osgm15 0.1.0

Read and interpolate OSGM15 geoid-separation grids (ASCII GTX format)

osgm15

A Python package that can be used to get the vertical reference for the UK.

Read and interpolate OSGM15 geoid-separation grids from ASCII GTX files.

OSGM15 is the Ordnance Survey's geoid model for Great Britain, Ireland, and surrounding waters. It provides the separation N between the GRS 80 ellipsoid and the local geoid so that:

orthometric height = ellipsoidal (GNSS) height − N

Installation

pip install osgm15

Or with uv:

uv add osgm15

Quick start

from osgm15 import OSGM15Grid

# Load the ASCII GTX file (download from Ordnance Survey)
grid = OSGM15Grid.from_file("OSGM15_GTX_ASCII.gtx")

# Single-point lookup — returns geoid separation N in metres
N = grid.height_N(lat=52.0905, lon=-2.2160)

# Vectorised lookup (NumPy arrays in, NumPy array out)
import numpy as np
lats = np.array([52.0905, 51.5074])
lons = np.array([-2.2160, -0.1278])
N_arr = grid.height_N_array(lats, lons)  # points outside grid → NaN

Obtaining the GTX file

The OSGM15 ASCII GTX file is published by Ordnance Survey and can be downloaded from:

https://www.ordnancesurvey.co.uk/geodesy-positioning/coordinate-transformations/resources

API

OSGM15Grid.from_file(path)

Read an ASCII GTX file and return an OSGM15Grid instance.

OSGM15Grid.height_N(lat, lon) → float

Bilinear interpolation of geoid separation at a single WGS 84 point. Raises ValueError if the point is outside the grid.

OSGM15Grid.height_N_array(lat, lon) → np.ndarray

Vectorised version — accepts array-like inputs and returns a NumPy array. Points outside the grid are set to NaN.

Development

# Clone and set up
git clone [e-mail]:phillipaarestrup/osgm15.git
cd osgm15
uv sync

# Run tests
uv run pytest

Example projects

TBA

License

MIT