This is the definition of the vertical reference frame for construction and maintenance of the UK High Speed 2 rail project. The EPSG code is 9304: https://epsg.org/transformation_9304/ETRS89-to-HS2-VRF-height-1.html
See existing Product Backlog Item 1987579. Please do contact me for geoid definitions.
Hi! We are happy to inform you that the vertical coordinate system HS2-VRF(HS2GM15) height is now available in MicroStation 26.0.1 (26.00.01.65).
You can download this update through the Bentley Connection Client or via the Software Downloads portal. Thank You!