Advances in engineering survey grid transformations for rail infrastructure

This paper provides a summary of innovative approaches developed on Snake Projection coordinate transformations that maximise data interoperability and compatibility within CAD, GIS and survey software and systems.

The project’s topographical survey, design and construction use an engineering coordinate system defined by the HS2 Snake Projection together with the HS2 Vertical Reference Frame. This approach has several advantages including minimal scale factor and height distortion at track level ^[1] with seamless continuity over the project.

For HS2, the zero-order survey control is defined by the Ordnance Survey OS Net™ network of Continually Operating GNSS Reference Stations (CORS). In 2016, Ordnance Survey updated the geodetic coordinates of OS Net ^[2] and the implications of these changes on HS2’s track design are potentially significant. For example, the update caused a 13mm horizontal shift and 25-30mm vertical shift in engineering coordinates. To ensure the project achieves positional stability and thus remains unaffected by these changes, both now and into the future, a description is presented of the development of a new geodetic transformation called HS2TN15 and a new equipotential geoid model, HS2GM15, which are applicable respectively to plan and height coordinates.

The HS2 Survey Grid transformations enable highest accuracy engineering survey for the project design, construction and operation; fully validated and authoritative for use, with significant benefits realised to date. HS2 has led knowledge sharing sessions with the wider industry, with the revised Snake Projection methodology being increasingly adopted on further infrastructure projects.