Tracing a Floor Plan to Scale From a Photo

The photo is not the problem. The scale is.

A phone photograph of a plan on a site table is perfectly good raw material. The lines are legible, the room names are readable, and the proportions are close enough to trace over. What it does not carry is any idea of how big anything is.

So the first thing you do after importing it is not trace. It is calibrate: point at two places on the image and say how far apart they are in the real world. From that one statement the software works out how many millimetres each pixel is worth, and every wall, every opening and every carpentry run you draw afterwards is measured in those units.

Get it wrong by five per cent and nothing looks wrong. The plan still draws, the walls still snap, the 3D still stands up. The kitchen is just quietly 300 mm shorter than the kitchen, and you find out when the carpenter does.

Three ways to set the scale

Which one you use depends on what the plan gives you, and they are worth knowing in order of preference rather than picking whichever is nearest.

• —Prefer a long dimension to a short one: the error in your two clicks is spread over more distance
• —Calibrate across a room, not across a door
• —If the plan carries two dimensions, set the scale from one and check it against the other

A printout is rarely at the scale printed on it

This is the one that catches people. HDB issues its residential floor plans as A4 at 1:100, and its own guidance is that to get the original 1:100 scale out of your printer you have to set the page sizing option to "Actual Size". The default in most print dialogs is "Fit to page", which shrinks the drawing by a few per cent to clear the printer margins and says nothing about having done it.

The result is a sheet that still has "1:100" written on it and is not at 1:100. Measure it with a scale rule and every reading is slightly short. Photograph it and calibrate from the paper scale, and you have imported that error into the drawing.

The defence is simple: never calibrate from the paper scale of a printout. Calibrate from a dimension written on the plan, because a printed number does not shrink when the page does.

Straighten it before you trace

A photograph taken standing over a table is never square to the paper. The plan is rotated a degree or two and often keystoned as well, and a degree of rotation across a six metre flat is about a hundred millimetres of drift from one end to the other.

Pick two points that you know are level in reality, the long edge of an external wall is the usual choice, and straighten the image against them. Do it before the first wall goes down, because rotating an underlay after you have traced half a flat means retracing half a flat.

Shoot it as square as you can while you are there. Stand over the middle of the sheet rather than at the edge, fill the frame, and take a second shot in case the first has a shadow across the dimensions.

Trace, then check two things

Tracing itself is the easy part: click the corners, chain the runs, type a number where you want the exact length rather than the traced one. Snapping catches the ends, the midpoints and the alignments, and ortho keeps you square where the building is square.

When the outline is closed, check two things before you go any further. First, measure one dimension in the drawing that is also printed on the plan and was not the one you calibrated from. It should match. Second, measure something you physically measured on site. That is the check that catches a plan that is out of date, which happens more often than a plan that is badly scaled, because somebody built a partition and nobody redrew anything.

• —Check a printed dimension you did not calibrate from
• —Check one dimension you took with a tape on site
• —If the two disagree, the site wins, and note where they disagreed

What the traced plan is actually for

It is not a submission drawing and nothing here is pretending otherwise. It is the thing that turns quantities from a matter of opinion into a matter of measurement, which is worth most at exactly the moment it is normally least available: before you have won the job.

A carpentry run on a scaled plan has a length. That length becomes foot run on the quotation, and the client can see where the number came from. A tiled area has an area. A ceiling has a perimeter. None of these has to be recalled from a photograph of a notebook at eleven at night, which is where the errors come from.

The same drawing then does a second job it costs nothing to do: it is already a model, so you can stand in it and walk the client through the room rather than describing it.