How the maps were made

Where the maps come from

Most of the maps were scanned from the Planning Department’s map books on its large-format scanner. They come in series, one for each edition of the map book: 1921, 1929, 1935, 1948, 1960, 1964, 1968, 1972, 1975, 1980, and the zoning, height and special-use maps of 1980–90, 1990–96, 1996–2000 and 2000. The 1948 atlas images come from the David Rumsey Map Collection.

How they were lined up with today’s streets

Old map sheets don’t sit on a modern grid. Paper stretches, and every street was drawn by hand. Aligning a sheet by hand means clicking matching street corners on the scan and on a modern map, one at a time. We wrote a program to do it instead.

First, find the sheet. An AI vision model reads the printed block numbers and street names on each page. The program matches them to today’s blocks and intersections from DataSF and places the sheet roughly, throwing out readings that disagree with the rest.

Then, fit it closely. It traces the drawn street and block lines and matches hundreds to thousands of block corners to today’s block outlines, not the eight or so a person would click. It bends the sheet with a smooth warp that corrects stretched paper and drafting error over whole areas.

Keep the history. Some blocks really have changed: streets were widened, shorelines filled, neighbourhoods like the Western Addition rebuilt. Where an old edge is consistently off from today’s in a way drafting error can’t explain, the program marks it as real change and leaves it where the old map put it, rather than pulling it onto the modern line.

Finally, stitch and tile. Sheets are joined into one layer per series, the white paper is made transparent so today’s blocks show through, and the result is cut into web map tiles.

How accurate they are

Each sheet is tested on blocks it was not fitted to: the program leaves some blocks out, fits the rest, and measures how far the left-out corners land from today’s. The hand-drafted maps themselves are only good to about ±4 feet at a street corner, so that is roughly the floor.

A sheet passes when half its corners land within 5 feet and 95% within 12 feet. Coarser scans get a proportionally looser bar: the 1980-and-later maps are office-copier scans at about 4 feet per pixel, and pass at about 7½ and 18 feet. A sheet also fails if it looks misplaced or the fit didn’t settle.

• 369map sheets
• 286passed automatically
• 83flagged for review
• 4–6 fttypical median error

Of the 83 flagged sheets, 26 had too few readable labels to place at all; they are missing from the maps until someone places them by hand. The rest appear, but may be off by more than the bar. Zoning boundaries will later be snapped to modern lot lines, which no warp of a hand-drawn map can match.

Source code and reuse

The code for the map pipeline and this site is open source, licensed under the GNU AGPL v3.0 and published on GitHub. The pipeline can align other cities’ historic maps too.

The story, the research and the processed maps are licensed CC BY-NC-SA 4.0: you can reuse and adapt them with credit, for non-commercial purposes, under the same license. That is not an open source license. The original zoning maps are, to the best of our knowledge, in the public domain.

Corrections, fixes and adaptations are welcome; the repository explains how to contribute.

Sources

Credits

• 1948 zoning atlas: David Rumsey Map Collection, David Rumsey Map Center, Stanford Libraries, licensed CC BY-NC-SA 3.0.
• 1921–2000 zoning maps: scanned from the San Francisco Planning Department’s map books on its large-format scanner; the 1921 ordinance is also held by the San Francisco Public Library (call no. 917.946 Sa52b 1921).
• Ordinances, codes and reports: many are digitised by the San Francisco Public Library on the Internet Archive.
• Base map: today’s blocks, built from DataSF parcels and street centerlines.
• Map alignment: sfzmaps, this project’s own georeferencing pipeline. See how the maps were made.
• About and license: sfzoning.com by Steven Bacio for GrowSF (https://growsf.org). © 2026 GrowSF. Code: AGPL-3.0, with the source on GitHub. Content and data: CC BY-NC-SA 4.0. Provided as is, without warranty, and not an official zoning record. GrowSF™ is a trademark of GrowSF.