AEC definitions
Drawing scale is the stated ratio between a distance on the sheet and the same distance in the building. A scale of 1/8 inch equals 1 foot means a line that is 1/8 inch long on paper stands for 1 foot of building. The scale is printed on the sheet. It is not a suggestion to measure the PDF.
Definition
Different drawings use different scales because they have different jobs. Floor plans are often 1/8 inch or 1/4 inch to the foot. Details are larger so a joint can be dimensioned. A graphic scale bar is a line you could, in theory, compare to a ruler if the sheet were plotted at the intended paper size. Most contracts say not to scale the drawings. Dimensions govern. A PDF zoomed on a screen has no reliable paper scale unless you verify the plot size.
In depth
In U.S. architectural practice, 1/4 inch = 1'-0" means one quarter inch on the plotted sheet equals one foot in the building. Engineers sometimes use a civil or decimal scale. Read the note on that sheet. Do not import the architectural scale onto the civil plan.
A detail can carry its own scale, different from the plan on the same sheet. The title under the detail is the one that applies. The sheet's primary scale does not win by default.
Sheets are reduced, tiled, and photographed. A PDF printed to 'fit' is no longer at the written scale. A dimension that was figured by the drafter is the instruction. A dimension you derived with a scale ruler from a fit-to-page plot is your own invention.
If a needed dimension is missing, send an RFI. Scaling is how walls land off the grid by an inch that nobody can defend later.
Riser diagrams, one-lines, and many schematics are NTS on purpose. The written sizes are the content. Line length is only graphic. Treating an NTS diagram as a small-scale plan double-counts the error.
A plan that was drawn NTS because it was hurried is not a schematic. It is an incomplete plan. Ask for dimensions.
The written scale assumes a paper size, often ARCH D or a stated sheet size in the title block. Half-size plots are common in the field, and a 1/8 inch scale becomes 1/16 inch on paper. People who know that carry a half-size ruler. People who do not, misread every dimension they scale.
The safe habit is not to scale either plot. Use the figured dimensions, which do not change when the paper shrinks.
In BIM the elements are modeled at full size and the sheet view applies a scale. A measurement in the model is not the same act as scaling a PDF. The contract still says which dimensions govern. A model measurement that contradicts a figured dimension on the sheet is a conflict to resolve, not a silent upgrade.
If the contract makes the model the deliverable, the agreement should say so. Otherwise the scaled sheet and its dimensions are what most contracts mean by the drawings.
Examples
A floor plan is noted 1/8 inch = 1 foot. A dimension string shows 30 feet. The bidder uses 30 feet, not a ruler on the PDF.
A detail on the same sheet is noted 3 inches = 1 foot. Its own title controls. The plan scale does not.
A half-size field plot makes the graphic scale look wrong under a full-size ruler. The figured dimensions are unchanged.
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