The BIGRID Control File

The BIGRID control file (*.CON) is used to set the gridding parameters that control the gridding process. It is an eleven-line free-format text file that can be created and modified with any text file editor. A blank-documented control file, "BIGRID.con", is provided for you to copy and modify in the \Geosoft\Desktop Applications \etc directory.

To see how to set this file up, we recommend that you use a text editor to display the "BIGRID.con" file.

The file is free-format, which means that the positioning of data within a line is not important. If more than one value is to be placed on any line, the values must be separated by a blank or comma. Each line, with the exception of the title line, must end with a slash character (/), after which any user comments may be entered on the same line. Any parameter not specified will assume a default value.

Following is a summary of the BIGRID control file parameters:

BIGRID Control File: Main Parameters

These parameters are written into lines 1,2,3 of the BIGRID control file.

Grid Title

The grid title is the first line of a Brigrid Control file and is placed in the grid file header. The grid title can be up to 48 characters. In common practice, the title is not used and may be left blank.

Grid Cell Size

Specify the grid cell size. The cell size is the distance between grid points in the X and Y directions. In most situations, the cell size can be selected as 1/4 to 1/8 of the line separation or the minimum station interval, whichever is greater. By default, BIGRID will choose a cell size based upon these criteria, although we do recommend that you choose the cell size directly.

A grid cell size of 2.5 mm at map scale is an ideal resolution for a good contour map. If the default cell size is too large, or you are unable to grid to this resolution because the original data is too sparse, the grid should be re-gridded before contouring.

Cells Beyond Edges

The number of cells to extend beyond the edges of the data. By default, the number of cells is set to 1 when gridding data, which ensures that the grid will always extend at least a fraction of a cell past the ends of valid data. When re-gridding grids, the number of cells is set to 0 by default to prevent growth of the grid. Setting the number of cells beyond the edges of the data to 2 or 3 and the maximum separation permitted between lines to the grid cell size will produce a strip grid. A strip grid will only have valid data in strips that frame the survey lines. This is an effective presentation when lines are too far apart to be gridded properly.

Sorting Option

Check off one of the following options.

No-Presort

Do not pre-sort the data (default setting).

Presort

Pre-sort the data. Presorting of the data will sort each line so that all data points are consecutive in the gridding direction. Caution should be exercised when pre-sorting because data entry errors may result in the data being sorted out of order.

Remove Backtrack

Remove back-track data. This option will cause data in the line that appears to backtrack to be removed. Use this option when processing airborne geophysical data.

Grid Area Limits

The limits of the area to be gridded.

Xmin, Ymin

The ground coordinates of the first grid point at the bottom left corner of the rectangular grid area. (Default, the minimum limits of the input data file are used).

Xmax, Ymax

The ground coordinates of the last grid point at the top right corner of the rectangular grid area. (Default, the maximum limits of the input data file are used).

BIGRID Control File: Trend Parameters

These parameters control overall trend enhancement and the storage sense of the gridded data. They are written into lines 4 and 5 of the control file.

Trend Angle and Output Direction

Trend angle is the angle of the predominant geological trend in degrees counter-clockwise from the +X axis of your reference system. If you want to emphasise certain geological features, select a trend angle that is equal to the trend of the feature. However, never set the trend angle so that it is parallel to your line direction. Ideally, the trend angle should be perpendicular to the line direction (but geologic features don’t always fall into this ideal case).

The resulting grid X axis will be in this direction, and the grid header will report this as the grid rotation. By default, a value of 0.0 is assumed for tra, which produces an output grid aligned with the user's standard reference system.

Output Grid Lines

The output grid lines parameter (represented by kx) determines whether the output grid lines will be parallel to the output grid X-axis (kx = 1) or the Y-axis (kx = -1). The variable kx should be chosen such that:

the first interpolation is close to the survey line direction;
the second interpolation, which defines the direction of the output grid lines, is across the survey lines.

If the survey lines run in the Y direction, kx should be 1; if the survey lines run in the X direction, kx should be –1.

By default, BIGRID will choose kx so that the second interpolation direction is closest to perpendicular to the average line direction (i.e. the average of all lines) unless a trend angle is specified, in which case kx will be 1.

Valid Data Areas

These parameters control the definition of valid and in-valid parts of the area being gridded.

Line to Line Minimum

The minimum separation permitted between data points and lines. Data points and parts of adjacent lines that are closer than this value are averaged. By default, one half the cell size is used.

