How can an old map be aligned to modern coordinates?
Linking a scanned historical map to modern positional data is called georeferencing. First establish the map's date, scale and likely coordinate reference; then match places that have remained recognizable on both the old map and a reliable modern layer as ground control points. A low software error number alone does not prove the historical drawing was accurate.
Control points and transformation choices
1. How trustworthy is the scan itself? Old paper may have been folded, stretched during copying, damaged at its edges or scanned crookedly. Marginal coordinates, sheet title, date, scale, legend and the producing organization provide initial clues. Line width and the surveying methods of the period limit positional detail. A mapped shore, river or road may genuinely have moved; disagreement with a present-day image is not automatically a registration error. Keep the original scan and source description unchanged, and produce a separate processed copy. Note whether a sheet has missing margins that could have contained coordinate ticks. 2. Which locations make good control points? The QGIS Georeferencer matches points on an unreferenced image with known positions. A permanent rock promontory, an old bridge abutment or a stable intersection visible in both datasets can be useful. A temporary sand bar, recently built house or re-routed bend is weak evidence. Points clustered in one corner can fit that corner while distorting the far edge; distribute them around the boundary and through the interior. Record every point's modern coordinate, the data source and why the feature is thought to be the same place. A visually neat fit can hide one mistaken identification. 3. Which transformation should be used? If the sheet is only shifted, rotated and uniformly scaled, a simple geometric model may be enough. If paper was stretched locally, a flexible polynomial may fit more points but can also bend the original shape unrealistically. QGIS requires an algorithm, target coordinate reference system and resampling method to be chosen together. Do not assume the datum of coordinates printed on an old sheet. Directly copying numbers between incompatible datums or projections can create a systematic offset even if the chosen control points look plausible.
Residual error, output and interpretation
4. How should the error table be read? Software shows each control point's residual after transformation. A very large residual can indicate a wrong match or a feature that moved. A small root mean square residual for the points used to create the model is not an independent validation of its accuracy. Test a few well-distributed points that were not fitted. The old map's drawing scale and original survey uncertainty can exceed a five-meter numerical residual by a wide margin. State whether the error uses map units or pixels; never present a fitting statistic alone as absolute ground truth. 5. How are new pixels made? Geometric transformation creates a new raster grid. Nearest-neighbor resampling can preserve the original map's discrete class colors. Bilinear or cubic methods may look smoother, but can mix those colors. Interpolation artifacts around an old line should not be counted as real historical shoreline movement. A georeferenced GeoTIFF or an image accompanied by a world file can carry location information, depending on the selected export. Retain the output resolution, model, point table and software settings so that someone else can repeat or critique the alignment. 6. What does overlaying old and new maps establish? Displaying an old and modern map with transparency can guide research into settlement and shoreline change. USGS historical chart recovery illustrates selecting relatively stable features and explaining transformation error. Nevertheless, not every mismatch along a river proves erosion or land reclamation: historic drafting, source datum, scanning and the date of the reference layer introduce separate uncertainties. Publish comparisons with dates, scale, source, coordinate systems and independent check points. The map is an interpreted record, not a time machine that reconstructs every vanished boundary exactly.