Site Heights - Orthometric levels (AOD) with GNSS, not BenchMarks
In 2000, the Ordnance Survey formally announced that it was abandoning the traditional BMs in favour of global positioning systems (GPS) plus a datum conversion model (OSGM02). With the national mapping agency of Great Britain no longer recognising BMs, their use ceases to be best practice for height referencing for general geospatial engineering and civil engineering work. An exception could be for very minor surveys that have limited extent, are self contained and have no impact on other projects.
Best practice since 2000 is that site-survey ground heights should be obtained by using survey-quality dual-frequency GNSS equipment to bring one or more Ordnance Survey datum height(s) into a job. The GPS position is then combined with a height correction modal (geoid model) to derive the final Ordnance Survey datum height. Spirit levelling is then usually used to promulgate that height around the site. GPS, combined with a geoid model, has developed sufficiently to provide more consistent and accurate heighting across the country than spirit levelling. That is not to say that, over short distances, relative heights can be determined to a much higher accuracy with spirit levelling; it is a combination of approaches. To obtain levelling measurements using GPS, two pieces of information need to be combined: Briefing sheets are provided free of charge to help increase knowledge and awareness. They may be freely copied. Care is taken to ensure information is correct, however readers are advised to consult source documents for authoritative information. Institution of Civil Engineers, One Great George Street, Westminster, London SW1P 3AA Registered Charity No. 210252
Briefing Sheet
• A GPS coordinate in the European Terrestrial Reference Frame 1989 (ETRF89). This is easily achieved either instantaneously through using a real-time GPS technique (such as Real-Time Kinematic: RTK) or through post-processing GPS data collected at the point to be heighted and at known base stations in the ETRS89 coordinate system. Data from OS base stations is the freely available ( http://gps.ordnancesurvey.co.uk/active.asp ). RTK and active stations are monitored daily unlike BMs, which were usually only observed once in their lifetime.
• Secondly, a transformation (currently OSGM02) is needed to convert the GPS ellipsoidal ‘altitude’ to a height above either Ordnance Survey Datum Newlyn for mainland Great Britain or one of the local Ordnance survey datums. This file is available for GPS500, GPS1200 & Viva GNSS instruments from http://www.leica-geosystems.co.uk/en/Support-Service-Customer-Downloads_25742.htm .
Source: ‘Ordnance Survey Benchmarks: Their very limited use for civil engineering survey work’, Prepared by ICE/ICES Geospatial Engineering Panel, Information note