Knowledge base
From PDF or paper drawing to model
Not every project comes with CAD files. A model can also be built from a PDF or a paper drawing, but that takes extra information and extra checking. How it works, where the limits are and what you should check yourself.
Why CAD is preferred
A DWG or DXF file contains the drawing as the designer made it: lines with real coordinates, often in the correct system and sometimes with levels already. A PDF is essentially a print. Even if the lines look sharp, the link with the coordinates is lost and levels only survive as text.
So first ask the designer or engineering consultant whether the digital files are available. That saves time and gives a more accurate model. If that is not possible, a PDF can still be used.
Scale and dimensions
A paper or PDF drawing is only correct at the scale it was drawn at. Printing, copying or scanning can make that scale drift slightly, and not always equally in both directions. A stated scale such as 1:200 is therefore a starting point, not a guarantee.
The drawing is scaled using known dimensions: a dimension line, a coordinate grid, the distance between two points with known coordinates. The more of these references are spread across the sheet, the better any scale drift can be detected and corrected.
What information helps
Because a PDF carries less information than a CAD file, supporting information matters all the more:
- levels and spot heights on the layout plan;
- long and cross sections, with chainage or distances;
- reference points or control stations with coordinates and height;
- a coordinate grid on the drawing;
- the coordinate system and height datum the project is drawn in.
How the model is built
The drawing is first georeferenced: scaled and placed in the right position in the coordinate system using the reference points or the grid. The relevant linework is then traced (digitised): centrelines, edges, kerbs, slopes.
Those lines are then given their levels from the spot heights and the sections. Finally, the result is checked against the dimensions and levels on the drawing. Where a measured distance or level does not match the drawing, it is reported rather than quietly adjusted.
Limits and what you should check
The accuracy of a model built from a PDF depends on the quality of the document, the line thickness, the scale and the number of reliable references. A scan of a creased or repeatedly copied drawing is the least accurate. Levels are usually more reliable than horizontal position, because they appear as numbers on the drawing.
Check the model with extra care before starting: on several known points, both in position and in height, ideally spread across the whole site. When in doubt about a dimension, the designer’s drawing prevails, not the model.