How to Modernize Your Paper Engineering Drawings
A White Paper From the Paper to CAD Experts
The Revision Cycle
Raster editing or drafting is the
simplest and most productive way to modify scanned paper drawings. It is the lowest cost
method to bring drawings into a format to modify or make changes to the data. This is
supported by the availability of many scanning service bureaus and the recent price
breakthroughs of large-format scanners. Raster drafting works best when simple updates are
required in nondimensioned or analysis-oriented drawings.
There is significant differentiation within the software products found in
todays market in terms of features, functionality and positioning. The more advanced
products are capable of snapping to or selecting and manipulating raster
"entities" just like vector CAD entities.
Conversion To Vector
Drawings with the highest degree
of corporate value are those used within Analysis and Modeling systems. These drawings
need to be in a fully vectorized format. Some examples include; a company that needs to
develop a three-dimensional model from an old drawing and run FEM or interference checks within the model; or a city planner that is
developing a new building and needs to use three-dimensional terrain models from old paper
drawings. Both environments require vector CAD models in its purest form and consequently
require full conversion to vector.
The process of automatically converting the scanned image
into a CAD drawing is called raster-to-vector conversion, or vectorization. Tools are
available to perform this function in an unattended batch manner, or operator-assisted
with line following or selective conversion processes. Conversion software will not
produce an unattended 100 percent conversion. It is best used as a component of the
conversion process rather than as a total solution.
The tools used to vectorize are:
Overlay Tracing - This is often referred to as
heads-up digitizing. A scanned image is loaded into the CAD system as a backdrop and the
image is "traced" over with CAD entities. This is very similar to the digitizing
except a digitizer table is not needed. Overlay
tracing is quicker than a complete redraw, but is still labor-intensive, tedious and
time-consuming. It is a good option when working with poor-quality drawings. Raster
snapping and heads-up digitizing improve throughput and accuracy over traditional
hand-digitizing methods which are still widely used.
Batch Coversion - Batch tools work with a set of
predefined rules to recognize unique settings such as text classification, width
separations and geometrics. This works best when drawing quality is very good, drawings
are consistent and the desired result is basic primitives. Results of batch systems will
often require cleanup to ensure the converted drawing meets the needs of the user.
Selective or Interactive Conversion - This is the most promising of the
CAD conversion techniques. It combines the intuitive knowledge of the user with an
interactive line-following or selective conversion process. These tools allow an operator
to isolate selected geometry and text, then work within the limiting factors of the
technology. As an example, a topographical map is
converted by selecting a raster contour, then the software traces it to an intersecting or
gap position, converting the pixels to CAD geometry. This process is repeated for the
entire trace. Then an elevation is assigned, creating a three-dimensional model for the
GIS system to take over.
Hybrid Process - A fully hybrid approach is where
scanned archives and CAD systems are maintained for a drawing. The term hybrid in this
case means a combination of both raster (scanned) and vector (CAD) drawings. Hybrid
editing means using both raster data and vector data simultaneously. Changes can be made
within either environment. Information can be exchanged back and forth between two
distinctive formats, thus offering the most efficient method for modifying the old within
the new. Calibration between the raster database and
vector drawing model is typically provided with a reference or resource file. This file
contains scaling and coordinate transformations to provide a real-world coordinate system
on an otherwise unintelligent raster database. Deteriorated drawings can be scanned, cleaned up and stored
in raster. Modifications can be made to the drawing in raster, or areas of the drawing can
be converted into CAD vectors as it becomes necessary. This combination of raster and
vector can also be plotted and stored within more advanced EDM/PDM systems. Working in a hybrid environment allows use of the scanned
drawings immediately. Decisions to modify, plot or vectorize can be made as needed.
Investing time and money to convert existing drawings can be accomplished on a "just
in time" basis.
With reduced labor costs and improved usage of CAD, the
benefits of revising drawings electronically are clear. What may not be clear is the
trade-off of investing in the up front conversion to full CAD versus taking advantage of
lower cost hybrid and raster CAD systems.
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GTXScanClean™ is protected by U.S. Patent No. 7,016,536
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