All articles · 7 min read

Why does a scan get less accurate the further I walk?

Because the phone is constantly working out its own position from the position before it — and each small error stacks on top of the next. Here is what drift is, where you see it, and how to keep it down.

What drift is

LiDAR measures the distance from the phone to the wall precisely. But to place that wall in the right spot in the home, the phone also needs to know where it was standing when it took the measurement. And it does not know that in advance — there is no indoor GPS signal accurate enough for centimetres.

Instead, the phone continuously works out its own movement: it recognises details in the camera image from frame to frame and combines that with the motion sensors. "Since the last frame, I have moved 8 cm forward and turned 2 degrees." Then again. And again, many times a second.

Each of these small calculations carries a small uncertainty. Because each new step builds on the last, the uncertainty accumulates. After ten metres it is small. After walking fifty metres through a house it is larger. That is drift: not an error in the rangefinder, but an error in the phone's idea of where it is standing.

Why one room is almost always better than the whole house

Scanning a single room might mean walking 12 metres in total. Scanning a two-storey house can easily mean 80-100 metres, in and out of rooms, up a staircase and back down again. The second route has far more links in the chain, and therefore more accumulated uncertainty.

That is also why the error typically shows up towards the end: the first room you scan is close to the phone's starting point and sits correctly. The last room has the whole journey behind it.

Practical consequence: if there is one room in the home you really need to get right — the one getting a new floor, or the kitchen that is being rebuilt — scan it first, or scan it on its own.

Where you can spot drift in the drawing

Drift looks different from the other scanning errors, and it is easy to recognise once you know what to look for:

• The house does not close up. If you walk all the way around a home and end where you started, the drawing should meet up cleanly. If there is a gap between the first and last wall — as if the plan does not quite add up — that is drift.
• Rooms measure right individually, but sit at an angle. The living room measures correctly, the kitchen measures correctly, but they are rotated a couple of degrees relative to each other.
• One long wall has a kink in it. An unbroken outer wall running past three rooms picks up a small bend exactly where you walked through a door.
• Floors are offset in a multi-storey home. The staircase is the longest and hardest stretch in the whole scan.

Note that drift does not make an individual room measurement wrong. Measure a living-room wall afterwards with a tape and it can match perfectly, while the plan as a whole is still skewed. These are two different kinds of accuracy, and they need to be checked separately.

What makes drift grow

1. Speed. Fast movement produces blurred camera frames, and a blurred frame has fewer details to recognise. Speed is the biggest cause and the easiest to remove.
2. Bare, plain surfaces. A long white corridor with no pictures, doors or furniture gives the phone almost nothing to lock onto. Empty, freshly painted homes are harder to scan than furnished ones.
3. Poor light. The distance measurement copes fine in the dark, but position tracking relies on the camera image — and a dark frame has few usable details.
4. Staircases. A long stretch, changing light, few recognisable surfaces, and movement in three directions at once.
5. Long, straight corridors. Everything looks the same all the way along, and the phone struggles to tell how far it has come.

The five things that keep drift down

1. Walk at a calm pace

Slower than you think. Turn corners by walking around them, not by swinging the phone. Pause for a couple of seconds in each corner of the room.

2. Close the loop

Finish your route where you started, and let the phone see the same corner again at the end. If it recognises somewhere it has already seen, it can correct its position against that — and some of the accumulated error disappears.

3. Keep the scan running through doorways

If you stop the scan in one room and start a new one in the next, the app has no way of knowing how the two rooms connect. Walk slowly through the doorway with the scan still running, and let the camera see the door frame from both sides — the frame is what stitches the two rooms together.

4. Break up large homes

A home of several hundred square metres, or spread over several floors, does not need to be one unbroken scan. Scan a floor, save it, and take the next one separately. Shorter routes mean less accumulated error in each — and you can always line them up afterwards against one known measurement, such as the position of the stairwell.

5. Turn on every light and leave doors open

Lights on everywhere, including rooms you only pass through. Open doors give the phone a view of something recognisable in the next room before you even get there.

How to measure how much your own scan has drifted

There is a simple check that catches drift — and it cannot be done by measuring a single room, because drift sits between rooms:

1. Find a measurement that runs across several rooms. For example, the home's total internal length, measured from an outer wall at one end to an outer wall at the other, or the distance from the front door to the furthest outer wall.
2. Measure it for real. If you do not have a tape measure long enough, measure it in stages and add them up — remember to add the wall thicknesses if you are measuring room by room.
3. Look up the same long measurement in the scan and subtract.

Example: the home's internal length measures 1,240 cm in reality. The scan says 1,262 cm. The difference is 22 cm, which is 22 ÷ 1,240 × 100 = 1.8% over a route of just over 12 metres. If you also check a single living-room wall and find it matches to within a couple of centimetres, you know what you are dealing with: the rooms are measured correctly, but they sit too far apart. That is drift, not poor distance measurement — and the fix is scanning in shorter routes, not scanning more carefully within a single room.

What drift means for what you actually use the drawing for

• Room areas and materials: largely unaffected. Drift shifts rooms relative to each other, but does not make an individual room larger or smaller.
• The home's overall outline and facade lengths: affected. If you need the home's outer dimensions, that is what you should double-check.
• Floors that need to line up above each other: most affected. The staircase is the stretch that accumulates the most error, and an offset floor is obvious once the plans are laid on top of each other.

What HouseSense does

HouseSense joins the rooms into one continuous home based on the geometry the phone measured, and states every measurement as an internal measurement, wall face to wall face, so you can check it with a tape measure. The app does not quietly move rooms around behind the scenes to make the plan add up — a drawing that closes the house up neatly by changing the numbers would be lying about what was actually measured.

The principle is that the drawing may be tidied up, the numbers may not: lines can be straightened so the drawing reads cleanly, but a measurement is never changed to make the geometry look right. If you spot drift in your scan, the answer is to scan again in shorter routes — or to draw the home on screen from your own tape-measure figures. A drawn home has no drift; it only has the measurements you typed in yourself.