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name: boule description: Calculate reference-ellipsoid geometry, normal gravity and geodetic/geocentric coordinate conversions with Boule. Use for gravity reference corrections and planetary ellipsoids; this does not transform between geodetic datums or compute terrain corrections. license: MIT metadata: version: "1.0.0" author: Geoscience Skills skill_type: domain tags: '["Geodesy", "Gravity", "Reference Ellipsoid"]' dependencies: '["boule==0.6.0"]' complements: '["harmonica", "ensaio", "verde"]' workflow_role: processing

Boule

Use the ellipsoid and coordinate convention attached to the actual observations. An ellipsoid is a reference surface; choosing WGS84 does not transform coordinates from another datum or reference epoch.

Inputs and conventions

For the 0.6 API, coordinates are a tuple (longitude, latitude, height) for geodetic input: angles in degrees, ellipsoidal height in metres. Spherical input uses geocentric latitude and radius in metres, not elevation. Orthometric height above a geoid requires a separate geoid model before use as ellipsoidal height.

Choose an existing ellipsoid such as WGS84 or GRS80 from source metadata, or define a documented body with consistent parameters. Do not infer the ellipsoid from a column merely called height.

Gravity and coordinate conversion

# example: ellipsoid-gravity
import numpy as np
import boule

longitude_deg = np.array([0.0, 10.0, 0.0])
latitude_deg = np.array([0.0, 45.0, 90.0])
height_m = np.zeros(3)
geodetic = (longitude_deg, latitude_deg, height_m)
gravity_mgal = boule.WGS84.normal_gravity(geodetic)
gravity_si = boule.WGS84.normal_gravity(geodetic, si_units=True)
spherical = boule.WGS84.geodetic_to_spherical(geodetic)
roundtrip = boule.WGS84.spherical_to_geodetic(spherical)

Default gravity is mGal: 1 mGal = 10⁻⁵ m/s². At zero height, reference gravity is about 9.7803253 m/s² at the equator and 9.8321849 m/s² at the poles. Mid-latitude geocentric latitude differs from geodetic latitude. Check both units and limits.

Applying corrections

Specify whether the requested quantity is observed gravity, normal gravity, disturbance or anomaly, with its height and sign convention. Subtraction of normal gravity alone is not a Bouguer, terrain or tide correction. Preserve instrument units and datum/epoch provenance through any handoff to a potential-field model.

Read the normal gravity guide and API for the selected body.