Every metre in a bill of quantities is a distance on the drawing times the scale. If the scale is wrong, every metre is wrong, by the same factor, and no single line shows it.
Why 1:100 is not always 1:100
The scale in the title block holds for the drawing at the paper size it was made for. An A1 drawing printed on A3, or a PDF that was scanned again, has shrunk on the way, and 1:100 is no longer 1:100. A photo of a drawing has no scale at all until you give it one.
Set the scale against a known size
The safe way is to measure against something whose length you know. Find a size printed on the drawing, ideally a long one: a building face, the distance between two grid lines, or a boundary with its length given. Click its two ends and type in the length. The longer the size, the less a click a few pixels out matters.
- Pick a size that runs along the drawing, not a short detail dimension.
- Zoom in before you click the end points.
- If the drawing has several pages, the scale only holds for the page you set it on.
Check before you measure
Then measure another known size on the same page and see that it agrees. If it is right to within a decimetre or so over ten metres, the scale is set correctly. If not, set it again before you draw a single run: everything drawn before then would otherwise have to be redone.
In Mängda you set the scale with two clicks and the length in metres, or pick it from the list if it is printed on the drawing. Each document has its own scale, so a section and a plan in the same project can differ.
Sections are their own thing
A section drawing shows width and height, in millimetres, not distances on the ground. It often has a different scale from the plan, and it is not for measuring lengths. Read the sizes off the section and the lengths off the plan.