Most of the maps people use are themselves assembled from other maps. A state geologic map is stitched together from county or quadrangle sheets; a national compilation is built from state maps; Macrostrat's global map is built from all of these. Macrostrat represents this directly, as compilations.
Before version 2, Macrostrat's multiscale map was assembled by hand-set priorities among its sources. Since October 2026, overlaps are resolved by the topology model described here, which is what lets a quadrangle sit inside a state map inside a continental map without seams or double coverage. The first compilations released this way are the USGS National Geologic Synthesis of the United States, the geologic map of Japan, and Arizona.
Maps and compilations
Every map in Macrostrat is a source. A source can have members: other sources it is assembled from. A source with members is a compilation; a source without them is an individual map. A compilation is itself a map: it has a footprint, can be cited, and can be a member of a larger compilation. Compilations can nest to any depth.
Members combine in one of two ways:
- Topological: members overlap, and where they do, a priority decides which one is shown. Typical of compilations that layer detailed maps over coarser ones.
- Mosaic: members partition the area between them without overlap, like the county sheets of a state map. Each member is shown inside its own footprint.
Some compilations arrive as a single published dataset that already merges many maps; others are assembled inside Macrostrat from maps ingested separately.
Footprints, territories and priority
Each map has a footprint, the area where it is defined. Where footprints overlap, priorities on the compilation's members decide which map wins, location by location. The area a map actually wins within a compilation is its territory. Territories are computed with a topology engine that maintains a non-overlapping coverage of faces, so that any location resolves quickly to the one map, and the one unit, that should be shown there. (The same engine underlies the Mapboard GIS editing app.)
Priority is authored, not inferred. Placing a detailed map above a regional one is an editorial decision, and recording it explicitly lets Macrostrat explain why any polygon on the global map comes from the map it does.
When a newer map replaces an older one, the older map is marked as superseded, and the replacement is recorded.
Served layers
The compilations that Macrostrat publishes as map tiles are served layers. The four base layers follow the scale buckets (tiny, small, medium, large) and gather the maps of each scale. The carto layers combine them into the map most people see: at each zoom level, the most detailed appropriate map is drawn, with coarser maps filling in wherever detailed mapping is missing.
Why compilations matter
Treating compilations as first-class maps solves several long-standing problems:
- Credit. A large compilation can hide dozens of contributing surveys behind a single citation. Recording its members keeps every contributing map visible and citable.
- Updates. When one member map is revised, only its area of the compilation needs to change, instead of rebuilding the whole global map.
- Transparency. Every polygon on the global map can be traced through the compilations it belongs to, back to the map it came from.
Earlier versions of Macrostrat built the carto map once, with implicit priorities. The compilation system replaces that one-time build with a live structure that can be inspected and revised.
