CHARACTERISTICS OF USGS DIGITAL LINE GRAPHS AND 
DIGITAL ELEVATION MODELS 
Characteristics of the three levels proposed by the USGS for encoding a 
digital line graph (DLG) are shown in table 3 and may be summarized for 
both the DLG and the digital elevation model (DEM) as follows: 
Level 1: This is the simplest structure which maintains the origi 
nal (raw) data in a standardized format, coded to pre 
scribed standards, and edited for normal input errors. 
The main purpose of this level is to meet three needs: 
(1) to provide a source of digital data quickly; (2) to 
provide data to users who can complete the structuring 
process; and (3) to provide data for plotting or display 
systems of low or moderate cartographic quality. 
Level 2: This structure is designed to support graphic display or 
plotting equipment of high cartographic quality. The DLG 
files contain extensive attribute codes that describe the 
graphic elements. The DEM files are enhanced for consist 
ency and filtered to reduce data volume. An illustrative 
diagram of a DEM is shown in figure 6. 
Level 3: The third structure is used for fully topologically struc 
tured data files designed to be integrated into geographic 
information systems. All topological relationships have 
been defined for the DLG data and the DEM data have been 
modified to be consistent with the corresponding planime- 
tric data (represented by the DLG data file). 
Table 3.—Characteristics of USGS Digital Line Graphs 
(Level 1, 2, and 3) 
CHARMTFRISTIC 
DLG-1 
DLG-2 
DLG-3 
COORDINATES 
MAY NOT MEET NBAS 
ACCURATE TO NttAS 
ACCURATE TO NMAS 
LINE COORDINATE 
SPACING 
VARIABLE 
VARIABLE BUT CONSTRAINED 
BY LINE CURVATURE TO 
PRESERVE ACCURATE SHAPE 
VARIABLE BUT CONSTRAINED 
3Y LINE CURVATURE TO 
PRESERVE ACCURATE SHAPE 
LINE ENDINGS 
MAY 3E APPROXIMATE 
PRECISE 
DEFINED BY NODES 
MAP PROJECTION 
PARAMETERS 
MAY BE ABSENT 
ACCURATELY DEFINED 
ACCURATELY DEFINED 
ATTRIBUTE COOING ! 
LIMITED 
ALLOHS LINE 
SYMBOLIZATION 
ALLOWS SYMBOLIZAT!ON 
AND COMPUTER ANALYSIS 
AREAS 
NONE 
NONE 
DEFINED AMD BOUNDED 
BY LINES (ARCS) 
LINE DIRECTION 
NOT SPECIFIED 
NOT SPECIFIED 
DEFINED INCLUDING AREAS 
TO LEFT AND RIGHT 
These three levels do not easily aggregate in an upward direction. Data 
collected under criteria and specifications for level 1 or level 2 cannot 
always be enhanced to level 3, and to achieve this level, it is often 
easier and more cost effective to recapture the data in level 3 form.