Motion Lab

Cairn / SN 4821 6630 / Est. 2016

Every number we give you comes with the error on it.

Anybody can hand you a contour drawing. The question is how far out it is, and the answer is a figure we are contractually happy to write down before we fly. Most of this page is that figure and where it comes from.

Request a quoteConcept work — fictional brand, built to demonstrate technique. Nothing is for sale.

Title block

Combined accuracy, 1σ
27 mm
Flight height
80 m
Ground control points
9
Ortho resolution, per px
15 mm

A survey is not a measurement. It is an estimate with a stated uncertainty, and a surveyor who will not tell you theirs has either not calculated it or would rather you did not know. The whole trade is arithmetic about how wrong you might be.

Land survey and drone mapping

The ground, at twenty centimetres

Scroll to fly the site. This is a wireframe of a surface with contour bands running through it at two-metre intervals — the deliverable, not a rendering of one. The mesh below is generated, but everything about the way it is drawn is how a real one is drawn: lines, not surfaces, because a surveyor reads a slope from the spacing of contours and a shaded render hides exactly that.

  1. Stage 1

    Ground control first. Nine targets on the site, each one occupied for twenty minutes, and everything that follows is measured against them.

  2. Stage 2

    Then the flight. Eighty metres, eighty per cent forward overlap, and about nine hundred frames for a site this size.

  3. Stage 3

    Contours last, at two metres. Where the lines crowd, the ground is steep — which is the one thing a shaded three-dimensional render will not tell you.

Where the error comes from

These do not add up. They combine in quadrature — the square root of the sum of the squares — because they are independent and it is very unlikely that all four are wrong in the same direction on the same day. That is also why halving the smallest term is a waste of a morning and halving the largest is the whole job.

The root of the sum of the squares of 8, 12, 22, 5 millimetres is 27 millimetres, at one standard deviation.

Combined, one standard deviation

  1. GNSS base8 mm9% of the varianceA two-hour static occupation on an OS passive station. Longer helps, but not much past two hours.
  2. Ground control12 mm20% of the varianceNine targets, each measured twice from different setups. This is the term we can buy down, and usually the one worth buying down.
  3. Photogrammetry22 mm68% of the varianceBundle adjustment residual at eighty metres. Fly lower and this falls; fly lower and the job takes two days.
  4. Target centring5 mm3% of the varianceA person putting a tripod over a nail. Small, irreducible, and the one nobody ever lists.

The figure in the title block above is this sum — 27 mm — and not a number typed in beside it, which is the same reason we would rather show you the arithmetic than the adjective.

What you actually get

Five files, and four of them are derived from the first. Surveyors talk about this as a stack because that is what it is: everything below is measurement and everything above is interpretation, and you should know which is which before you build off it.

  1. Contours

    DWG / DXF · 18 MB

    Two-metre intervals, half-metre where you ask for them. The file everyone actually opens.

  2. Orthomosaic

    GeoTIFF · 2.1 GB

    Nine hundred frames, corrected for lens and terrain so you can scale off it. 15 mm per pixel.

  3. Digital terrain model

    GeoTIFF · 640 MB

    The same, with everything that is not ground classified out. This step is judgement and we will tell you where we made a call.

  4. Digital surface model

    GeoTIFF · 780 MB

    The cloud, gridded at 50 mm. Includes the hedge, the shed and the parked van.

  5. Point cloud

    LAS 1.4 · 4.2 GB

    The measurement. Every other file here is an opinion about this one.

    ↑ Everything above this line is derived from this one

What a survey costs

Priced by what has to be occupied on the ground, not by hectares flown — the flight is the cheap part and always has been. A second visit to the same site inside a year reuses the control and comes in at about half.

Topographic, under 5 ha
5 points
4 working days
£1,450
Topographic, 5–25 ha
9 points
6 working days
£2,900
Volumetric stockpile
4 points
2 working days
£880
Measured building survey
scanned
8 working days
£3,400
Repeat visit, same site
reused
3 working days
£1,320

Before you compare quotes

Ask the other quote what their combined accuracy is

It is a fair question and it has a number for an answer. If it comes back as "survey grade" or "centimetre accurate" without a figure or a confidence interval, that is not a specification — it is a word. Ours is twenty-seven millimetres at one sigma and it is in the contract.

Send a site boundaryConcept work — fictional brand, built to demonstrate technique. Nothing is for sale.