Subsurface and Office Representation in Mining Geology 999 



Areal Map Grid System 



In order to bring about efficiency and coordination of surveying, field 

 mapping, sampling, office map preparation, ore-reserve calculation, in- 

 dexing, and filing, it is necessary to subdivide the entire district into sys- 

 tematic rectangular subdivisions. All map layout, indexing, and filing of 

 data are based on the grid system described below. This grid system is 

 based on the assumptions that letter-size field note sheets containing an 

 8" X 10" map area and office sheets containing 20" x 40" map areas are 

 used. These standardized sheets are used for both plan and sectional rep- 

 resentation. 



Note Sheet Grid — The size of rectangle included in an 8"xl0" note 

 sheet area at 1" = 50' scale is 400' X 500'. This area is the basic unit 

 area or "quadrangle" into which the entire district is subdivided. These 

 areas are laid out in a grid system with the long dimension of the rectangle 

 oriented north. 



The boundaries of these unit areas are determined by the coordinate 

 system. The north and south boundaries of the unit areas are made to 

 coincide with ordinate values whose last three digits are alternately 000 

 and 500; thus, the border local ordinate values are always evenly divisible 

 by 500 and increases as follows: 47,500N^lS,00ON-48,5OON^9,0O0N- 

 etc. The east and west boundaries of the unit areas are made to coincide 

 with ordinate values which are evenly divisible by 400. Thus, the se- 

 quence may be 52,000E-52,400E-52,800E-59,600E-60,000E-60,4.00E- 

 etc. 



A rectangle that is four unit 50' scale areas long and four unit areas 

 wide, contains sixteen of the 50' scale unit areas, and is l,600'x2,000'. This 

 is the unit area used for 200' scale mapping on the letter-size standardized 

 note sheet. The north and south regional ordinate boundaries of these 

 200' scale unit areas are always evenly divisible by 2,000. The east and 

 west boundaries are evenly divisible by 1,600. The boundaries of every 

 fourth 400'x500' rectangle coincides with the boundaries of this larger unit 

 area. (fig. 529) 



If the 200' scale rectangles are combined in multiples of five in north- 

 south and east-west directions, a 1,000' scale unit results in a map area of 

 8,000'xl0,000'. The north and south regional values of the boundaries are 

 evenly divisible by 8,000; the east and west ordinate boundary values are 

 divisible by 10,000. 



In a similar manner 1,000' scale unit areas are combined to form 

 5,000' unit mapping areas, which are 40,000'x50,000'. The 50-scale 

 400'x500' unit rectangles are subdivided where necessary into 10-scale 

 8'xlOO' rectangles for representation of extreme detail. These areas may 

 be further subdivided into 1' scale rectangles of 8'xlO' size. 



Sectional Grid System — The same type of letter-size note sheet is used 

 for both plan and sectional representation. Whenever possible all sec- 



