Sanborn Map geoferencing

This page provides a workflow for bringing Sanborn Fire Insurance Maps into web geographical coordinates using Open Source software.

Sanborn Maps are available for urban areas in the United States from 1867 to the present and they are especially useful for documenting the footprint and location of buildings and adjacent structures. Sanborn maps have additional information such as building construction material, zoning information, density, and sometimes a neighborhood's predominant ethnicity. .

While Sanborn Maps were produced commercially under copyright the maps over 97 years old are in the public domain and available for OpenHistoricalMap.org mapping. Over 500,000 color Sanborn maps are available from the Library of Congress by the end of 2020. Some university libraries have access to Sanborn maps through their library proxy through Proquest.

Confirm that the georeferencing work hasn't been done already and available among the Fire Insurance maps at DavidRumsey.com.

Georeference in Mapwarper.net

Instructions on OHM Wiki page

See detailed instructions using Wikimedia and Mapwarper

Best practice for most OHM mapping is to georeference using Mapwarper.net and have these maps available for tracing in JOSM or iD editor however Sanborn Maps and other large scale basemaps may benefit from the method described below.

Approach using QGIS 3.x and JOSM open source software

This alternative workflow allows you to snap to county parcel lines which speeds up the process of georeferencing Sanborn maps.

An example of aligning warping GCPs to parcel vertices.

Initial Steps

1. QGIS (Free and available for Mac/Linux/Windows) is a widely used Geographical Information System. There are a number of QGIS Georeferencer tutorials online and walk throughs on Youtube including this Georeferencer tutorial that provides the basics. This approach will work in Arcmap or ArcPro as well.
2. For compatibility with web mapping georeference into Web Mercator (EPSG 3857) coordinate system
3. Acquire public domain color Sanborn maps from the Library of Congress. Save Index and individual Sanborn pages locally as JPG2000 files. Batch rotate JPG files so North is up if necessary. If Sanborns aren't available at LOC check DavidRumsey.com or via a paid library subscription at Proquest.
4. Acquire Parcel lines from county or city GIS centers (free and open in most jurisdictions).
5. Reproject county parcel lines into EPSG 3857 (or rely on Reproject on the fly).
6. Download Buildings from best source. Compare with OSM or Microsoft building footprint project, or from the county or city web sites themselves. To tag footprints with Year_Built field from County or City sources use a spatial join to attach attributes to the best building footprint geometry. Use the Year_Built tag to filter out newer or modern buildings from your map.
7. In QGIS 3.x start a project and use the QuickMapServices plugin to load a Bing Satellite layer, Parcel data, and use these to georeference the Sanborn index and map pages.

QGIS Georeferencer for each Sanborn page

1. In QGIS turn on Snapping - Intersections on the Parcel vector layer then Use the parcel boundaries on the Sanborn and snap to parcel vectors. Beware of corners as some are modified and do not reference well to historical parcels so mid-block parcel edges are good.
2. A general QGIS Georeferencer tutorial is available. Set the Transformation settings: In QGIS Georeferencer choose Settings > Transformation Settings > 1st Order Polynomial. If this is not generating a good fit due to distortions then consider Thin Plate Spline transformation.
   • Dealing with residual error
   • Place at least 4 Ground Control Points (GCPs), preferably 6 - 8 or more on parcel intersections (and create a few extra GCPs so you can throw out the bad ones). Click the Start Georeferencing... button to see coordinates and results in the GCP table.
   • Ideally you should have no more than 2 or 3 pixel residual on any GCPs. Sort by the Residual column to see the highest residual GCPs. Are those GCPs on one side of the map? Consider if the unused street portion is not to scale, there are creases on the original, or other distortions. Remove the highest residual points and see how the other GCP residual values respond.
   • Consider clipping the map into individual city blocks (use an image editor such as GIMP), or in lieu of clipping create several versions of georeferencing of the same Sanborn page (e.g., North and South versions).
   • Encircle critical areas with GCPs on parcel intersections for the best results. Save to GeoTIFF file and save the GCP points CSV table as well in case it has to be re-done.
3. In Transformation settings check the box Use 0 for transparency when needed" so that you can view adjacent georeferenced Sanborns together by removing the collar around the scan. After georeferencing this is set in Layer Properties > Transparency > Additional no data values: 0
4. Consider uploading these georeferenced map .TIF files to Commons as {{map data type for use by others. The GeoTIFF header tags will be preserved for use by others and it is possible to batch upload and tag so this step goes quickly. Please make sure to add "Category:Sanborn_maps" as a category in Wikimedia Commons to be able to find them again.

Digitizing building footprints in JOSM

While it is possible to digitize buildings from Sanborn Maps directly in QGIS using JOSM provides tools for digitizing orthogonal buildings, tagging, topological validation, and uploads directly into OpenHistoricalMap.org to share with others.

1. Download and install Java OSM Editor (JOSM)
2. Start a new project and from the Imagery menu enable OSM Carto and sharp satellite imagery layers in your area
3. Load GeoTIFF layers of georeferenced Sanborn pages into JOSM using the ImportImage JOSM plugin.
4. Read in Shapefiles containing the parcel vectors or other useful reference layers using the OpenData JOSM plugin.
5. Trace, orthogonalize (Q), then immediately tag each house with addr:housenumber for the street address
6. When you complete a street group tag all the street footprints with addr:street to add street name.
7. Before uploading group tag entire map page footprints with the other tags such as

• start_date=before 1911
• end_date=1923-09-17
• source=1911 Sanborn Map
• wikidata=Q4561337
• wikipedia=en:1923 Berkeley Fire