Estimate grid values by inverse distance
This function allows you to fill the Grid Model by inverse distance interpolations. This function estimates values based on the weighted values of data points closest to the grid point. The weighting is the inverse distance of the data point from the grid point raised to a specific power.
To run this function: Choose Surfaces > Contouring > Estimate grid values > By inverse distance, or...
A 2 dimensional ellipse may be used in the estimation process.
The source of the sample data must be from a string file.
You may interpolate either z values or a numeric value from one of the string file D fields (eg gold grade, silver grade).
You may define spatial constraints (eg inside a closed string, above a certain y value etc), to either:
(a) Select a subregion of the sample data to use to fill the grid in this particular pass; or
(b) Select a subregion of the grid to be filled by the sample data in this particular pass.
You will find that by using the spatial constraints and the facility to fill the Grid Model in multiple passes that you can have great flexibility in modelling complex surfaces, in particular surfaces containing faults.
Location
Enter the location of the string file for the source data.
ID Range
Enter the ID range of the string file for the source data.
String Range
Enter the string range of the string file for the source data.
Z or D field?
Enter Z to interpolate the z coordinate or D to interpolate one of the string file numeric attributes.
Enter D in the Z or D field? field to display:
D field
Enter the ID of the particular D field containing the sample data.
Ignore data outside grid?
Enter Y if you wish to ignore data that lies outside the grid when performing the interpolations. Enter N if you wish to use both the data which lies inside the grid AND the data which lies outside the grid. Here outside the grid means outside the grid mesh boundary.
Constrain data?
Enter N if you wish to use all the source data. Enter Y if you wish to define spatial constraints, eg inside string or above a certain x value, to define a subregion of the source data to fill the grid in this pass.
Enter Y in the Constrain data? field to display the ENTER CONSTRAINTS form.
Choose Cancel on the SELECT STRING FILE SOURCE form to return to the GRID menu or Apply to display the
Search type
Enter the search type. An elliptical search will create an estimate at a grid point if the minimum number of samples is found within the search ellipse. Elliptical searches are used for most data sets. An Octant search will divide the horizontal plane into eight equal areas, and will allow only n/8 samples per octant, where n=max number of samples allowed. If there are too many empty octants surrounding a point, no estimate will be made. Octant searches are used when there is significant data clustering.
Minimum number of samples to select
Enter the minimum number of samples to select. If less than the minimum number of points are located within the search parameters, no estimate will be made for the grid point.
Maximum number of samples to select
Enter the maximum number of samples to select. If this is n, then the nearest n samples will be used to create the estimate.
Maximum search radius
Enter the maximum search radius. This is the maximum search distance along the major axis.
If you entered OCTANT in the Search type field, you must enter the following field:
Maximum number of adjacent octants with no samples
This field will only be requested when an OCTANT search is specified. An octant search will divide the horizontal plane into eight equal areas, and will allow only n/8 samples per octant, where n=max number of samples allowed. If more than the specified number of adjacent octants do not contain data, no estimate will be made.
SEARCH ELLIPSE SPECIFICATION
Bearing of major axis
Enter the bearing of the major axis of the search ellipse. The major and minor axes are mutually perpendicular. The major axis is the orientation of greatest continuity of the data set. If the bearing of the major axis is zero then the major axis will be parallel to the y (northing) axis and the minor axis will be parallel to the x (easting) axis. The bearing of the major axis should be entered in the range 0 to 360 degrees. When in a ZOOM ALL view (ie looking directly from above), the angle is measured relative to the y (northing) axis with clockwise being positive. Note that the bearing of the search ellipse is independent of the rotation of the grid itself. For example, if the grid rotation angle is 40 degrees this does not effect the bearing of the search ellipse, which will still be zero. You must define the search ellipse bearing separately.
ANISOTROPY RATIO
major/minor
Enter the anisotropy factor. The search ellipse defines the zone within which samples must be found to be used to create an estimate. If the anisotropy factor is set to 1, an ISOTROPIC search ellipse is used. If the anisotropy factor is greater than one, an ANISOTROPIC search ellipse is used. The maximum search distance is along the major axis of the search ellipse. The length of the minor axis is dependent upon the anisotropy factor. Example: Assume that the bearing of the major axis is zero. The length of the major axis is 60m. The anisotropy factor of the minor axis is 3. The major axis will be parallel to the y (northing axis) and the minor axis would be 20m long and parallel to the x (easting) axis.
Choose Cancel on the SEARCH PARAMETERS form to return to the SELECT STRING FILE SOURCE form or Apply to display the INVERSE DISTANCE PARAMETERS form.
Inverse distance power
Enter the inverse distance power to be used in the interpolations. The lowest power that can be used is zero.
Constrain interpolation?
Enter N if you wish to fill the whole grid. Enter Y if you wish to define spatial constraints, eg inside string or above a certain x value, to define a subregion of the grid to be filled in this pass.
Enter Y in the Constrain interpolation? field to display the ENTER CONSTRAINTS form.
Choose Cancel on the INVERSE DISTANCE PARAMETERS form to return to the SEARCH PARAMETERS form or Apply to display a progress feedback bar while the interpolations are performed.
Result
Once the calculations are completed the current grid is displayed on the screen as point markers in the graphics layer grid_display. The layer grid_display can be saved to a string file using the SAVE FILE button on the GRID menu. You can also toggle its display using the LAYERS Status button on the GRID menu.