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What is the difference between measuring with LiDAR, with camera photos, and drawing it yourself?

LiDAR measures distance directly, with light. Camera-based methods compute distance from images. Draw it yourself, and the measurements come from your tape measure. The three methods fail in different places — here is where.

LiDAR: light, timed

LiDAR is a small rangefinder sitting next to the camera lenses on the iPhone Pro models from iPhone 12 Pro onwards, and on iPad Pro from 2020 onwards. It sends out short, invisible pulses of light and measures how long they take to hit something and come back. The speed of light is known, so the time becomes a distance.

The strength: it measures directly. A plain, single-colour white wall in a dark room is the hardest thing there is for a camera, and completely unproblematic for LiDAR. It has no opinion about what it sees — only how far away it is.

The weakness: glass and mirrors. The light pulse passes through the pane or reflects onward, so a large glass wall can look as if the room continues out into the garden. Glossy black surfaces also return too little light. And the range is limited — a very large hall is best measured by walking closer to each wall.

Camera-based methods: distance computed from images

The other family of measuring apps uses only the camera. They compare how points in the image shift as you move the phone, and from that compute how far away things are. Some ask you to tap the corners yourself; others try to find them automatically.

The strength: it works on any phone. No special sensor is needed.

The weakness: the method needs something to recognise. A plain white wall with no pattern gives no points to follow, and a dimly lit room gives a noisy image. Precisely those two conditions — white wall, dim light — are extremely common in homes. Move the phone too fast, on top of that, and the computation loses track.

You know the same difference from the phone's Portrait mode: the depth there is computed from the image and good enough to blur a background, but not to establish that the wall is 3.42 metres away.

Drawing it yourself: the measurement comes from your tape measure

The third method has no sensor at all. You drag the rooms out on a sheet of squared paper on the screen and type the measurements in, in centimetres — measured with an ordinary tape measure or a laser distance meter.

The strength: it works on any device, it works without you being in the home, and it can draw what does not exist yet — the wall you are considering removing, the extension that is on the drawing board. No sensor can measure a wall that hasn't been built.

The weakness: it only knows what you yourself have seen. The pier behind the bookcase, the recess behind the door, the chimney breast in the kitchen, and the 6 cm the room is out of square — all of it only comes along if you spotted it and measured it yourself.

In HouseSense, the drawing board works in internal measurements, from wall face to wall face, in whole centimetres — the same language as the tape measure. Walls snap to their neighbour and become one shared wall, so two rooms never count the same square metres twice.

The three methods, side by side

Dark room: LiDAR works. Camera-based method struggles. Tape measure works (turn the light on).

Plain white wall: LiDAR works. Camera-based method has nothing to recognise. Tape measure works.

Large glass wall: LiDAR is weak — the light passes through. Camera-based method is also weak. Tape measure works.

Wall behind a built-in wardrobe: both sensors measure the cabinet front. Tape measure works, if you open the cabinet.

Room you are not in: no sensor works. Only drawing.

Recesses and projections you didn't know about: LiDAR finds them. Camera-based method can find them. Drawing never finds them on its own.

A wall due to come down next year: no sensor can draw it away. Only drawing.

What decides the precision — for all three

None of the methods has one fixed precision figure, and you should be sceptical of anyone who promises you one. What actually moves the number is:

• What the sensor hit. A 2 cm skirting board, a 5 cm curtain or a 10 cm radiator in front of the wall makes the room smaller than it is — whichever technology is measuring.
• What height it was measured at. Real walls are not vertical. Measure at around 1 metre's height; that is the height at which furniture and kitchen units meet the wall.
• Whether the room is out of square. A 5-8 cm difference end to end is normal in older houses and invisible to the eye.
• Whether anyone checked afterwards. A takeoff that hasn't been checked against a single tape measure is a guess in tidy packaging.

The sums: when the difference matters

A room measures 3.60 × 4.20 m = 15.12 m². If the width is 5 cm too small, you get 3.55 × 4.20 = 14.91 m² — a difference of 0.21 m², or a good 1 %.

If you're painting or laying flooring, that 1 % disappears into the 5-10 % waste allowance you're adding anyway. If you need a 3.58 m run of kitchen units to fit along that wall, those 5 cm are the difference between it fitting and it not.

So the same rule applies regardless of method: use the floor plan to plan, and the tape measure to order.

What HouseSense does

HouseSense has two ways in: a LiDAR scan on the devices that have the sensor, and the drawing board on every other device — and for anything that isn't built yet. There is no camera-based computation in between, precisely because white walls and dim light are everyday reality in a home, and because a computed measurement that looks measured is more dangerous than a measurement you took yourself.

Once the rooms are in, there is no difference between the two paths: area, floor plan, 3D model, sunlight, takeoffs for contractor quotes and reports are all worked out from the same geometry. It all lives in your own iCloud, with no account and no server.