Example survey types/use

Transcription

1 Measured surveys 3 rd ed Survey accuracy banding In previous editions of this guidance accuracy has been determined relative to the scale at which a survey map or product will be output. While this is still an important consideration for the capture of survey features, technological changes brought about by digital data handling have made this approach less useful to survey users. In this edition RICS has introduced the concept of survey accuracy banding. This concept is introduced for both survey detail, the features surveyed and displayed on maps/drawings, and survey control, the reference points which connect all the features to a common grid and determine their relative and absolute accuracy over the full survey extents. Note Accuracy in surveying is generally understood as the closeness a measurement has to its true or infinitely perfect measurement. It is not to be confused with resolution, which is related to the smallest increment of measurement (i.e. kilometres, metres, centimetres millimetres) or precision, which is related to the closeness of repeated measurements of the same point. Accuracy values in surveying are generally expressed as standard deviations or sigma (σ) values. Standard deviation is based on the normal distribution of random errors and has certain statistical properties. One standard deviation or one sigma (σ) means there is 68.3 per cent confidence that all measurements will have errors not greater that this value. At 2 times the standard deviation value or 2 sigma there is an 95.4 per cent confidence all measurements will be equally to or better than double the std dev. In small samples it is assumed that all measurements are expected to adhere to the 3 sigma (σ) rule. Standard deviation of normal distribution does not include systematic errors (biases) and gross errors (mistakes) which should be removed from survey data by the survey supplier.

2 Measured surveys 3 rd ed Survey detail accuracy banding The survey detail accuracy banding table shall be used to define what accuracies are applicable to surveyed features independent of scale. Where a client requires a bespoke or customised accuracy band they shall complete the row for custom accuracy in band X-Y for plan and band Z for height on the table. The accuracy band for each feature shall be confirmed in the survey feature tables listed throughout this specification to ensure the client s accuracy requirements for different features are clearly defined. The accuracy banding table includes guidance for users by way of examples of the types of survey and scale of output that may be appropriate for different accuracies. However, this is not an exhaustive list of examples. Clients should ensure that both the accuracy band chosen and the survey output are appropriate to their needs. Where a client is unsure they should seek professional advice from competent survey practitioners, such as a chartered geomatics member of RICS. Note The accuracy banding table does not determine the level of detail shown for each feature, although it does indicate the minimum size of feature that will be shown true to scale and not symbolised. In general, features will be surveyed by the minimum number of points required to show their geometric position or extents. For example, a tree can be described by a centre point with trunk diameter, spread diameter and a ground and crown level or height. Where clients require more specific levels of detail on a feature to be surveyed, above the minimum required to describe the location and extent, they should confirm this in the type of survey outputs required (i.e. imagery, scan clouds, bespoke measurement or output requirements).

3 Measured surveys 3 rd ed Survey detail accuracy banding table Plan accuracy (X,Y) Height accuracy (Z) Note Band Std dev. Band Hard detail std dev. Soft detail std dev. Example survey types/use Legacy Output Scale Min size of feature shown to scale (not symbolised) A +/- 2mm A +/- 2mm N/A Monitoring, high accuracy engineering setting out and fabrication surveys B +/- 3mm B +/- 3mm N/A Monitoring, high accuracy engineering and measured building surveys and setting out C +/- 5mm C +/- 5mm N/A Engineering surveying and setting out, high accuracy measured building surveying, heritage recording D +/- 10 mm D +/- 10mm +/-25mm Engineering surveying and setting out, measured building surveys, high accuracy topographic surveys, determined E +/- 25 mm E +/- 10mm +/- 50mm boundaries Measured building surveys, topographic surveys, low accuracy setting out, net area surveys, valuation surveys F +/- 50mm F +/-25 mm +/- 50mm Low accuracy measured building surveys, topographic surveys, high accuracy utility tracing, gross area surveys G +/- 100mm G +/- 50mm +/-100mm Topographic Surveys, low accuracy measured building surveys, utility tracing surveys, boundary mapping H +/- 250mm H +/- 125mm +/- 250mm Low accuracy topographic surveys, national urban area mapping, geotechnical mapping, tree surveys I +/- 500mm I +/- 500mm +/- 500mm Low accuracy topographic mapping, national non-urban mapping, general boundary mapping, asset mapping J +/-1000mm J +/-1000mm +/-1000mm Low accuracy route/corridor planning surveys, GIS mapping, asset mapping (Custom ) O Custom (Custom) O Custom Custom 1:5 4mm 1:10 5mm 1:20 10mm 1:50 20mm 1:100 50mm 1: mm 1: mm 1: mm 1: mm 1: mm

4 . Measured surveys 3 rd ed Note: Legacy scale is shown to provide a comparison of the typical accuracies achieved from older scale derived survey outputs. Survey bands should now supersede the legacy output scale for determining feature accuracies. Relative accuracies apply to the measurement of adjacent features and are taken to have a maximum range of 100 metres. All accuracies are quoted as standard deviation (std dev - 1 sigma or 1σ) of normal distribution which has 68.3 % statistical confidence of all measurements falling within this accuracy. If greater confidence is required the client should select a band which can achieve the required accuracy at greater multiples of std dev or sigma (e.g. 2 sigma or 2σ = 95.5% confidence). As a general rule the accuracy of survey control which is used to map survey detail should be an order of accuracy higher (i.e. twice as accurate as the accuracy specified for survey detail).

5 Measured surveys 3 rd ed

6 Measured surveys 3 rd ed