An additional savings can be realized if the preparation of 
the plate of one layer of map information, the PLSS, can be 
expedited further through the proposed use of interactive 
graphics and computations. 
DIGITAL LINE GRAPHS 
The digital line graph (DLG) product of the National Map 
ping Program of the USGS is derived by encoding, digitiz 
ing, and storing graphic map features in a computer com 
patible form. In its most structured form, the DLG level 3 
(DLG-3), the two-dimensional data is both geometrically and 
topologically correct. In other words, relationships among 
map features that are explicit when a map is examined 
visually are retained in the coding of the computer file. 
Spatial analyses of the data can therefore be accomplished 
without the aid of a hard-copy of the map. 
Creating a DLG requires the digitizing of encoded map 
(graph) elements. The first element, a node, represents a 
point: a) at which a line begins or ends, b) at which two 
or more lines intersect, c) depicting any arbitrary point, 
d) locating a point feature, or e) indicating a significant 
location on a line. A node is encoded by digitizing its 
X-Y coordinates and associated numeric descriptor codes 
(attributes). The second element, the line, is a graph 
feature connecting two nodes. Its digitizing syntax 
requires that a starting node, ending node, area on the 
left and area on the right be referenced, in addition to 
associated attribute codes. The third element, an area, is 
digitized by encoding the X-Y coordinates of an arbitrary 
location chosen to represent the entire enclosed area and 
its attribute codes. This procedure depicts numerically 
the spatial relationships among nodes, lines, and areas 
exhibited in the map (see figure 1). 
Figure 1. Node, Line, and Area Relationships.