The second stage of the survey usually consists of drilling on a large and more or less regular grid. In general, this grid will cover what is considered to be the mineralized zone and should be sufficient to evaluate the overall in situ resources (tonnages and mean grades). If promising results are obtained from this second stage, the survey may be extended to some in-fill drilling or by drilling in adjacent zones. At this stage, the geologist and project engineer are faced with two fundamental quantitative problems.
The first one is how to deduce the required global estimations from the available information, which consists of qualitative geological hypotheses and quantitative data from core samples, trenches, pits, etc. In general, estimations of a geometric nature such as mineralized surface and yardage will make more use of qualitative information than estimations of a ``concentration'' nature such as mean grades and densities.
The second problem is to express the confidence that can be given to these estimations. In practice, it is common to classify the in situ resources in different categories such as, for example, those recommended by the US Bureau of Mines: measured ore, indicated ore and inferred ore. The boundaries between these categories do not always appear clearly. The geologist knows from experience that the precision of an estimation, such as mean grade, does not only depend on the number of the available assays but also on the in situ variability of the grade in the deposit and the manner in which this variability has been sampled. He also knows that the concepts of continuity and zone of influence, which are used in the evaluation of resources, depend on the particular deposit and the type of ore considered. And, finally, he knows that for a given ore the variability of assays of core samples, cuttings or channel samples can be markedly different.