The three measurements are all thicker than the 14-inch minimum thickness of coal considered when calculating anthracite and bituminous coal resources. O. An arc AB is drawn between the predicted 14-inch points using either AO or BO as a radius. The arc AB locates the 14-inch isopach according to the available data on thickness and length of outcrop. Areas A to P are ready for planimetry of acreages, for calculation of coal tonnages after weighted average thickness of coal for the areas are determined, and for subsequent entry on tonnage tables according to their assignment to a thickness of coal, thickness of overburden, reliability category, and to a county. Table 4 lists the areas of reliability by thickness of overburden category, by a single thickness of coal category, and by counties.
Subsequent figures are more complicated because more thickness categories are introduced.
This method of determining the area of coal under a ridge is commonly necessary when thickness measurements are sparse and an estimation of coal resources beneath a ridge is required. Table 5 enumerates the areas assigned to the varous thicknesses of coal and overburden and reliability categories. Figures 16 and 17 and table 6 show how to construct the 14-inch minimum thickness limit of anthracite or bituminous coal resources, other coal isopachs, and the areas of reliability and overburden isopachs for a simple valley reentrant in a coal bed outcrop. Figures 16 and 17 and table 6 are important because they show how to treat a valley reentrant in a coal outcrop characterized by minimal thickness data. Figure 19 also illustrates the areas of reliability, over-burden categories, and coal isopachs.