This entry is part of the Chart Index, the reference library for the Chart Design Field Guide.

A geographic base map provides spatial context without assigning a quantitative value to every region. It is the foundation beneath point, route, flow, and annotation layers, and sometimes the map itself is the answer when the task is simply to locate boundaries or places.

The base map's discipline is restraint. A choropleth argues; a base map orients. Everything on it — coastline, borders, the faint fill of land — exists so that whatever is drawn on top of it can be read in its right place, and the moment the base competes with the marks it has failed.

What it is

The chart renders geographic features (usually GeoJSON polygons) through a map projection, then draws marks over them. Unlike a choropleth, regions do not need a measure; their geometry, boundaries, and labels are the information. The most common overlay is the proportional symbol: a circle at each location whose area encodes a quantity, so the map answers where and the circles answer how much.

The hero chart above puts Australia's twelve largest urban centres on a quiet base of state outlines, with circle area proportional to residents. The reading is immediate and two-fold: the country lives on its south-eastern coast, and two cities hold two in five of its people. Randomise the populations and the circles resize in place — position is fixed by geography and never moves, which is the base map's contract.

When to use it

Base maps and proportional-symbol maps are the right choice when:

  • The question begins with where: locations, service areas, routes, assets, or administrative boundaries.
  • Other marks — points, flows, annotations — need geographic context to be read at all.
  • Region geometry is meaningful even without a quantitative fill, as in a store network or a coverage map.
  • The quantity belongs to places rather than areas — a city's population, a port's throughput, a store's revenue — so it should sit at a point rather than fill a polygon.
  • The reader will recognise the geography and use it to locate the marks.

When not to use it

  • Location adds no analytical value. If the reader will never use the geography — if the question is simply which city is biggest — a sorted bar chart is faster and more precise.
  • Region size will dominate attention while the real task is comparing values. Use a cartogram or a tilegram.
  • The available geometry is too detailed for the display size. A national coastline at every inlet is noise at 400 pixels; simplify or choose coarser boundaries.
  • The marks overlap badly. When the interesting points cluster in one corner — as Australian cities do — symbols collide and the map needs an inset, a smaller size range or a different form.

Design principles

Keep the base quiet

Boundaries, land, water, and labels should support the analytical marks. A loud base competes with the data; a quiet one disappears into context. The base map in this entry is a single pale fill with hairline canvas borders and no labels, and that is usually enough — a reader who recognises the outline of Australia does not need the states named to find Perth.

The base alone: geometry as the only information
No values, no marks. The map still answers a question — where are the boundaries, what is the shape of the country — and nothing on it will fight with whatever is drawn next.

Encode quantity as area, not radius

A proportional symbol is read by its area, so its radius must scale with the square root of the value. Scale the radius directly and a city four times the size becomes a circle sixteen times the area — the most common and most damaging mistake in symbol maps. The two figures below plot the same cities; the second scales the radius to the value and so exaggerates every difference.

Area proportional to population: the honest map
Sydney and Melbourne are about twice Brisbane and Perth, and the circles say so. Hobart, Darwin and Cairns are small but legible, because the smallest circle has a floor.
Radius proportional to population: the exaggerated map
Values squared before scaling, so the radius — not the area — grows with population. Sydney and Melbourne are unchanged, because they set the top of the scale; but Brisbane and Perth, at half their population, have shrunk to a quarter of their area, and the northern cities have collapsed to dots. Same data, a false ratio.

Choose the projection for the task

Projection is not decoration. Every flat map distorts something — area, shape, distance or direction — and the projection decides which. Mercator preserves angles and inflates the poles; equal-area projections keep every region's area true at the cost of shape. For a regional map, choose a projection centred on the region so that distortion is small and even; for a global one, state which property is preserved.

Mercator, upright
The familiar atlas view with the projection's rotation switched off. Tasmania and Victoria, far from the equator, are stretched relative to the Top End — mild at this latitude, but present.
Equal Earth, upright
An equal-area projection of the same cities. Every state now covers its true share of the canvas, and the outline settles into a slightly different shape. Neither is wrong; the choice is which distortion the reader can afford.

Draw the big marks first, and give the small ones a floor

Symbols overlap where places cluster, and a small circle hidden beneath a large one is a missing datum. Draw the largest marks first so the small ones sit on top, keep the fill translucent so a covered edge still shows through, and give the size scale a minimum radius so that the smallest value remains a target the reader can hover.

Bulk cargo through the major ports, million tonnes
Port Hedland's iron ore dwarfs everything, so it is drawn first and the east-coast coal ports sit above it. Translucent fills let Newcastle and Port Kembla overlap without either vanishing; the smallest ports — Adelaide, Darwin — keep a legible circle from the size floor.

Simplify geometry at small sizes

Dense coastlines and boundaries add file weight and visual noise. Match geometry detail to the final rendered scale: the state boundaries in this entry are simplified from the ABS 2021 set until they hold up at a few hundred pixels and no further. A boundary vertex the reader cannot see is a cost with no return.

Preserve geographic conventions

Use recognisable orientation, labels, and boundary hierarchy unless the analytical purpose clearly requires otherwise. North stays up; state borders are lighter than the coastline; place labels sit to the right of their marks. The reader has spent a lifetime learning these conventions, and every one you keep is attention saved for the data.

Label the marks, not the base

If anything on a symbol map is to be named, it is the symbols. Region names on the base compete with them and rarely earn their place; a reader who needs to know which state Townsville is in can hover. When labels are needed on every mark, keep them mono, small and to one side, and let the marks themselves stay unlabelled when the pattern is the point.

Anatomy

The composition of a proportional-symbol map
An anatomical guide

GeoJSON features supply geometry and properties; the projection converts coordinates to screen position; quiet fills and hairline borders establish context; and the symbols carry the data, sized by area and drawn largest-first so that nothing is hidden.

  • Choropleth — regions filled by a quantitative measure. Use for rates, not totals.
  • Value-by-alpha map — colour and opacity encode separate measures.
  • Hexbin and H3 map — uniform spatial cells for density; the right form when the points are too many to symbolise.
  • Cartogram — gives up the base geometry so that area can carry the value.
  • Bubble chart — the same area-scaled circles on measured axes instead of geography.
  • Flow map — lines or arcs between points on a base map; the base map with a second layer.

Reading list

  • Snyder, J. P. (1987). Map Projections: A Working Manual. The reference on what every projection preserves and distorts.
  • Robinson, A. H., Morrison, J. L., Muehrcke, P. C., Kimerling, A. J. & Guptill, S. C. (1995). Elements of Cartography. The standard text on base-map design and symbolisation.
  • Slocum, T. A., McMaster, R. B., Kessler, F. C. & Howard, H. H. (2009). Thematic Cartography and Geovisualization. On proportional-symbol scaling and the area-versus-radius problem.
  • Bertin, J. (1967). Sémiologie graphique. Size as a visual variable, and why area must carry quantity.
  • Cairo, A. (2016). The Truthful Art. On projection choice as an editorial decision.