sign symbols--organized into one object. This is simi-
definitions and attributes are on layer 0, this can make
lar in structure to a block, but it cannot be converted
the conversion process quite difficult.
into GIS data and should be avoided.
GEOREFERENCING AND COORDINATE
Multilines and other entities
SYSTEMS
A multiline is a special type of line that can be de-
fined as sets and patterns of parallel lines treated as a
Several additional points of interest should be noted
single line. These lines cannot be converted using the
as they apply to the conversion of CAD data. Most no-
DXFARC conversion command in ARC/INFO. Rays
tably, GIS data need to be associated with a specific
and Xlines are construction objects in CAD and should
map projection and datum. For this reason, the CAD
never be used to symbolize GIS information.
data should be digitized in a coordinate system that is
agreed to by all parties involved and in an agreed-upon
Hatching
set of units (meters, feet, etc.). If the drawing data are
An AutoCAD hatch object is a specially defined
not referenced directly, then there should be at least
pattern used to fill in an enclosed area. The hatch is
four control points on the drawing at which the geo-
created as an anonymous (unnamed) block in the data-
graphic coordinates are known.
base placed at coordinate 0,0,0. Hatch patterns cannot
be converted and should not be used to convey any in-
METHODOLOGY
formation about an enclosed area.
Adoption of a standard and agreed-upon method to
External references
create GIS data for use in ARC/INFO will make the
An AutoCAD external reference (XREF) is a refer-
data provider's job, and the GIS professional's job,
ence to another AutoCAD drawing, basically placing
much easier and more efficient. In this section, we dis-
its contents into another drawing as a block. XREF
cuss several concepts that can be very useful.
objects cannot be interpreted by ARC/INFO and should
Layering standards and conventions
not be used.
The adoption of a consistent and agreed-upon layer-
Dimensions and leaders
ing standard is critical to the success of the conversion.
A dimension (DIM) is a complex object consisting
In this way both the data provider and the GIS user
of text, lines, and other symbols used to indicate sizes,
know what to expect in the drawing without having to
distances, and other quantities. DIMs cannot be con-
be experts in both systems. One of the most effective
verted and should not be used. Simple text objects
and simplest concepts is to isolate GIS and non-GIS
should be used instead. A leader is an entity that is used
data into their own named sets of layers. For example,
to draw an arrow to point to an object. It should be
all GIS data could be stored on layers beginning with
treated as a dimension and similarly avoided.
the prefix GIS_. Data for street centerlines could be
stored on a layer called GIS_ STREET_LINES. All lay-
Blocks as grouped graphics
ers that do not start with the GIS_ prefix, although still
Blocks are very useful for symbology and locating
useful to the CAD drafter, could be ignored by the ARC/
point features, but a block entity should not be used
INFO user performing the conversion. Another useful
when it is created as a large-scale collection of graph-
standard is to place all feature classes (point, arc, poly-
ics, say, from another drawing. If the CAD designer
gon, and annotation) onto separate layers. The layer
inserts a large portion of another drawing into the
GIS_STREET_LINES implies that the layer con-
existing one, he can insert it with the explode option,
tains only line (arc) data for the street centerlines.
which will break up the graphic elements individually.
Text for the street names could be stored on
This is always the necessary procedure. If blocks are
GIS_STREET_TEXT. Again, this makes it easier to
used as symbols, the BLOCKS subcommand option of
convert the data. Text especially should be placed on
the DXFARC command can be used.
an individual layer. It is even useful to place blocks
and attributes on individual layers. If there is a layer
Layer 0
for each attribute tag in a block, the GIS user can easily
Layer 0 (zero) is a special layer in an AutoCAD
extract the pieces needed to complete the conversion.
drawing, and it is best not used to draw any objects to
Color and line type schemes
be converted via DXFARC. Blocks should be defined
on specific layers, as should their attributes. If all block
As mentioned earlier, color can be used to catego-
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