Series Index - Setting Up in the Real World
[updated with Jim Turner's comments; thank you Jim!]
SnakeGrid takes a linear alignment and lays it out on a flat plane maintaining low distortion along a strip approximately 20km wide each side of the alignment. This allows accurate measurements of distance and angle within this strip.
This is not a Geographic Coordinate System (GCS) that relates to established geoids, but a self-contained coordinate system unique to each individual alignment. If, for example, two SnakeGrids cross at a common real world location, the SnakeGrid coordinates will be totally different for each alignment.
It is possible to use dhp11's SnakeGrid Transformer (link checked Feb 2022) to transform some MicroStation elements between SnakeGrids, and MicroStation GCSs such as OSGB or project specific grid GCSs such as the London Survey Grid and (for example) the Reading project grid. This involves creating transformed copies of the selected elements (which can only be from a subset of MicroStation elements, those that are/can be defined by vertices) so is suitable only for simpler 2D geometry and 3D triangulated surfaces. These elements, including the alignments, are exploded to their most basic geometric parts, their vertices are then transformed to the target GCS.
Complex 3D geometry would also need to be exploded to give access to the vertices, reducing the result to a useless mass of lines; so there is no point attempting to transform such complex elements.
Simpler elements, such as track alignments, can be transformed and would also be exploded to constituent elements, but being simpler would still be useable.
Once transformed to a known GCS, geometry can be referenced using MicroStation geo-referencing techniques for work with facilities/buildings that relate to the alignment.
Maintaining duplicate copies of data is inevitable using this workflow. The SnakeGrid version will generally be the master, the transformed copy a child read-only version. Version management is crucial to the success of this workflow, possibly enforced by a ProjectWise workflow that requires the child to be updated before changes to the master can be committed.
A customised transformation from the HS2 SnakeGrid to GCS has been created and made available in CONNECT edition applications:
This note from Jim Turner is quoted in full:
"The HS2 SnakeGrid (phase 1+2a) can now be defined within the Microstation GCS library using an NTv2 transform + Transverse Mercator projection. It will be available in Microstation update 13 however will require manual installation of the transformation files. MS CE Update 14 will have the files distributed. Two GCS will be available: HS2_Snake_2002 and HS2_Snake_2015. These are essentially the same GCS - the difference is the transformation used for conversions into other GCS: Use HS2_Snake_2002(HS2TN02) when converting to/from OSTN02.BritishNatGrid Use HS2_Snake_2015(HS2TN15) when converting to/from OSTN15.BritishNatGrid Use HS2_Snake_2015(HS2TN15) when converting to/from ETRS89 OS Net v2009 latitude/longitude (current OS Active Network)Note that the HS2TN02 transform is equivalent to using the HS2P1+14 SnakeGrid parameter file. If converting into lat/lon using HS2TN02/HS2P1+14 then these coordinates are only compatible for use with the legacy OS Active Network (OS Net v2001)."
To work with AECOsim Building Designer we need to model within the Solids Working Area. The recommended workflow is to model individual buildings/facilities near to the Design File Centre unrotated then use geo-referencing to reference the real-world context to the AECOsim Building Designer models. Geo-referencing can also be used to relate individual buildings/facilities to each other as well as the context, as shown in this illustration:
Drawings and visualizations that include the site context can be created from these geo-referenced composites.
The individual buildings/facilities along with their footprints and/or setting out/location points can be referenced into the transformed site model from where the footprints and/or setting out/location points could be transformed back to SnakeGrid for coordination purposes. (Potentially the edges of simplified building/facility envelope volumes could also be transformed back to SnakeGrid for volumetric coordination, the volumes would need to be rebuilt from the edges after the transform.)
For more information on Geo-referencing with MicroStation, AECOsim Building Designer and other related applications please refer to product help and the articles in this blog series: Series Index - Setting Up in the Real World
Just to clarify some points on this article:
It is possible for SnakeGrid to be a GCS that is installed in the AECOSim GCS library and related to an established datum (WGS84). See this post for clarification on the HS2 SnakeGrid: https://communities.bentley.com/products/microstation/f/microstation-forum/148712/geographic-coordinate-system-missing/522043#522043
Consequently, in this case, the known GCS will be SnakeGrid and a helmert transform directly implemented to the SWA.
Furthermore, the zero-distortion band on a SnakeGrid is not 1 mile; it is typically up-to 20km either side of the alignment (depending on height changes)