Layers
Line types
A layer is a free-form storage entity capable of hold-
LineType libraries are used in AutoCAD to indicate
ing data in various symbolic patterns and containing
varying attributes. For example, a dashed line might be
data of various object types. Although there is no
used to indicate an intermittent stream. This method of
requirement to do so, users often organize their layers
showing breaks in a linear feature is acceptable for trans-
into different types of information. This organization
lation; however, physically "breaking" the line to place
is commonly grouped by some scheme meant to lend
text or another symbol there is not acceptable, because
structure to the drawing. For example, a drawing could
the continuous nature of the linear feature must be pre-
be broken up into CAD layers called parcels, streams,
served.
streets, and hydrants. The parcels layer could contain
Line width
the polylines that make up the boundary of the parcels,
as well as the text that describes the parcels. The streams
In reference to the example in the Layers section
layer might contain the lines delineating the streams,
above, water lines could be drawn using specific line
as well as the stream names. The hydrant layer could
widths to represent the pipe diameter: a line with a width
contain AutoCAD block symbols that have a hydrant
of 4 could mean a 4-in.-diameter pipe. This methodol-
symbol, as well as some block attributes describing the
ogy can break down rather quickly when there is a large
hydrant. Layers can be named using alphanumeric
amount of data or a diverse number of diameter values,
names. An effective layering scheme is essential for
such as 42 values requiring 42 different line widths.
the conversion process. When possible, all text objects
should be on layers independent of the entities they
PROBLEMS TO AVOID
describe. For example, street centerline entities could
be placed on a layer called GIS_STREET_ LINES, and
There are certain entities that should never be used
text labeling for these streets would be stored on a layer
for representing CAD data to be converted using ARC/
called GIS_ STREET_TEXT. Using separate layers for
INFO. They can be used, but they need to be isolated
data that will be stored in different feature classes in
on non-GIS layers.
the coverage is very helpful in the conversion process.
Using 3-D data
All non-GIS data should be isolated on layers that can
be easily identified and excluded from the conversion
Three-dimensional data can be used on GIS layers,
process. All GIS layers should contain only one feature
but there are some limitations. Polylines, or any other
class, such as POINT, LINE, or TEXT, again to facili-
object that has multiple parts on different elevations,
tate conversion.
will be reduced to the elevation of the first entity. Con-
tour lines and other entities that are at a single eleva-
Colors
tion (Z coordinate) can be converted properly.
Proper use of color can convey much information
Model space and paper space
about the data in a drawing. Colors can be assigned on
a layer-by-layer basis (BYLAYER), meaning that the
Paper space is an environment in the AutoCAD
layer has a default color and all objects drawn on it are
drawing in which multiple viewports can be created to
of that color, and they can be assigned on an object-by-
contain actual drawing data that has been rotated,
object basis (BYCOLOR). Colors can be used to fur-
clipped, and annotated for use in plotting to hard copy.
ther refine the definition of objects on a layer. A layer
Paper space is, by definition, in the coordinate space of
called WATER_LINES could contain the water pipe
the plotted output and can have no connection to the
information on a site plan. The pipe material could then
geographic coordinate system. While these data are
be symbolized by colors on this layer; for example, red
useful in CAD, paper space objects and viewports are
lines could represent PVC, blue lines cast iron, and
invalid for use in ARC/INFO and cannot be converted.
green lines ductile iron. This provides a way for the
Therefore, model space should always be used.
AutoCAD user to carry pseudo-attributes on each
Details and title blocks
object without having to design object data tables
with another application (such as AutoCAD Map) or
Any details, insets, and title block information per-
writing a custom application. Often text objects are
tinent to the drafting of the drawing should be isolated
used to indicate additional attributes, such as pipe
onto non-GIS layers.
conditions or installation dates or both. This text,
Groups
however, has no connection to the water pipe object,
rather it simply provides a visual aid on the drawing
An AutoCAD group is a collection of heterogeneous
canvas.
objects--such as a street line, its name text, and two
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