Vincenty distance calculator

This capability takes two latitude and longitude pairs, runs the classic Vincenty iteration with fixed WGS-84 semi-major axis and flattening, and returns metres, kilometres, and initial and final bearings as deterministic JSON.

What Vincenty distance measures and how to supply the two points

Aliases such as from_lat, to_lat, from_lon, to_lon, latitude1, and longitude1 are accepted when your client already uses those names. Any coordinate outside those ranges raises invalid_input with a message that names the field and the bound, so callers never silently clamp bad GPS noise into a fake geodesic. Identical inputs always produce identical JSON because the path is pure arithmetic with fixed WGS-84 constants, no wall clock, and no external map service.

Reading metres, kilometres, bearings, iterations, and convergence

Prefer metres for survey corridors and runway-scale work; prefer kilometres for intercity and oceanic legs. On success the response echoes every input coordinate, then reports distance_m and distance_km as the ellipsoidal geodesic length. initial_bearing_deg is the forward azimuth at the first point measured clockwise from true north on the zero-to-three-hundred-sixty interval; final_bearing_deg is the corresponding reverse azimuth at the second point after the geodesic arrives. iterations counts how many longitude updates the solver needed before the residual fell below the published epsilon, which helps diagnose borderline pairs without re-running the loop by hand. ellipsoid is always the string WGS-84, a_m is the semi-major axis in metres, and f is the flattening, so downstream code can assert the datum without hard-coding constants in every client. method is always vincenty_inverse and converged is true on every successful response. When two points are nearly antipodal the classical iteration may never settle; the engine stops at a hard iteration cap and raises invalid_input with a message that names near-antipodal failure rather than returning a half-finished distance. Validation failures for missing fields, non-finite numbers, and out-of-range coordinates likewise fail closed and are not charged under the prepaid API.

API usage, pricing, GIS workflows, and practical limits

Send a JSON body with the four required coordinates, then call the free in-browser Vincenty distance calculator for classroom demos, chart checks, and quick mental verification without installing PROJ, GeographicLib, or a full GIS stack. Use the prepaid API when a logistics platform, flight-planning tool, cadastral workflow, or simulation harness needs a stable contract under KIT balance at $0.002 per successful item; validation failures and non-converging antipodes never consume balance after the engine rejects the request. Pair this endpoint with your own geocoder when you need place names: this product only measures geometry on the WGS-84 ellipsoid. For ordinary continental separations Vincenty is accurate to well under a metre relative to modern geodesic libraries; for the thin band of near-antipodal cases where the 1975 iteration diverges, switch to a specialized antipodal solver or a Karney-style geodesic rather than forcing this endpoint.