Concept

Meridian — where it appears

A line of constant longitude, running from pole to pole. It is a great circle, so a geodesic, a rhumb line and a straight line on a conic sheet along it are all the same curve — which makes a boundary described as a meridian the one description with no geometric ambiguity in it.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

One sentence, three readings of it. "a straight line from the initial point on the Rio Grande to a point on the Colorado" — Treaty of Mesilla, 1853. Every curve here answers to those words. the geodesic, the rhumb line, straight on the sheet, drawn between the same two monuments on a conformal conic fitted to the segment itself. The widest pair, the geodesic against the rhumb line, are 7.53 kilometres apart at their worst and enclose 3,925 km². The shading is that ground. It is not an artefact of the drawing: the same figure of the 141st meridian shows one line, because on a meridian every one of these readings is the same curve.

One sentence, and the ground between its readings

Every land boundary in the world is defined by a sentence, and a sentence naming two monuments does not name a curve. "A straight line" between the Rio Grande and the Colorado admits at least three answers 7.53 kilometres apart at their worst; "the forty-ninth parallel" admits four, 96 kilometres apart, with 130,972 square kilometres between the extremes.

practice · Boundary

One meridian, two datums, and the ground between them. "the meridian line of the 141st degree of west longitude" — Anglo-Russian Convention, 1825. The line whose longitude is exactly 141° west, drawn twice: once on nad27 and once on WGS84, with the east–west separation exaggerated 3,000 times. The two are 129.0 metres apart, and remarkably constant — the shift changes by five centimetres over nine degrees of latitude — so the strip between them is a ribbon 129 metres wide and 1039 kilometres long, which is 134.0 square kilometres. The sentence has not changed. The surface the number refers to has.

A meridian boundary moves when its datum does

The 141st meridian is the one boundary description in this collection with no geometric ambiguity in it: every reading of it is the same curve, exactly. It has a different one. A longitude refers to a datum, the 1825 convention named none, and the line of longitude exactly 141° west sits 129.0 metres apart on NAD27 and WGS84 — a ribbon 1,039 kilometres long and 134 square kilometres in area.

practice · Boundary

Named alongside it

The objects these essays reach for when they reach for this one.

Area Boundary Convention Tolerance Verification Arcsecond Central meridian Datum Ellipsoid Geodesic Geodetic datum Great circle

All concepts