Calculate grade in polygons
This function provides a selection of techniques to assign average grades to closed ore outlines. The grade assigned is an average of the assays for all samples lying inside the outline. The samples can be treated as points whereby each sample is given the same weighting for the grade calculation, or as interval samples in which case the grade is weighted by the length of the sample.
To run this function: Choose File tools > Calculate grade in polygons, or...
Fields on the CALCULATE GRADES FOR ORE ZONES form.
(I)nterval or (P)oint averages
You must enter here what type of data you are going to use to calculate the average grades. For example, your data may be drill hole assays where each value has an associated length and so the average grade calculation must weight each assay value by its length. You can extract interval assay data from a drill hole database. In this case you would enter `I' for interval data. The program assumes that the assay value is stored in the coordinate at the end of each sample and so calculates the assay length from the previous point in the string file which should be the coordinate of the start of the sample. Any missing assay intervals are ignored in the calculation.
Note: This will overestimate the grade unless you take steps to set missing samples to zero or some other value.
(P)oint data might consist of centroids from a block model where you wish to calculate the average value inside a polygon. In this case, all of the sample values are assumed to have an equal weighting in the calculation.
Define the range of sections to process
Section range
You must enter the range of ID's of the string files that you wish to process. Although the program asks for the section range, this may of course refer to layers as well.
Location of assay files
Enter the location of the string files that contain the assay data.
Location of ore zone files
Enter the location of the string files that contain the closed ore outlines.
Location for resultant zones
Enter the location for the string files to store the resultant ore outlines with the assigned average grades.
Overwrite D-fields (Y/N)
If you want to overwrite the contents of the previous D-fields with the calculated grades select 'Y'. If you want the previous D-fields appended to the calculated grades select 'N'.
Compute full samples only (Y/N)
Generally, the ore outlines are designed to pass through points equal to the sample start and endpoints, but if they have been digitised from a tablet, then the intersection points are rarely exact. Enter 'Y' here and the program will assume that the outline passes exactly through the sample point nearest to the actual point of intersection of the outline with the drill hole trace. Enter 'N' and the sample length is taken from the exact point of intersection of the outline with the drill hole trace, and so the assay is weighted by the reduced length of the sample. This entry is only used by the program for interval averages.
Dilute blank or negative samples (Y/N)
In certain conditions, samples may be missing from the string files containing the sample values. The missing samples are represented by either assay values which are less than zero or blanks in the description field that contains the assay values.
There are two different methods available to handle missing samples in the calculation process:
- Y, the sample will take on an assumed value of 0 (zero). This has the effect of reducing or diluting the final average grade.
- N, the sample will be ignored completely and it will have no effect on the final result.
(M)ean or (S)ichels
This entry is only used for point averages. If you enter `M', a simple arithmetic mean of all the assays within the closed outline is applied. This calculation assumes the assays are normally distributed.
If you enter `S', a Sichels t estimate of all of the assays within the envelope applied. This calculation assumes a lognormal distribution of the assay values. The Sichel t estimator is a good estimate of lognormal distributions and overcomes the tendency of the arithmetic mean to overestimate the average. It is a function of the median, the number of samples and the logarithmic variance. It should be used with great care with geologic data populations, since even though these characteristically show a highly skewed distribution, they are rarely true lognormal distributions.
Number of assays to process
Enter the number of different elements for which you want to calculate average grade. The different elements are stored in description fields of the assay string files. The function assumes that the first assay is in the D1 field, the second in the D2 field and so on.
Complete the CALCULATE GRADES FOR ORE ZONES form and choose Apply to start processing.
The ore zone strings are written to an output string file where the average grades for individual closed segments are stored in the various description fields of each closed segment.
A message is displayed to inform you which section is currently being processed. As each section is finished, the average grades of the string polygons, and string file where they are stored are displayed in the message window.