Earth, latitude and longitude

What does a coordinate actually locate?

A latitude and longitude pair is an address on a defined Earth model.

Start from the reference surfaces

The Earth is approximately an oblate ellipsoid: its equatorial radius exceeds its polar radius. A sphere is useful for many examination calculations, but it is an explicitly simplified model. The geoid is a gravity-defined surface close to mean sea level; an ellipsoid is a smooth mathematical surface. They answer different questions.

Geodetic latitude is the angle between the local ellipsoid normal and the equatorial plane. Geocentric latitude uses the line from the Earth’s centre. These differ away from the equator and poles. Operational chart and GNSS coordinates normally use geodetic latitude on a stated datum, commonly WGS84.

Read the hemispheres before the digits

Latitude runs from 0° at the equator to 90°N or 90°S; longitude from Greenwich to 180°E or 180°W. A parallel is a circle of latitude. A meridian is half a great circle joining the poles. Only the equator is a great-circle parallel.

Convert degrees and minutes with 60 minutes per degree. Thus 25°30′N is 25.5°N, not 25.30°. Retain the hemisphere or signed convention consistently; a swapped N/S or E/W entry can be numerically plausible and geographically disastrous.

Plot, then challenge the address

Suppose a route point is written 48°15′N 002°30′E. Its decimal form is latitude +48.25°, longitude +2.5°. Write the hemispheres before entering the digits. Reversing the pair gives a valid-looking latitude of 2.5°N and longitude of 48.25°E, but a completely different region. A format validator alone cannot catch that mistake.

Now ask what the map should show: the point should be north of the equator and just east of Greenwich. Compare the loaded waypoint with its intended nearby airport, coastline or route segment. This geometric cross-check tests the meaning of the address, whereas reading the same digits twice tests only transcription. Keep the datum and source alongside the coordinate when transferring it between publications.

Why can correct digits still put a fix in the wrong place?

Position accuracy depends on reference datum, precision and entry discipline.

A datum makes the address reproducible

A horizontal datum defines the Earth model and its relationship to the actual Earth. Coordinates from an old local datum cannot be treated as WGS84 simply by copying their digits. The offset may matter most during an approach or near terrain. Use the datum and conversion stated by the approved source.

Precision describes the resolution of the written value; accuracy describes its closeness to the intended position. More decimal places do not repair a wrong datum, wrong hemisphere or wrong waypoint. Compare identifiers, route geometry and an independent reference before accepting an entry.

Turn rounding into a distance check

On the spherical training Earth, 1′ of latitude is about 1 NM. A coordinate rounded to the nearest 0.1′ has a maximum rounding error of 0.05 NM north or south. Longitude distance also contracts with cosine of latitude.

At 60°N, 0.1′ of longitude spans about 0.05 NM. The same written angular precision therefore gives different east–west distance resolution at different latitudes. Keep the source precision; do not imply survey accuracy from a rounded chart position.

Estimate the rounding footprint

For a coordinate rounded independently to the nearest 0.1′, the north–south rounding component is at most 0.05 NM on the spherical model. At 60°N, the east–west component is at most 0.025 NM. Their combined corner distance is √(0.05² + 0.025²) ≈ 0.056 NM. This is a rounding footprint, not the total position error.

A source survey error or datum mismatch can be much larger and need not be random. The distinction matters when comparing two coordinates: a difference within the printed rounding footprint may reflect representation, while a larger systematic displacement calls for investigation of reference and source accuracy. Never use the extra precision of one display as evidence that it identifies the correct physical point.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.