the data as is (the default)

grid log(Z), where any Z-value less than logmin is replaced by logmin.

grid log(Z) outside the range ±1, and use the original Z in the range ±1.

With logopt set to +1 or +2, the output grid will be logarithmic. This means that, if an original Z value was 100, the value in the grid will be 2. Grids produced with logopt = 1 can be contoured and labelled correctly by setting the logopt value in CONTOUR to 1.

Options -1 and -2 will cause RANGRID to place 10Z-value (ten to the power of the gridded Z values) in the output grid. This can be an effective way to reduce the effect of extreme values in highly skewed data, i.e. it converts the data back to a more linear form.

The minimum allowed Z value for logopt=1, or the amplitude around zero for linear gridding with logopt=2. This must be a positive non-zero number. The default is 1.0.

Any data less than LOGMIN is set to LOGMIN, then all data are divided by LOGMIN before the logarithm is calculated.

Initial sampling and coarse grid parameters are selected using this control file line.

The de-sampling factor as a multiple of the grid cell. This factor effectively acts as a low-pass filter by averaging all points into the nearest cell defined by this factor.

For example, a factor of 3 would first average database points into a coarse grid whose cells size is 3 times the final cell size. The default is 1, producing no pre-filtering other than de-aliasing by averaging points within a cell. If the resulting grid is too noisy around data points, increase the desampling factor.

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.  

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. 

The default is four times the coarse grid size defined by icgr.

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.

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.

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.

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. 

When entering a number for the tolerance, it should be the absolute value, not the percentage value. For example, find the range of your data and take a percentage of that number, then enter this as your tolerance.

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

The quality of the final result is easier to control using the tol and itrmax parameters.

Maximum number iterations allowed to solve the minimum curvature function. itrmax should be increased only if the maximum number of iterations is exceeded and a more accurate grid is desired.

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.

The coarse grid size relative to the final grid size. This factor can only be 16, 8, 4, 2, or 1.

The default is 8.

The optimum factor is close to half the nominal data spacing, although in most situations the default is fine. The icgr parameter only effects the length of time RANGRID takes to find a solution.

Using a factor that is too low will result in significant increases in processing time while erring on the high side will only increase the processing time moderately.