About This Tool

This tool provides high-precision, bidirectional coordinate transformations between three major UK survey grids:

• OSGB36: The familiar Ordnance Survey National Grid.
• ETRS89: The standard GPS-compatible system for Europe (effectively WGS84).
• LSG 2025: The London Survey Grid, used for major infrastructure projects.

Methodology

• Horizontal (2D): Conversions between OSGB36 and ETRS89 utilize the definitive OSTN15 transformation grid provided by the Ordnance Survey. Conversions to/from LSG use the standard 7-parameter Helmert transformation and Projection parameters defined in TfL Standard S1026.
• Vertical (Z): Level conversions are fully rigorous.
  • OSGB36 (ODN): Uses the OSTN15/OSGM15 geoid model to convert to/from ETRS89 ellipsoidal heights.
  • LSG Levels: derived using the specific London Survey Grid Geoid Model 2025 (LSGGM2025). This uses bilinear interpolation across the official TfL geoid grid files to precisely calculate the Geoid Separation at any given location.

Please consult the Transport for London Lead Surveyor for more information on LSG.

This tool has been built using the following resources:

Ordnance Survey Developer Resources

Transport for London Standard S1026 (A4) Topographic Surveys and Mapping

Accuracy

The tool is designed to meet survey-grade accuracy requirements. In validation testing, the calculator achieves 3D coordinate agreement typically within 2mm.

Disclaimer: While every effort has been made to ensure accuracy using definitive transformation parameters, this tool is provided "as is" without warranty. Always validate critical survey data against known control points.