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further and tagged (fig. 7) on an Intergraph edit station, with final structuring to a digital line graph format occurring on an Amdahl mainframe computer. It is believed that this production system will enable us to produce, at a high rate, large amounts of digital cartographic data.

Features Included in the Data
Base

The data base will provide comprehensive coverage of transportation and hydrographic features. A complete list of the features to be included is given in table 1. These features provide a basic

Table 1. Features included in 1.100,000-scale digital data base

Transportation features
Airport, airfield, landing strip
Heliport
Pipeline

Power substation
Power transmission line
Railroads and related features
Bridge
Carline
Ferry

Roundhouse
Sidings
Snowshed
Station
Tracks, narrow gage

Juxtaposition

Multiple

Multiple, abandoned

Multiple, dismantled

Multiple, under construction

Single

Single, abandoned Tracks, standard gage

Juxtaposition

Multiple

Multiple, abandoned

Multiple, dismantled

Multiple, under construction

Single

Single, abandoned

Single, dismantled

Single, under construction
Tunnel

Underpass, overpass
Yards
Roads and related features
Bridge
Class 1
Class 2
Class 3
Class 4
Class 5 (trail)
Dead-end road
Ferry

Interchange
Parking area

Paved service and rest areas
Tunnel
Under construction, class 1

Underpass, overpass Ski lift, tramway, incline railway

Hydrographic features

Alkali flat

Aqueduct, conduit, flume,

(elevated) Aqueduct, flume, and so forth

(underground) Aqueduct tunnel Area subject to

controlled inundation
Area to be submerged
Breakwater, pier, wharf
Canal, flume, aqueduct,

or perennial ditch
Canal, intermittent
Canal lock or sluice gate
Canal, navigable
Channel in water area
Cranberry bog
Dam, masonry
Dam with lock
Ditch, intermittent
Drydock

Dry lake or pond
Falls

Filtration plant
Fish hatchery
Gaging station
Glacial crevasses
Glacial or permanent

snowfield Lake or pond,

intermittent Lake or pond,

perennial
Lock, shipping canal
Mangrove
Marsh or swamp
Rapids
Reservoir
Salt evaporator
Seawall

Sewage disposal
Shoreline
Siphon
Spring

Stream, braided
Stream, disappearing
Stream, intermittent
Stream, perennial
Stream, unsurveyed

perennial
Submerged marsh or swamp
Wash
Watermill
Water surface

elevation Water well Windmill

framework of geographic information needed by major users (such as the Bureau of the Census) to perform various types of spatial analyses.

DATA STRUCTURE

The digital planimetric data are produced and distributed in the form of digital line graphs. The digital line graph concept is based on graph theory in which a graph can be represented as a set of nodes and links that explicitly records the spatial relationships inherent in the graph.

The digital line graph's topologically structured data file accommodates all of the categories of data (that is, point, line, and area data types) included in

table 1. Each distinct major data category (such as transportation and hydrography) is stored as a separate data file in the data base. Details on the digital line graph structure can be found in U.S. Geological Survey Circular 895-C: listings and explanations of the attribute (feature) codes are contained in U.S. Geological Survey Circular 895-G.

This data base, when complete, will contain an immense amount of spatial information. Not only are a large number of features identified and coded, but the level of feature portrayal also is quite detailed. Figure 8, which is a plot of a portion of the road network digitized from the Tallahassee, Florida, l:100,000-scale map, provides an indication of this detail.

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Anticipated Uses

The U.S. Bureau of the Census has a requirement for digital cartographic data describing roads, railroads, streams, and other geographic features to enable it to automate its geographic support system for the 1990 census. Census is developing its Topologically Integrated Geographic Encoding and Reference (TIGER) System to automate many of the geographically based aspects of data collection, processing, and publication activities associated with a decennial census. According to an article by R. W. Marx in American Demographics, "TIGER will store and identify the hierarchical relationships of all geographic areas represented in each census or survey: the 50 states, 3,137 counties, more than 18,000 minor civil divisions or equivalent areas, more than 20,000 incorporated places, and all other areas. . . .TIGER must accurately

portray the boundaries of all political and statistical areas recorded in the file, plus the streets, roads, and other map features needed for field operations." The l:100,000-scale digital cartographic data base will form the cartographic foundation for the TIGER files. One example of a geographic product that Census will produce from the system is shown in figure 9. The Geological Survey and the Bureau of the Census have agreed to work jointly in the production of this data base.

The l:100,000-scale digital cartographic data base will provide the geographic framework for geographic information systems through the rest of the century. The addition of street names and census-keyed geocodes by Census will allow spatial analysis of vast amounts of socioeconomic data. The Geological Survey is adding information on land ownership and mineral resources to the base categories to form

Figure 9. Proposed enumerator map as it will be created by the U.S. Bureau of the Census' Topological^ Integrated Geographic Encoding and Reference System.

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