ST 
The net error of the above four controlled sources of +4-mil 
error will average together statistically to be approximately 
+ /4 x 4) or +8 mils -- sufficient to meet current cartographic 
map standards. Further improvement in precision of each system 
component will not improve the total error materially. As a 
result, the +8 mil standard system error is recommended for use 
in designing digital mapping systems using general purpose processors. 
Estimating Map File Content 
  
Another major factor now to be defined in processing line 
map data is the practical size of the digital files to contain a 
line map which for economy is reduced to incremental vector form. 
Figure 4 shows several examples of a square inch from a map overlay. 
For simplicity, one inch of a straight line will be equated by a 
start and ending pair of incremental vectors in X or Y direction 
(Figure 4). A maximum curvalinear line can have 250-unit vectors 
per inch 4 mil in length. À unit X Or Y vector, is always 4 mils 
  
by definition. An average for an incremental vector is 20 mils 
in any 8 directions -- 0°, 45°, etc. Estimates of the incremental 
vectors are shown (Figure 4) for each typical, square-inch 
example to indicate the reasoning for selecting an average number 
of incremental vectors per square inch. 
A standard for a map "clutter" maximum of 240 incremental 
vectors per square inch is reasonable. The estimated average 
for an 8-color map is defined as about 1/3 of this maximum. Each 
of eight overlays would thus have an average of about l10 incre- 
mental vectors per square inch for an average map. This average