How to tell which one you have
- Zoom in a long way. Lines in a vector PDF stay sharp at any zoom. Lines in a scan turn into blocks of pixels or go soft.
- Try to select text. A PDF exported from CAD software usually has selectable text. A scan is a picture, so its text cannot be selected, unless an optical character recognition (OCR) layer was added afterwards.
- Look at the edges of the page. Shadows, a slight tilt, or a curved edge mean the plan was scanned or photographed.
What vector plans are good for
In a vector plan every wall is a set of lines with exact coordinates, and the dimension text is real text. That makes it possible to measure to a small fraction of an inch, provided the scale is right. Even so, a vector plan can mislead you: a drawing can be marked not to scale, a sheet can hold several views at different scales, and a PDF can be resized after it was drawn. Always confirm the scale against a printed dimension. The guide to measuring a PDF floor plan shows how.
What limits a scanned plan
- Resolution. One pixel on a 150 dpi scan is 1/150 inch of paper. On a 1/4" = 1'-0" plan, that is about 0.32 inch of building. At 300 dpi it is about 0.16 inch. A 6 inch wall is only about 19 pixels wide at 150 dpi, so its edges cannot be placed exactly.
- Stretch. Scanners and copiers can stretch a page slightly, and more in one direction than the other. Test the scale along both the long and the short side.
- Skew and perspective. A page fed in crooked is rotated, and a photograph taken at an angle is distorted, so parallel walls are no longer parallel. Straighten the image before you measure it.
- Small text. OCR can misread small print, including the scale note itself. Check a scale that was read from a scan by eye.
- Faint or hand-drawn lines. Thin pencil lines and sketches leave the edges of walls open to judgment.
A practical rule for each
For a vector plan, confirm the scale against a long printed dimension, then measure. For a scan, confirm the scale in both directions, straighten the page first, expect more error, and round the result to what the scan can support: a figure good to about a foot should not be quoted to the inch.
When the numbers feed an order
Material quantities magnify measurement errors. A scale that is 3 per cent off makes every area about 6 per cent off, because area grows with the square of length. That is why it is worth ten minutes to confirm the scale before measuring a scan, and to keep a record of how it was confirmed.
How QWIKIT Plans treats each kind
QWIKIT Plans reads floor-plan PDFs, including scanned or photographed pages, which it can straighten first. It will make mistakes sometimes, especially on scanned, hand-drawn or crowded plans, so results are checked by hand during the pilot, and you should check them too. The software is not released yet.