Figure 1. CAD and GIS software data storage.

DATA STORAGE

MODELING ARC/INFO FEATURES WITH AUTOCAD ENTITIES

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Figure 1. CAD and GIS software data storage.

in a LAB file that is linked internally to the point

1-D, and 2-D features of points, arcs, and polygons.

attribute table (PAT).

These will then map through the conversion from

AutoCAD points, block inserts, lines, polylines, light-

Arc features

weight polylines, circles, arcs, and other drawing enti-

Arcs represent both linear features and the borders

ties. Each ARC/INFO feature is connected to its fea-

of polygon features. Linear features represented by arcs

ture attribute table via an internal ID system. This is

can have length but no area, such as a contour line, or

not a major concern to the AutoCAD user. However,

can be long narrow features whose width is not appar-

creating a way to uniquely identify each entity by a

ent at a given map scale, such as a river. Each linear

system-dependent identifier might be a concern. For

feature may be made up of many arcs. Since arcs also

example, each fire hydrant in a drawing may have some

form the basic linear infrastructure from which com-

unique number assigned by the Public Works Depart-

plex GIS processes are derived, such as tracing and

ment. This ID might be placed in the drawing to iden-

surface computation, it is very important that an entity

tify the hydrant. A CAD user might be satisfied to place

be continuous. An arc is a continuous string of X,Y

this on the drawing to be plotted out. The GIS would

coordinate pairs (vertices), beginning at one location

need to store this value with the hydrant in the feature

and ending at another, stored in an ARC file. Geometri-

attribute table, so it can join it to other databases in the

cally, an arc is similar to an AutoCAD Polyline entity,

future. As we will see, there are various methods to

which is often used to create ARC/INFO arc features.

capture this type of information into ARC/INFO.

The topological structure is created so that the starting

Point features

and ending points are special locations known as nodes.

The in-between points are the vertices that define the

Points represent geographic features that have no

area or length, such as sewer manholes, bank locations,

arc's shape. This structure is used to trace linear sys-

tems and to form the boundary of polygon features. This

accident locations, or features that are too small at a

means that there should be no break in the continuity

given scale to be represented by their dimensions. Points

of entities drawn in AutoCAD to represent features in

are stored as single X,Y coordinate pairs. In a GIS,

ARC/INFO. If a text entity is used to indicate the

descriptive data are stored in a database, then related to

diameter of a water line, it should not break the conti-

the spatial database that holds the coordinates. As shown

nuity of the water line and should be placed above or

in Figure 2, the spatial database stores the coordinates

LAB (Internal File)

WELL#

X-COORD

Y-COORD

150.75

209.45

254.12

400.23

PAT

AREA

PERIMETER

WELL#

WELL-ID

0.00

Figure 2. ARC/INFO point feature.

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