Digitizing is the process
of converting paper-based graphical information into a digital format.
When you digitize a map, you use drawing commands to trace data
from the paper map into a DWG file.
Digitizing lets you
take information from raster images or paper maps, and enter it
into a digital map. Move your cursor over the image to see the results.
Planning for Digitizing
Before you begin to digitize,
consider the following:
- Suitability of source maps
- Global coordinate system
- Tiling maps
- Layer organization
- Data storage: internal or external
- Representation of node, network, and
If possible, plan on
completing all digitizing for one map in one session because the
map media may distort over time.
Digitizing Linear Objects
Linear objects are
objects such as lines, arcs, and polylines.
- If you plan to use topography later to
generate 3D views from digital terrain models, place linear objects
at the elevations (Z- values) they represent.
- If you use the SKETCH command to trace
an irregular line, make sure the variable SKETCHINC is set to a
reasonable value, because each line segment ends at the interval
set by SKETCHINC. The SKETCH command can create huge files for one
small line when SKETCHINC is set to a small value.
- When digitizing irregular curves with
PLINE or MAPDIGITIZE,
the spacing of the selected vertex points should depend on the curvature
of the line. Straighter segments require fewer points.
Examples of digitized
- However accurately you work, you lose
data when you digitize a curve. You need to digitize more points
when you create sharp curves to ensure that the line is as accurate
as possible; however, while you reduce the data loss, you increase
file size and complexity. If you know the parameters used to define
a regular curve, such as the radius or length, use the Arc option
of the PLINE and MAPDIGITIZE commands
for digitizing. Irregular lines, such as topography contours, should
be continuous polylines. They can be smoothed with the Fit option
of PEDIT if necessary. Set the PLINEGEN system variable to 1 (on) before digitizing,
so that any dashed linetypes are evaluated correctly.
- When you finish digitizing a segment,
mark it on the paper map so you do not repeat the digitizing. Double
digitizing increases file size.
When digitizing data
that will be used to create a topology, follow these principles
to achieve the most accurate results.
- Boundaries (or other polylines) should
be completed with the Near, Intersection, or Endpoint object
snaps to ensure that closed areas such as parcels, buildings, and
water bodies are in fact complete polygons.
- Line segments should be snapped to existing
end points where they intersect.
- When you are digitizing data for network
topology, do not duplicate objects. For example, do not double-digitize
boundary lines separating adjacent polygons. It's better to digitize
adjacent polygons on the same layer with common lines defining common
boundaries. If one edge serves two or more purposes, digitize the
line once, then use the COPY and CHPROP commands to put a duplicate
line on a different layer.
After you digitize the
linear elements that form the basis of the topology, you should clean
up any problems before you create the
Digitizing Control Data
Points and Monuments
When you are trying to
match digitized maps with existing digital maps, you can use some
known-to-be-accurate points common to both maps.
- Control Data Points — A
system of geodetic control points covers the entire United
States. The latitude and longitude, and often elevation,
are established for these points. Similar systems exist for other
countries, such as Bench Marks and Trigonometry Points throughout
the United Kingdom.
- Monuments — If you are working
with maps for a city or county, points used for establishing locations
for all maps probably already exist: these points can include features
such as public buildings, hill summits, and parts of highways.
When you are digitizing
a map, use the following procedures to establish known control points:
- Create a layer called REFERENCE. On it,
digitize at least four points corresponding to real-world coordinates
such as the coordinate intersections of latitude and longitude lines.
These points should either appear at the corners of your map sheet
or surround the map features to be digitized. Be careful to note
on the drawing the location of these reference points and their
real-world coordinates. Use these points to register the map with
the TABLET command, as described in Registering
- To ensure accuracy, you can also digitize
other points such as control points and monument locations that
have known positions. Digitizing more control points is important
Map Edges or Rubber
Sheeting Two Maps operations.
While you are digitizing,
you can add text to indicate nodes or important locations on a map.
Use the STYLE command to define a text style that uses a simple
font, such as isocp.shx, with a fixed text height
so that you do not have to enter a text height each time you enter
text. You can modify the text style and height when you finish digitizing.
Use the TEXT command
to enter text as you digitize. Text should be single-line entries
on the same layer as the feature it describes. If required, enter complex
or lengthy text with the MTEXT command after you finish digitizing.
For more information, look up "text" in the Help index.
Try to avoid overlaying
the insertion point of the text and end points of the objects you