HS2 Engineering Coordinate Systems

High Speed 2 Engineering Coordinate Systems

09th July 2019 OS National Grid The Ordnance Survey National Grid (OSNG) is the standard coordinate reference system used for geospatial data and mapping in Great Britain.

Typical map distortion HS2 Phase 1: • London 190 PPM or 20cm per km • Birmingham 400PPM or 40cm per km

To correct for the distance variation: introduce a scale factor, with a different scale factor being applied every 10km The Requirement

Design – True Scale Projection We require a coordinate system that has the low-distortion characteristics of engineering grids.

We want it to be continuous, without Build – on the ground zones, able to extend for hundreds of kilometres.

Aim is for unity scale factor along the line of the project at ground level. HS2 Survey Grid Snake Projection The Snake projection is a method for projecting the curved surface of the Earth onto a flat plane and is designed specifically for long linear sinuous routes such as rail. Each projection is tailored for the target project.

This forms a unified grid system for the whole project which removes the need for further corrections.

The SnakeGrid innovation was developed in 2005 by University College London. HS2 Survey Grid Snake Projection

The HS2 Survey Grid defines the engineering project grid for phase 1 and 2a: HS2P1+14

• At ground level along the whole route the CRS distance distortion meets the specification of < 20 PPM. • 100.00m ‘in the grid’ is virtually identical to 100.00m ‘on the ground’ HS2 Survey Grid What’s the impact?

Polyline approximating the route from London to Birmingham

Length in OS National Grid: 175,425.94 m Length in HS2 Survey Grid: 175,485.23 m Difference: +59.29 m

HS2 Survey Grid is principal coordinate system used for design, construction and asset location (chainage). HS2 Survey Grid Re-implementation: NTv2

• National Transformation version 2 (NTv2) de-facto standard for transforming geodetic coordinates between datums. Originally developed for conversions in North America. • Transformations stored in a ‘GSB’ format file. • Ordnance Survey also release NTv2 files which define the British National Grid. • HS2 SnakeGrid can now be represented using :

Fully interoperable coordinate system compatible with GIS/CAD/Survey HS2 Survey Grid – Single Source of Truth

Model: HS2 SnakeGrid

On-the-fly Conversion +

Base Mapping: OS Grid

Full CAD and GIS Interoperability Ordnance Survey Geodetic Control

OS GNSS Active Network • Network of Continuously Operating Reference Stations. • Geodetic stations that enable accurate engineering survey. • Contribute to the definition of the Europe- wide reference frame for GNSS surveying. • Station coordinates are periodically updated. Ordnance Survey National Grid Managing Geodetic Change The OS release new geodetic transformations for converting to/from OSGB British National Grid: • OSTN02 • OSTN15

This strategy minimises the change in coordinates due to the OS Net updates. HS2 Survey Grid Managing Geodetic Change

HS2P1+14 SnakeGrid Parameter File • Updates to the OS Active Network are not managed. • The majority of the holdings are in the HS2 Survey Grid within OS Net v2001 (A), so are not fully compatible with the current OS Active Net (v2009).

HS2P1+14 Parameter File HS2 Survey Grid Managing Geodetic Change

Complete alignment with Ordnance Survey Strategy. Transformations: • HS2TN02 NTv2 transform • HS2TN15 NTv2 transform

And use the same widely recognised format and method used to implement British National Grid: • NTv2 transformation and Transverse Mercator projection. Currently applicable to HS2 Phases 1 and 2a. HS2 Survey Grid

HS2P1+14 Managing Geodetic Change Parameter File

HS2TN02 is equivalent to using the HS2P1+14 SnakeGrid Parameter File

HS2P1+14 SnakeGrid Parameter File only to be used with OS Net v2001 lat/lon

Currently applicable to HS2 Phases 1 and 2a. HS2 Survey Grid Re-projection Guidance • Conversions to/from OS Grid using OSTN02 to use the respective HS2TN02 transformation. • Conversions to/from OS Grid using OSTN15 to use the respective HS2TN15 transformation.

And for use with current OS Net v2009: • Conversion from HS2 Survey Grid to/from ETRS89 latitude/longitude to use the HS2TN15 transform. HS2 Survey Grid – Information Management Grid Reference System Spatial Indexing - grid reference system • Defined for HS2P1+14 Survey Grid. • Identifies tile/squares. • Used for data management: base mapping; LIDAR; DTM; DSM

Example: HS2AG005777 100m square with lower left coordinate (200500, 377700) HS2 Survey Grid NTv2 Benefits • Distortion-free coordinate grid for High Speed Rail design and construction • Aligns with the Ordnance Survey strategy for maintaining a stable coordinate system. No loss of data currency due to geodetic changes. • Aligns with the Ordnance Survey technical implementation. • Creates an open source interoperable technical solution compatible with majority of geospatially-enabled systems. • Reduces need for data duplication; enables on-the-fly re-projection. • Enables ‘tagging’ of geospatial data and integrated data discovery/spatial searches. • EPSG Geodetic Parameter Registry Questions? Copyright notice

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