Line to Line Maximum

Maximum Line Separation is the maximum separation distance permitted between lines. Areas enclosed by lines that are farther apart than this distance are represented by dummy values in the output grid. By default, the maximum separation is set to 1.5 times the average line separation. If the line to line separation maximum is too narrow, the output grid will consist of data strips that frame each survey lines, with blank grid areas in between. The width of the data strips will depend on the number of cells that extend beyond the edges of the data in line 2 of the control file.

Station to Station Maximum

The maximum separation to allow between stations on a line. Gaps in lines wider than the station to station maximum are not interpolated. The default is to use the maximum line to line separation value.

Bigrid Control File: Translation Parameters

These parameters can be used to apply plane translation to the input coordinates before gridding. It is recommended to use the XYZ Utilities for this task, since the data would then be properly registered for use by other programs in the Mapping System. These parameters are written in line 6 of the control file.

By default, no translation is applied. To set any one of these parameters, all of the parameters must be specified.

Multiplier for Scaling

A scale factor to convert the input X,Y survey coordinate system units to the units to ground units system. The units are survey X,Y units per standard system unit.

Ground Coordinates of Survey Origin

X-axis

The location of the origin of the input survey coordinate system in the desired ground coordinate system. If a standard coordinate system separate from the field survey coordinate system is not being used, the default values of 0,0 are applicable.

Y-axis

Rotation Parameters

The angle measured from the X axis of the desired ground coordinate system to the X axis of the local field coordinate system. The default angle is 0 degrees, which makes the local field coordinate system square to the reference system. Positive angles are measured counter-clockwise. The units are in degrees.

BIGRID Control File: Data Parameters

Data selection and logarithmic gridding options. These parameters are written in line 7 of the control file.

Selection Tool

This tool allows you to select which lines are to be included in your gridding process.

Minimum Z Value

The minimum allowable Z value when using the logarithmic option. It must be a positive non-zero number. The default is 1.0. Any data less than zmin is set to zmin.

Channel Gridding Option

No logarithm

Grid the data as is (default).

Log (Base10) with Clip to ZMIN

Grid the log (base 10) of the data. Data less than zmin is clipped to zmin. Data with a very high dynamic range, such as geochemical data, will produce a better result if the log of the data is gridded. The output grid will contain the log of the original data. When contouring this data, you can use the logarithmic contouring option in CONTOUR to have the contour labels labelled using pre-log units.

Log (Base10) to ZMIN Envelope

Grid the log (base 10) of the data above zmin and below -zmin, and use the original data within the range ±zmin.

As above then convert to original

These options are the same as the options above them respectively, except that the output grid values are converted back to original units before being placed in the output grid. Note that since the output grid is limited to integers in the range from -32766 to +32767, there may be some loss of resolution in the low-amplitude areas of the data with the use of this option.

BIGRID Control File: Filter Parameters

Linear Filter Options

These parameters select the linear filtering parameters. Linear filtering is accomplished by designing a convolution filter that is applied to the data before the gridding process. These parameters are written into line 10 of the control file.

Filter Wavelength Cutoffs

The short and long-wavelength cut-offs. These are specified in the same distance units as the XYZ file. The default is to apply no filters. To apply a low-pass filter, specify only the short wavelength cut-off. To apply a high-pass filter, specify only the long wavelength cut-off.

Resample Interval and Tolerance

If the distance between each data point on a line differs by more than the tolerance ft (expressed in %), the line is first re-sampled at an interval using the down-line spline. The data is then filtered and splined again to the grid cell size. By default, resample interval is set to the smaller of the grid cell size or the smallest data interval on the first survey line, and ft is 2%.

Number of Coefficients

The number of coefficients to use in the linear filter. The default is enough to cover one wavelength of the longest wavelength being filtered and should be adequate for most situations. The maximum is 512 points.

Non-Linear Filter

These parameters control the application of a non-linear filter. These parameters are written into line 10 of the control file.

Non-linear filtering uses logic to determine if a given data point is part of the short wavelength information to be removed or not. The decision is based on wavelength only. If the data is considered part of the short wavelength information, and if it exceeds a tolerance, it is replaced by a smoothly interpolated value based on the neighbouring values. Because of this, the non-linear filter is excellent for removing data spikes. The cut-off wavelength used for the non-linear filter is the same as the wavelength specified for the linear filter (see above section).

