Geodesy
Grid Shift Transformations (NTv2, NADCON, GeoTIFF)
Summary
A grid shift transformation stores latitude and longitude (or height) differences at regular grid nodes and interpolates them at each point. It models the irregular distortions of classical datums far better than a single set of Helmert parameters, often reaching centimetre accuracy. Software must have the grid file installed; otherwise it may silently fall back to a coarser transformation.
Definition
Grid shift: a transformation that applies interpolated coordinate offsets from a regular grid of known differences between two datums.
- Formats: NTv2, NADCON, GTX, GeoTIFF
- Bilinear interpolation
- cm-level where well fitted
- Missing grid ⇒ fallback
How it works
For each point the four surrounding grid nodes are found, and the shift is interpolated bilinearly. Horizontal grids give Δφ and Δλ; vertical (geoid) grids give the geoid separation N used in H = h − N.
Formats and examples
| Format | Examples |
|---|---|
| NTv2 (.gsb) | Canada NAD27→NAD83, Australia AGD66→GDA94, UK OSTN15 (as NTv2) |
| NADCON / NADCON5 | US NAD27↔NAD83 |
| GTX / GeoTIFF | Geoid models (EGM2008, national geoids), PROJ-data grids |
In GeoCAD Bridge
The browser engine (Proj4js) applies Helmert parameters only. When a datum change is involved, results include a warning that no grid is applied. For survey-grade work, use desktop PROJ with the appropriate grid and validate on control.
FAQ
Why do I get different results with and without the grid?
Without the grid, the software uses a fallback Helmert or ballpark transformation, which can be metres different.
Where do I get grids?
From the national mapping agency or the PROJ-data package, which bundles many official grids.
Related tools and articles
Tools
Articles
- Datum Transformations: Helmert, Grids and AccuracyA datum transformation converts coordinates between two geodetic datums, for example Ain el Abd 1970 to WGS 84…
- Vertical CRSReference topic
- Geodetic Reference FramesReference topic
References
- PROJ contributors. PROJ coordinate transformation software library, proj.org.
- IOGP. Geomatics Guidance Note 7-2: Coordinate Conversions and Transformations including Formulas, IOGP Report 373-7-2.
Cite this page
GeoCAD Bridge. “Grid Shift Transformations (NTv2, NADCON, GeoTIFF).” GeoCAD Bridge Knowledge Center, updated 2026-10-05. https://www.geocadbridge.com/knowledge/coordinate-systems-grid-shifts/