Size & scale

The map projection

Greenland is not bigger than Africa. The map is.

a.k.a. Web Mercator · EPSG:3857 · pseudo-Mercator · the basemap · conformal projection · area distortion · the flat map

A globe cannot be flattened without something being stretched, and every projection chooses what it will sacrifice. Web Mercator — the one under nearly every map on the internet — keeps angles and local shape, and pays for it in area: it stretches both directions by the secant of the latitude, so it stretches area by the square of that. Every square kilometre is quietly multiplied before you see it, by 1 at the equator, 2 at 45°, 4 at 60° and 33 at 80°, and the eye reads the ink rather than the coordinates. The same arithmetic escapes the picture and turns up as a figure with units on it, because a GIS asked for an area in the coordinate system it happens to be drawing in will answer in square metres of the projection plane and never mention that the plane is not the ground.

How to spot it

Ask which projection. On the web the answer is almost always Web Mercator — EPSG:3857 — and it is not equal-area.
Look at the top of the world: Greenland the size of Africa, Russia half the page, Antarctica a wall along the bottom.
The multiplier is sec² of the latitude — 1.3× at 30°, 2× at 45°, 4× at 60°, 15× at 75°, 33× at 80°. Two shapes at different latitudes are drawn at two different scales.
For any area figure that came out of a GIS, ask which coordinate system it was measured in. Measuring in the display projection is the commonest way a wrong square-kilometre number reaches a slide.
Dots, bins and choropleth shading on Mercator give northern ground more ink per unit of whatever is being counted, before any colour scale is chosen.
An equal-area map gives itself away by looking slightly wrong — Africa enormous, Greenland modest, the continents squashed a little toward the poles. That is what correct looks like.

The fix

Pick the projection for the question. If the question is about amount — area, density, share of the ground — use an equal-area projection and name it: Equal Earth or Mollweide for the world, an Albers or Lambert equal-area conic for one country. If the question is about position or bearing, Mercator is the right answer and always was; a straight line on it is a constant compass course, which is the whole reason it exists. Measure areas on the ellipsoid rather than in whatever plane the map is drawn in — in PostGIS that is the difference between ST_Area on a 3857 column and ST_Area on the same geometry cast to geography, which is to say the difference between a number and that number times sec² of the latitude. Put the projection in the caption the way you would put units on an axis, because a map without one is a chart whose scale is a secret. And where the comparison actually matters, take it off the map altogether: two shapes at different latitudes will never be comparable by eye, and a sorted bar chart of the areas settles in a glance what a world map cannot settle at all.

In the gallery

Dark navy board slide, masthead “Northlight Earth Observation · Monitoring programme review · Board pack, FY27”, headlined “The Arctic is four fifths of the programme”. A subhead reads: every square kilometre of the FY27 monitoring commitment, measured the only way the platform can measure it — the four theatres’ national boundaries, taken whole, each one’s area computed by the same function, from the same boundary file, in the same coordinate system the operations map is drawn in; nothing sampled, nothing estimated, nothing set aside. Filling the left of the slide, under the label “FY27 monitoring theatres · Natural Earth 1:50m boundaries · platform basemap (EPSG:3857)”, is a Web Mercator world map on a dark ocean, with a faint graticule at 15° parallels and 30° meridians. Greenland and Norway, including Svalbard, are filled gold and are enormous — Greenland alone dominates the northern half of the map. The Democratic Republic of the Congo and the Indonesian archipelago are filled blue and look modest by comparison. A legend reads “Ice & snow theatres” in gold and “Forest & peatland theatres” in blue. In the centre column, under the label “Ground under watch · square kilometres”, four horizontal bars stand on a zero-based axis running 0 to 40 millions: Greenland 35,395,741, a gold bar running nearly the full width; Norway 3,452,856, a short gold bar; DR Congo 2,359,632 and Indonesia 1,885,417, two shorter blue bars. Down the right-hand side, a gold-bordered card carries 43.1 M km² over the line “Total ground under watch in FY27, across all four theatres”. A card headed “Ground by theatre · FY27” lists Greenland 35,395,741, Norway 3,452,856, DR Congo 2,359,632, Indonesia 1,885,417 and a total of 43,093,646. A gold card headed “Read-out for the board” argues that Greenland is not one theatre among four but is the programme, carrying 82% of the ground under watch, 15 times the Congo Basin and 4.6 times the other three theatres put together, and recommends that FY27 headcount follow the ground — both new analysts to the ice desk, the equatorial theatres held at current staffing. A card headed “How the areas were measured” says one boundary file, one function, one coordinate system: ST_Area over the Natural Earth 1:50m admin polygons, in the platform’s own CRS, the same projection the operations map is drawn in, so the map and the table cannot disagree with each other. A final card, “Basis of measurement”, gives four theatres, 1:50m generalisation, coordinate system EPSG:3857, and four of four territories measured whole. Three ticked boxes run along the bottom: “Zero-based axis. Every bar starts at 0. No truncation, no break, no log scale and no second axis anywhere on this slide”, “Nothing cherry-picked. All four FY27 theatres, whole national territories, complete boundaries — not a sample, not a window, nothing set aside”, and “One unit throughout. Every bar is square kilometres, from one boundary file, one function and one coordinate system, computed once”. A footnote credits Natural Earth 1:50m Admin 0 – Countries, notes that the Norway polygon includes Svalbard and Jan Mayen, records that the areas were computed by us on 22 August 2026 as planar polygon areas in EPSG:3857 (WGS 84 / Pseudo-Mercator, sphere R = 6,378,137 m), the coordinate system of the basemap above, and states that the slide is a demonstration drawn by us in the manner of a board pack and has never been published anywhere.

The 43 million square kilometres that were really 6.7 million

The map projection