Cartographic Projection
every map is wrong, and by exactly how much
Concept

Survey network — where it appears

A set of marked points whose relative positions are computed together from observations between them. Its degrees of freedom are the same 2n − 3 an embedding has, and the observations beyond that number are redundancy — the surplus that makes a check possible.

Named by 3 essays across 3 fields — each of them below, with the objects they name alongside it.

The share of its distances a flat picture can hold. A picture of n places on a plane has 2n coordinates and is unchanged by two translations and a rotation, so 2n − 3 numbers in it are genuinely free; each distance held exactly is one equation. So at most 2n − 3 of the n(n−1)/2 distances can be right, whatever the places are and however hard the map maker tries. The share falls as 4/n: five of six for four places, thirteen of twenty-eight for eight, and 197 of 4,950 — 4.0% — for a hundred. The dashed curve is 4/n, which the count approaches from below and never crosses.

Five distances of six, and never more

A flat picture of n places has 2n − 3 free numbers and n(n−1)/2 distances, so it can be exactly right about 2n − 3 of them and no more — five of six for four places, twenty-nine of a hundred and twenty for sixteen, and 3.98 per cent for a hundred. The bound is reached by a construction with no fitting in it, and an azimuthal equidistant map spends only n − 1.

impossibility · Embedding The line a commission can actually run. A boundary described as a parallel of latitude and marked by monuments 220 kilometres apart, with the offset exaggerated 700 times so that it can be seen at all. A commission cannot run a parallel: it can set a monument, sight a straight line to the next and clear the trees between, and a straight line between two points of equal latitude is a geodesic, which passes POLEWARD of the parallel everywhere between them. So the marked line lies north of the described one, by 1176.5 metres at the middle of each of its 9 chords, and encloses 1611.2 square kilometres that the words put on the other side. At the 20-kilometre spacing this ladder measures at, the same offset is 9.03 metres.

The line a commission can actually run

A boundary commission cannot run a parallel of latitude. It can sight a straight line between monuments, and a straight line between two points of equal latitude passes poleward of the parallel — by s² tan φ / 8R, which at a mile of spacing is fifty-eight millimetres and at a hundred kilometres is two hundred and twenty-six metres. The described line and the marked line are different curves, and the marked one governs.

practice · Boundary How long a network has before it breaks its own tolerance. The years until the worst baseline in a 400-kilometre network exceeds five millimetres plus one part per million of its length, which is an ordinary first-order specification. A boundary zone breaks it in 16 years; a plate interior takes 1819; a rigid plate never does, at any speed, because a rigid body keeps every distance it has. Bars are clipped at five thousand years.

A grid stops fitting the ground it was laid on

A hundred-kilometre baseline across a plate boundary changes by 815 millimetres in fifty years, and the same baseline on the fastest plate in the model changes by nothing at all in the same fifty. The interval before an ordinary first-order specification is broken is six years in one place and never in the other.

datums · Strain

Named alongside it

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

Closed formToleranceVerificationArcsecondAreaAzimuthalBaselineBoundaryConstraintConventionDatumDegrees of freedom

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