How the overlays are made
Every overlay is a scanned historic map warped onto real coordinates using control points read from features that still exist: lake tips, river mouths and junctions, towns. The method and the measured error are published for each map.
1. Source and rights
We start from an archive scan (or, failing that, a photograph) and record the rights status: public domain, permission on file, or restricted. Restricted maps never build for the public site.
2. Control points
We read pixel coordinates of features on the sheet and pair them with modern coordinates. Hydrography is preferred over village dots, because the cartographer traced the rivers and lakes from a base map but placed villages by hand. Each point carries a weight for how confidently it was read.
3. Warp
For printed maps in a consistent projection a second-order polynomial fit is enough (Wallace's Pennsylvania sheet: 82 points, RMS 2.0 km). For loose hand drawings a single fit is not: the 1962 redraw of Morgan's map reaches only 8 km with a polynomial, so it is rubber-sheeted with a weighted thin-plate spline pinned to 54 points (RMS 5.7 km). The residual at every control point is published in each overlay's control-points file.
4. Layers
The scan is separated into layers: dark ink (villages, names, rivers, boundaries), the trails (red dotted lines on Morgan's map, numbered dotted lines on Wallace's), and shading. Text panels, legends and decorative drawings are masked. The "tinted" layer recolors trails magenta and ink plum on a transparent background; the "scan" layer is the warped original.
5. Tiles
Both layers are cut into Web Mercator tiles (WebP with alpha) packed into single-file PMTiles archives served from object storage with HTTP range requests, so there is no tile server to run. The base map is a self-hosted Protomaps extract of OpenStreetMap data, served the same way.
Reading the accuracy
The stated accuracy is the typical distance between a feature on the overlay and its true position. It combines the original cartographer's error with ours. For the 1851/1962 map, treat a trail as "this valley", not "this side of the road"; village symbols on that sheet can sit 10–15 km from the archaeological site.
Current overlays
| Overlay | Method | Points | RMS (km) | Stated accuracy (km) |
|---|---|---|---|---|
| Gallatin's tribes of North America, 1836 | Single third-order polynomial fit to 21 coastal and lake features across the continent; the sheet is a small conic projection | 21 | 45 | 100 |
| Iroquois trails and villages, 1851 | Thin-plate spline rubber sheet pinned to lake tips, river mouths and junctions; weighted control points | 54 | 5.7 | 5 |
| Indian & Oklahoma Territories, 1894 | Second-order polynomial fit to 49 towns read from the sheet against their modern locations; the sheet is a GLO-derived polyconic at 1:760k | 49 | 6.8 | 10 |
| Southeastern trail system, 1928 | Third-order polynomial fit of thirty modern city locations to the plate's city symbols (symbols detected automatically, matched by graduated tolerance); the plate is a small-scale polyconic sheet, so trails are corridors, not surveyed lines | 30 | 6.3 | 10 |
| Nicollet's Upper Mississippi, 1843 | The map is two separately registered sheets; each fitted with a second-order polynomial to lakes, river junctions, falls and towns (22 control points kept), samples routed by sheet | 22 | 17 | 25 |
| Warren's map of the West, 1857 | The map is four separately registered sheets; each sheet fitted with its own polynomial (65 control points total: coastal capes, river confluences, forts, towns and boundary corners), samples routed to the sheet whose frame contains them | 65 | 35 | 50 |
Corrections
Found a misplaced feature or a better source? Use the corrections form. Corrections ship as a new overlay version with a changelog entry; permalinks always show the latest version.