998 Subsurface Geologic Methods 



utilization of 50' scale representation results in smaller-sized office maps. 

 Such "quadrangles" require a 25" x 50" sheet if posted to 40' scale and 

 a 20" X 40" sheet if posted to 50' scale. The smaller 50' scale 20" x 40" 

 map books are much more convenient to handle; consequently wearing 

 longer before replacement becomes necessary, and the cost of tracing cloth 

 is 40 percent less. At many mining properties stope-book floor plans are 

 prepared with a plan map for every 8' or 10' vertical interval. Such stope 

 books sometimes contain 200 or 300 pages, and cover the entire area of 

 the piine. It can be appreciated, therefore, that the saving of expenditure 

 for cloth is a large item. 



If 200' scale representation is utilized for detailed correlation maps, 

 100' scale maps serve no useful purpose. In actual practice the detail 

 that can be represented on 100' scale maps is not much more than can be 

 shown on 200' scale maps. If plotting is on a sheet of constant size, 100' 

 scale maps cover an area only one-fourth as large as that covered by the 

 latter scale. 



Mapping and representation scales of intermediate values between 

 200' and 1000' scale can readily be dispensed with. It seems that in map- 

 ping and representation practice, a leveling off in the ratio of the detail 

 representation possible to the rate of scale increase takes place above the 

 200' scale value. It is therefore possible to show nearly the same detail in 

 1000' scale mapping as in 400' or 500' scale mapping. 



Mapping on smaller scales than 1000' = 1" is often useful to determine 

 the general geology of an area and to delineate the "hot" areas for more 

 detailed mapping. Data useful for mineral exploration may be secured 

 from published maps and plotted on small-scale regional maps. Local 

 geology can be better evaluated if the regional geologic, metallogenic, or 

 tectonic environment of an area is known and clearly indicated on maps. 



Coordinate System 



Coordinate values are first determined when initiating a geologic rep- 

 resentation system. As stated previously, a coordinate system must be such 

 that all points within a district may be located in terms of positive (north 

 and east) values. The north and east zero values must, therefore, be well 

 outside the mineralized area. In order to avoid negative values, numbers 

 less than five digits are never assigned to the initial point. It is desirable 

 to derive local coordinate values from a national, provincial, or state-wide 

 system that has previously been established by federal or state agencies 

 or bureaus. These coordinate systems are of such regional nature that 

 values are too large to be used with convenience in actual local practice; 

 it is, therefore, advisable to drop the first few digits and limit local values 

 to the last five or six digits. Thus, if the regional value of a point is 

 5,654,432N and 6,476,451E, this point can be assigned a local value of 

 54,432N and 76,451E and used as the initial point for the local system. 



