HOW ACCURATE IS A SURVEY? RICS ACCURACY BANDS, LOD AND SCALE EXPLAINED

What does survey accuracy actually mean?

It means how far a measured point can be from its true position, and how confident the surveyor is in that figure. A tolerance of ±20 mm tells you the size of the possible error. It doesn’t tell you how often a point might fall outside it, or what the point was measured against. Both matter.

The RICS guidance answers both. First, every accuracy figure is taken relative to the survey control: the network of fixed points the whole survey is tied to. Spatial Dimensions ties its control to Ordnance Survey grid and datum as standard. Second, accuracy is stated statistically, using a measure called sigma, or standard deviation:

• 1 sigma: about 68% of points fall within the stated figure.
• 2 sigma: about 95% of points fall within it. This is the figure most clients mean when they ask for an accuracy.
• 3 times the 1 sigma figure: about 99.7% of points fall within it.

So “±10 mm at 1 sigma” and “±10 mm at 2 sigma” are different promises. The first is half as tight as the second. When you compare two quotes, check which one each surveyor means.

What do plan and height accuracy mean?

They are measured separately, because a survey can be tighter in one direction than the other. Plan accuracy (X, Y) is position on the ground or floor. Height accuracy (Z) is level.

Height is then split again:

• Hard detail: firm, well-defined surfaces such as kerbs, floor slabs, walls and steps.
• Soft detail: surfaces with no crisp edge, such as grass, planting and natural ground.

Soft detail is always looser, because there is no single true surface to measure to. In the RICS table, Band D allows ±10 mm in height on hard detail but ±25 mm on soft detail. A good survey states both.

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What are the RICS accuracy bands?

They are a set of standard accuracies, from Band A (the tightest) to Band J (the loosest), that the client chooses from for each part of a survey. They were introduced in the third edition of the RICS guidance to replace the old habit of setting accuracy by plot scale, because digital survey data and BIM models don’t have a fixed scale.

The bands most building and site projects use are C to F:

The RICS guidance gives its own example of choosing a band: a client who needs 10 mm plan accuracy at 95% confidence should choose Band C, because Band C is ±10 mm at 2 sigma. The legacy scale column is only there to help people who learned surveying by plot scale. It is not part of the specification.

A client can also set a custom band for one feature or one direction. For example, a tighter height band for floor levels in a lift shaft than for the rest of the building.

How do accuracy bands relate to LOD and drawing scale?

They describe different things, which is why they get mixed up. An accuracy band says how close each point is to its true position. Level of Detail (LOD) says how much is drawn or modelled, and our LOD guides attach a tolerance to each level. Drawing scale is how the result is printed, and since the third edition it no longer sets the accuracy.

For 3D models and point clouds, Spatial Dimensions publishes each LOD with its RICS band. These are the 2 sigma (95%) plan figures:

For 2D measured building survey drawings (plans, elevations, sections and reflected ceiling plans), our 2D LOD guide sets a separate tolerance for each level:

The two tables don’t match, and they shouldn’t be blended. Two mistakes come up often:

• ±5 mm is not a plan accuracy for a model or point cloud. At LOD 4, a 3D model is ±10 mm in plan. The ±5 mm figure is its height accuracy on hard detail, or the tolerance for LOD 4 2D drawings.
• An LOD 3 model is ±20 mm in plan, not ±10 mm. The ±10 mm figure is the LOD 3 tolerance for 2D drawings, or the model’s height accuracy on hard detail.

The safe rule is to name the deliverable in the same sentence as the figure. “LOD 4 elevation drawings to ±5 mm” is right. “An LOD 4 point cloud to ±5 mm” is not.

One more term to keep apart: deliverable “Level”. On a 3D topographical survey, Spatial Dimensions offers four deliverable levels: 3D points, 3D contours, a 3D mesh (a digital terrain model) and a BIM model. That describes what you receive, not how accurate it is. A Level 4 BIM model is delivered at LOD 3.

How accurate is a topographical survey?

For a 3D topographical deliverable, Spatial Dimensions works to LOD 3, which is RICS Band D: ±20 mm in plan, ±10 mm in height on hard detail and ±25 mm on soft detail. For a 2D topographical drawing there is no single published tolerance. The RICS guidance has the client choose a band, and topographic surveys usually sit in Band D or Band E. We state the band in writing at quotation for each job.

The control matters as much as the band. A survey can meet its band perfectly and still be wrong if it sits on an arbitrary grid. Our infill topographical survey article covers why control on and off the site, tied to Ordnance Survey, is worth asking about. There is more on our topographical surveys page.

How accurate is an underground utility survey?

Less accurate than a building survey, and honestly so, because the pipes and cables are underground. Spatial Dimensions states utility depth as a percentage of depth: ±15% when two detection technologies, electromagnetic location (EML) and ground penetrating radar (GPR), are used together, and ±40% with one technology.

