in its ability to allow the user to manipulate graphic
tional mode desired. As we will see, this is both a
objects quickly and easily to create a finished hardcopy
strength and a weakness when it comes to integrating
product. Traditionally, AutoCAD is concerned with two
data. The user can organize data into logical layers and
types of data: each object in an AutoCAD model stores
impose a methodology for the symbolization if desired,
its location (geometry) and the characteristics of its
but this is not required. Although there is a great deal
appearance (graphics). A line may be drawn in a spe-
of control over the geometry of the graphics, there are
cific color or using a specific line style (dashed, dotted,
few built-in methods to attach descriptive information
etc.) that conveys information about its use. Further-
via a database. This makes AutoCAD an attractive tool
more, AutoCAD uses an organizational approach in its
to use for the capture of data because of its powerful
drawing (.DWG) files via a system of layers. These are
editing engine. The task now becomes translating that
places where objects can be stored, with the user then
work into a format that can be converted easily into
being able to manipulate their display as a group. Lay-
ARC/INFO to facilitate advanced GIS data manipula-
ers can be assigned colors, although individual objects
tion. The coordinate space upon which an AutoCAD
can have their own color. AutoCAD, being primarily a
drawing exists is a double-precision, 3-D system, origi-
graphic system, uses the graphic characteristics of an
nating at 0,0, and extending out in all directions.
object to carry information about itself. Primitive meth-
ARC/INFO 7.2 coverages
ods exist in AutoCAD to associate data with drawing
entities, such as blocks and object data. One method is
As mentioned, ARC/INFO stores data into logical
to simply create text in the drawing that visually
units called coverages. A coverage stores a set of vec-
describes an entity but is not rigidly associated with
tor-based geographic features, such as roads, soil bound-
the drawing object. Thus, the CAD package uses a
aries, zip codes, buildings, city locations, sample wells,
graphic database for symbology and a geometric data-
rivers, etc. Coverages are stored in a georelational for-
base for the coordinates of the dimensions of the
mat, which means the vector features are associated with
objects. AutoCAD stores true geometry for circles, arcs,
a database containing descriptive attributes of the fea-
ellipses, and other types of mathematical curves.
tures such as zip code number, soil type, or stream name.
A coverage, taken as a whole, is a directory of files, all
ARC/INFO
related together into a model that supports such topo-
ARC/INFO is the most widely used professional GIS
logical relationships that may need to be created to sup-
software package. Professionals in many disciplines use
port network tracing, polygon overlay, and other spa-
ARC/INFO to analyze relationships among the geo-
tial operations. This topology is created by ARC/INFO's
graphic locations of features and the information
BUILD and CLEAN commands and generates tables
describing these features. A GIS is a database in the
to describe the relationships between coordinates that
fullest sense, relating tables of information that describe
define linear features (arcnode topology) and tables
features to point locations, such as monitoring wells;
to describe the relationships between linear features that
linear features, such as rivers or roads; and area fea-
define closed polygonal areas (polygonarc topology).
tures, such as county boundaries, parcels of land, or zip
These relationships create a seamless, nonredundant
codes. The databases, spatial and attribute, are inter-
database that describes the geometry of the geographic
related and can be used to create ad hoc symbology to
data. Another key component of a coverage is that it
represent the state of the data, trends in the values, or
has a specific coordinate system associated with it,
specific values searched on or queried for. ARC/INFO
which can be described by its projection and datum.
stores these data in a series of georelational units called
The datum is the ellipsoid upon which the surface of
the earth is approximated, and the projection is the
make up the geometry of the features as well as the
mathematical method via which the surface is "flat-
database that describes them. Coverages store this infor-
tened" into a Cartesian coordinate mapping space. Fig-
mation as points (single coordinate locations), arcs (lin-
ure 1 illustrates data storage concepts for both AutoCAD
ear connected segments), and polygons (closed areas),
and ARC/INFO.
and more complex features such as regions.
ARC/INFO coverage features
ARC/INFO is capable of storing data in many simple
DATA STORAGE
and complex features. Some examples of complex fea-
AutoCAD R14 drawings
ture classes are routes (paths through multiple arcs),
AutoCAD R14 stores its data into files called draw-
regions (collections of polygons in potentially overlap-
ings. A drawing can contain any design of graphic
ping areas), and raster data sets such as images and grids.
objects that the user wishes to create, in any organiza-
We will concern ourselves here with the simple 0-D,
2
Previous Page
