Sichel point average grade
With this function you can estimate the grade inside all closed segments of a string. The function works by finding all the sample points which are inside the segments of the selected string and then using a Sichels t estimator to estimate the grade value using these samples.
To run this function: Choose Design > Grade control > Sichel T grade of polygon, or...
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. When the Dilute blank or negative samples check box is ticked on the Recall and Display Samples for Grade Control form, the sample will take on an assumed value of 0 (zero). This has the effect of reducing or diluting the final average grade. Otherwise the sample will be ignored completely and it will have no effect on the final result.
It is only appropriate to use this function when the spatial distribution of the samples is reasonably consistent and the distribution of the sample assays exhibits lognormal behaviour.
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.
Generally all segments must be clockwise in direction except in the special circumstance where one or more areas of exclusion need to be defined within a clockwise segment. Note that anti-clockwise segments are only permitted if they are contained within a clockwise segment. This function will consider anti-clockwise segments which are contained inside clockwise segments as areas of exclusion and any sample points which are inside the anti-clockwise segment are excluded from the grade estimate.
Select the point of interest
Position the cursor and select a point on the closed segment within which you wish to estimate the grade. After you have selected the segment the grade estimate is made and a message similar to the following is displayed in the message window.
String 3, Net Grade=12.298, Net Volume=884.353, Net Mass=884.353, Net Area=884.353
Clockwise segment IDs=1,3 Volume=1044.043, Mass=1044.043, Area=1044.043
Anticlockwise segment IDs=2 Volume=-159.691, Mass=-159.691, Area=-159.691
After the grade estimate is displayed you will be prompted to selected another point. This function will continue in this manner until you choose Cancel.
The flexibility of this grade control system becomes apparent when you use the grade estimation functions. If the grade estimated is too low then you can use the GRAPHICS EDIT functions to alter the shape of the segments to include or exclude samples from segment and hence alter the grade estimate until you achieve a result which is acceptable.
The next stage of the Grade Control task is to mark the newly delineated ore zone out in the pit. Setout notes to aid in this task can now be easily generated by using the Setout functions which can be found on the INQUIRE menu.
No clockwise string found for anticlockwise
stringA segment of the selected string is anti-clockwise in direction but it is not contained inside a clockwise segment. This is not permitted. Check the directions of the different segments of the selected string.