30
The full register
Every entry, by section
The complete roster: eight sections, thirty entries, each naming what a mapping choice keeps and what it wrecks.
01 — Projections
What a projection gives up
You cannot flatten a curved surface without tearing or stretching it. Every projection therefore preserves at most one of angle, area and distance, and wrecks the others — which makes the choice a statement about what the map is for.
Conformal, equal-area, equidistant
Three families, three things preserved, and no overlap between them.
Reading the indicatrix
Circles drawn on the globe and projected: their shape on the flat sheet is the distortion, made visible.
Choosing for a continent
Latitude range and shape decide it more than preference does.
02 — Datums & Coordinates
The Gulf of Guinea problem
Coordinates that default to zero land at the origin, off West Africa. The commonest sign that a coordinate system was never set.
Ellipsoid and geoid
One is a smooth mathematical figure, the other is where gravity actually is. Height depends on which you mean.
Latitude first, or longitude first
Two conventions in wide use, and the axis swap that follows.
Sixty zones, six degrees each
A projected grid that works well inside a zone and badly across one.
03 — Scale & Generalisation
The length of a coastline
Measure with a shorter ruler and it gets longer, without limit. Which means a coastline has no single length, only a length at a scale.
Tolerance, and what it deletes
Simplification thresholds remove the smallest features first — which are often the ones that identify the place.
Simplify, smooth, displace
Three different operations regularly confused for one.
One geometry, many scales
The same data cannot serve every zoom without being generalised for each.
04 — Tiles & Zoom
Why the map arrives in squares
Fixed-size images on a fixed grid cache well and can be served by anything. The whole delivery model follows from that.
The pyramid, level by level
Each level down holds four times as many tiles as the one above it.
Resolution at the equator
Ground distance per pixel is quoted there and is smaller everywhere else.
Seams, labels and edges
Features that cross a tile boundary are the hard case.
05 — Vector & Raster
Geometry on arrival, or pixels in advance
Send the shapes and style them in the client, or send a picture already drawn. The choice moves the work and moves the constraints with it.
Where the styling decision sits
Server-side is fixed and cacheable; client-side is flexible and heavier.
Continuous fields
Elevation and temperature have a value everywhere, which suits a grid, not a polygon.
06 — Classification
Two stories, one dataset
The same numbers, classified two ways, produce maps that support opposite readings. The method belongs in the legend.
Equal interval and quantile
One divides the range, the other divides the count. They rarely agree.
Counts, rates and denominators
A map of counts is usually a map of population unless you divide by something.
How many classes
Beyond about seven the reader stops distinguishing them.
07 — Map Furniture
What a scale bar promises
On most projections it is only true along particular lines, which is rarely stated.
Legends that need reading twice
Order, breaks and units, and what a legend leaves the reader to infer.
Three norths
True, grid and magnetic, and the angles between them.
Lettering on a sheet
Placement rules that were settled long before screens.