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How to Modernize Your Paper Engineering Drawings
A White Paper From the Paper to CAD Experts

The Revision Cycle

Raster Drafting

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 today’s 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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