Measurements and Accuracy

 

Unlike CAD formats where basically the line data is stored as if all lines have zero widths for the endpoints the PDF format stores the endpoints of lines based on their thickness, endcap styles, and line merge styles.  CAD was meant for high accuracy, where PDF was meant to be a simple paper based format.

 

So basically in CAD a 10 inch long line is always going to measure 10 inches where in PDF a 10 inch line that is 0.25 inches is thick with round endcaps is going to measure as 9.75 inches (10 - (0.25 * 0.5) * 2) where a line 0.125 inches thick would measure as 9.875 inches.

 

Also CAD formats store the values in high double precision accuracy based on real world coordinates and a scaled viewport where the PDF format is a paper based format based on the DPI it is created at.  So a 400 dpi PDF would at the best have an accuracy of 0.0025 inches (1/400).  While this may seam like a high resolution when you start talking about building or roadway design and the scale factors they use it can quickly throw off your measurements.

 

Because of this we always say that you have to think of PDF formats as being similar to your manual drawings and using an engineering scale.  You can get a close measurement off them but never an exact measurement when you start looking at large scale factors used in engineering.

 

And while we could look at automatically rounding the display of the dimensions we have chosen to stick to the way that Adobe works with their measurement annotations in their PDF files.  Some of our competitors will round the dimensions but if you open that PDF in Adobe Acrobat or most other PDF Editors and someone moves the annotation or something it reverts back to the non-rounded value based on the actual scale factor to use.