Amplitude Tolerance

The amplitude tolerance that signal must exceed to be considered noise. If the non-linear filter tolerance is specified, the system uses theactual data units, i.e. 5 gammas. If no value is specified, the default uses a values equal to apercentageof the range of data in the grid or the xyz file.

The amplitude tolerance parameter in the linear filter screen operates only with the non-linear filter.

BIGRID Control File: Spline Parameters

Spline Type

These parameters specify the splines to be used in the down-line and across line directions. These parameters are written into line 11 of the control file. The options are:

Linear
Minimum curvature (cubic) spline (default)
Akima spline.
Nearest

For data that is sampled at a high density relative to the grid cell size, such as is often the case for airborne data, linear interpolation is usually sufficient for the down-line spline and will produce some saving in computation time.

The default for the both the down-line and across-line interpolation option is a minimum curvature spline.

In cases where the data contains rapid changes in gradient, the minimum curvature (cubic) spline may produce undesirable highs or lows in the final grid. This most commonly occurs across lines with poor line-to-line correlation of the data. The Akima spline method does not suffer as much from this problem, although it does tend to produce sharper corners around actual data points and the resulting grid tends to be less smooth.

Stereo Correction

These parameters control a stereo correction factor, which can be applied to the location of each data point before gridding. This allows BIGRID to make stereo image pairs. These parameters are written into line 12 of the control file.

Correction Factor

The correction factor in metres per Z unit. The correction factor is multiplied by the difference between the Z value of a point and the base defined by the base focus value. The location of the point is moved left this distance. To move points right, enter a negative correction factor.

By default, no stereo correction is applied. If a correction factor of 0 is specified, BIGRID will select an appropriate factor based on the range of data. This will result in the greatest amplitude anomaly being moved 3 grid cells to the left.

Base Focus Value

The Z level which represents the base focus level. Data points above this level move left and data points below this level move right. The default is the minimum point in the data set. The default should be acceptable in most cases.

To use the stereo correction feature, create two grids, one with a positive control factor, and one with a negative control factor. Apply shading to the results and plot the two grids separately using grey scale colour tables. You can then view the results using stereo viewing glasses.

You can also view stereo grids by contouring and plotting one grid using cyan lines, and the second grid using magenta lines, both on the same plot, the resulting map can be viewed with 3-D glasses to obtain a 3-D image of the data. However, the plot colours must match the filter colours in the glasses very closely for this effect to work.

BIGRID Control File: Projection Parameters

These filename parameters control the input and ouput projection definition files. By default, the input and ouput grids are assumed to be in the same projection as the input data. If they are different, you must define the projections in two .PRJ files. This provides you with a method to re-project a grid.

When creating projection files, please note:

False northing standard for UTM should be 10,000,000 in the southern hemisphere.

Latitudes should be negative in the southern hemisphere, longitudes should be negative in the west (e.g. North America).

BIGRID Control File Examples

The Most Common Case

The most common application of BIGRID is to produce a grid of data in the local coordinate system of the survey with no unusual filtering or trend enhancement required. For this case, the BIGRID control file is as simple as:

A simple grid:

25 /CS,NEX,ISRT – grid cell size

The first line is a title and the second line is the grid cell size in the user's standard reference system.

The following lines could also be part of the control file, but in this case they can be omitted since all their parameters assume default values.

Regridding with Trend Enhancement

Assume that after viewing the resulting grid from the first example, it became apparent that the over-all geologic strike direction is oriented at 75 degrees to the +X axis of the coordinate system. The XYZ data should be re-gridded to take this into account using the following control file:

Note that the only addition is the trend angle. The resulting grid will be oriented at an angle of 75 degrees to the +X axis and the trends in this direction will be enhanced.

This grid may be sufficient for final contouring, but often it is desirable to reorient the grid square to the original coordinate system for map registration purposes. To do this run BIGRID again using the latest grid file in place of the XYZ file with the control file changed to request an orientation of 0.0 (or leave blank since the default is 0):

The trend was established in the grid as a result of the first gridding operation. Consequently, the process of re-gridding a grid will not significantly change the interpolated grid points. The final grid will have the 75 degree trend in-tact.

An example using match strings

The following command line will run BIGRID to create a 12.5 metre grid of the third Z column, with a 15 degree trend and a 100 metre low-pass filter:

BIGRID qbigrid mag mag15 .cs.=12.5 .tr.=15 .iz.=3 .ws.=100

The following command would re-orient the grid to be square to the coordinate system:

BIGRID qbigrid mag15.grd mag0

In this case, no parameters need be defined since the default trend angle is 0.