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.

What it is

The chart renders GeoJSON features through a geographic projection. Unlike a choropleth, regions do not need a measure; their geometry, boundaries, and labels are the information.

When to use it

  • The question begins with where: locations, service areas, routes, assets, or administrative boundaries.
  • Other marks need geographic context.
  • Region geometry is meaningful even without a quantitative fill.

When not to use it

  • Location adds no analytical value. A chart using position on ordinary axes will usually be easier to read.
  • Region size will dominate attention while the real task is comparing values.
  • The available geometry is too detailed for the display size.

Design principles

Choose the projection for the task

Projection is not decoration. Use a locally appropriate projection for regional work and state material distortions in global views.

Keep the base quiet

Boundaries, land, water, and labels should support the analytical marks. A loud basemap competes with the data.

Simplify geometry at small sizes

Dense coastlines and boundaries add file weight and visual noise. Match geometry detail to the final rendered scale.

Preserve geographic conventions

Use recognisable orientation, labels, and boundary hierarchy unless the analytical purpose clearly requires otherwise.

Anatomy

GeoJSON features supply geometry and properties; the projection converts coordinates to screen position; fills, borders, labels, and overlays establish context.

Reading list

  • Nivo GeoMap documentation - projections, GeoJSON features, and interaction.
  • Snyder, J. P. (1987). Map Projections: A Working Manual.