The RICS table shows where each PAS 128 quality level sits. PAS 128 is the British Standards specification that grades utility surveys from Quality Level D (a desktop search) to Quality Level A (verified by exposing the service). In the RICS table, QL-A verification sits in Band E, QL-B1 detection in Band G and QL-B4 detection in Band I. In plain terms, a detected service is located to tens or hundreds of millimetres, not to the few millimetres of a measured building survey. Where that isn’t enough, QL-A trial holes confirm the position. Our underground utility surveys page sets out the quality levels in more detail.

Which accuracy do you need?

The one that matches what the survey will be used for, and no tighter. A tighter band means more instrument set-ups, more checks and more time on site, so it costs more. The RICS guidance advises clients to take advice on the cost and quality implications of the band they choose.

As a starting point:

• Planning drawings and rights to light: 2D drawings at LOD 2 (±20 mm).
• Architectural design: 2D drawings at LOD 3 (±10 mm), or a 3D model at LOD 3 (Band D).
• Heritage recording and inspection: 2D drawings at LOD 4 (±5 mm), or a 3D model at LOD 4 (Band C).
• Topographical surveys for design: Band D or Band E, agreed at quotation.

It also helps to know how accuracy behaves between two points. The RICS guidance combines the accuracy of both points, then adds an allowance for the control over distances beyond 100 m. Its own worked example, a Band B point and a Band D point 200 m apart, comes to 13 mm. So the distance between two features can be slightly less accurate than either feature on its own. On a long building or a large site, that is worth knowing before you set a tolerance on the drawings.

What can’t an accuracy figure tell you?

An accuracy band doesn’t decide what gets surveyed. The RICS guidance is clear that the band sets how accurately each feature is positioned, not how much detail is shown. A Band C survey can still leave out the cornice you needed, if nobody asked for it. Level of Detail and the drawing schedule do that job.

It also can’t be proved without spare measurements. The RICS guidance notes that where there are no repeated or redundant observations, survey accuracy cannot be verified. That is why Spatial Dimensions checks every survey before it is issued, and why it’s worth asking any surveyor how their data is checked.

Finally, the figure only covers what could be seen and reached. Dense vegetation, stored materials or a locked room limit what can be measured at all. We say before the survey where access or clearance will affect the result, rather than leaving gaps to be found later.

How much does an accurate survey cost?

A measured building survey from Spatial Dimensions starts from £850, and most jobs fall between £1,500 and £15,000. Accuracy is one of the price drivers, along with the size of the building, the Level of Detail and the deliverable.

The ranges are the span across each whole service, not one job. A price match guarantee applies to measured building, topographical, utility and 3D model work. All prices exclude VAT. We will come back with a written quote within 24 hours.

What does this mean for architects and design teams?

It means asking for accuracy in writing, in a form you can check. On every quote, look for four things: the deliverable, the Level of Detail, the RICS band (or the tolerance) and the confidence level. If a quote says only “accurate survey”, or gives a ± figure with no deliverable attached, ask.

Spatial Dimensions states all four at quotation, uses permanent in-house surveyors and CAD technicians rather than subcontractors, and carries £10m of insurance across professional indemnity, public liability and employer’s liability.

Survey accuracy FAQs

What is survey accuracy?

It is how close each measured point is to its true position, measured against the survey control and stated with a confidence level. Spatial Dimensions states accuracy as a tolerance or an RICS band for each deliverable, in writing at quotation.

What are the RICS survey accuracy bands?

They are standard accuracies from Band A (the tightest) to Band J (the loosest), set out in the RICS guidance note Measured surveys of land, buildings and utilities, 3rd edition. The client chooses a band for each feature, instead of setting accuracy by drawing scale.

What is the difference between 1 sigma and 2 sigma accuracy?

About 68% of measured points fall within a 1 sigma figure, and about 95% within a 2 sigma figure. So ±10 mm at 2 sigma is a tighter promise than ±10 mm at 1 sigma.

Is Level of Detail the same as accuracy?

No. Level of Detail sets how much is drawn or modelled, and each level carries its own tolerance. An accuracy band sets how closely each point is positioned. Spatial Dimensions publishes both for every LOD.

How accurate is a Spatial Dimensions 3D model?

At LOD 3, a Spatial Dimensions 3D model or point cloud is ±20 mm in plan and ±10 mm in height on hard detail, which is RICS Band D. At LOD 4 it is ±10 mm in plan and ±5 mm in height on hard detail, which is Band C.

Which accuracy band should I ask for?

The one that matches the survey’s use, and no tighter, because tighter bands take longer and cost more. Most measured building and design work sits in Band D, heritage recording in Band C, and topographical surveys in Band D or E.

Spatial Dimensions has carried out measured building, topographical and utility surveys for architects, engineers and developers since 2012, with permanent in-house teams of surveyors, CAD and Revit technicians and project managers. Our offices are in Sevenoaks, London and Nottingham. Our Managing Director, Claire Fenwick FCInstCES, is President of the Geospatial Survey Association. Call us on 01732 458799 or get in touch.

Specifying a measured building survey? Download our Level of Detail in 2D Measured Building Surveys guide.