PDF inaccuracies from CAD files


Software version: 2020

Last updated May 28, 2020 by Tekla Support US tekla.support.us@trimble.com

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PDF inaccuracies from CAD files

This article attempts to help explain how accurately produced CAD drawing files are critical in order for printed PDFs to be used in 3D modeling platforms as overlay references. Many times, firms will insert a 2D CAD file into a 3D model space in order to properly lay out the basic grid and shape of a structure. This "overlay" needs to be accurately created and inported for this to be beneficial.

There are several considerations to be made before deciding to go down this path of 2D information collaboration:

  • Drawing accuracy: Was the original CAD file produced accurately and not just created as a best guess document? Meaning, does 12 feet 3 inches on the drawing actually equal 12 feet 3 inches, or is it just “close?”
  • Scaling: Was the PDF printed at a specific scale, or was the file produced at a fit to paper scale?
  • Raster vs. Vector graphics: Is the produced PDF file a raster or vector graphics file?

These questions need to be addressed before moving forward with producing a “construction accurate” PDF drawing from a CAD drawing file.

Drawing accuracy

Many drafters, engineers, and architects will produce a set of drawings with dimensions that may simply be close to exact. Meaning, they drew an object at 12 feet and simply edited the dimension to read 12 feet 3 inches as per design. This is a quick way to produce drawings when a deadline is near, but is not very useful or helpful later in the BIM process. That said, this method is generally against most firms’ policies, but nonetheless is many times done in order to produce the drawings on time. This can cause modeling errors if using the PDF as an overlay in a model.

Example:
 

Scaling

Scaling is another factor that plays into the accuracy of the BIM process and is directly affected by dimension editing previously mentioned. Assuming the dimensions are exact, the best options for producing a PDF for an overlay is to print the PDF at a one to one scale. This way the file will not have to go through any factor scaling that could cause an inaccuracy. Consider, a PDF printed as “Scale to fit.” When this technique is employed, the original drawing - no matter how accurate - is now scaled in multiple directions simply to fit the drawing on a sheet of paper.

Take two dimensions, one in the “X” direction and one in the “Y” direction: When measured after scaling, the dimensions may not return the correct result. This can cause lines to not intersect where they should because the software is trying to maintain the correct length in multiple directions. Notice in the image below that the lines do not completely meet at the same spot. This case was caused by the scale to fit scenario just mentioned.

Example:
 

Raster vs. Vector graphics

It is critical to know if the PDF file you are using is a Raster or Vector image file. A raster file is essentially a collection of pixels on the PDF that appear to form the desired shapes, lines, and text clearly. When simply viewed in a PDF viewer/editor, this is not an issue, but for a file that needs to be converted to a CAD file this is a problem. When the conversion process is initiated on a raster image, the software comes to an intersection of two lines, curves, etc. and the system cannot decide what the pixels (dots) are associated with and cannot create the CAD object needed.

Vector on the other hand, is specifically designed to create mathematically clean and crisp lines and shapes. Vector images will maintain that accuracy because of these calculations. Therefore, when the file is converted to a CAD file, the lines and shapes will transfer accurately.

Example:
 

Generally speaking: If the PDF was printed from a CAD software, it will be a vector file. If the PDF was creaed by scanning a paper drawing, it will probably be a raster file. 

Other tools and considerations

After determining that you want to use PDF insert and conversion for overlays in your 3D models, you will want to check the files after they are inserted with the measure tools inside Tekla Structures. When inserting a PDF to Tekla Structures the system will create a DXF file from the PDF based on the vector graphics. This tool is available through File > Import > Insert PDF. Learn to insert a PDF by clicking this link: Insert PDF

 

If needed, there is a tool on the Warehouse that will help scale a PDF after conversion to DXF named, “Reference model scale tool.” This tool will use two user-selected points to determine the correct scale of a drawing based on a user-input dimension. This tool will appear in the Applications & components side pane. See this link to learn how to use this tool: Reference model scale tool

 

After inserting the PDF and scaling the file properly, you will need to move the file into place by selecting the overlay and using the Right-click > Move option.

A recommendation for using the 2D file inserted into a 3D model is to get a copy of the original CAD file and simply import the DWG, DXF, DGN to Tekla Structures. These files will not suffer the same fit to paper or raster vs. vector issues to which PDFs are susceptible. We know that this is not always an option, so we do offer the PDF conversion path discussed in this article.

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