Methodology, sources and limits

This page exists so that you can check us. It states exactly what we show, where each document came from, how accurately it is placed on a modern map, and what does not follow from any of it.

What PlaceLayers does

We reproduce public-domain historical documents about places in the United States, together with the catalogue record for each one, and we describe what the conventions on those documents mean. That is the whole product.

What we never do: we do not state or imply that a property is contaminated or clean; we do not produce a risk score, a rating, a grade or a colour-coded verdict for any property; we do not aggregate documents into a conclusion about a parcel; and we do not present our pages as an environmental site assessment, a survey, or advice of any kind.

Sources

Sanborn fire insurance atlases

From the Library of Congress Sanborn Maps collection, Geography and Map Division. The Library states that Sanborn maps published before 1931 are in the public domain and may be downloaded without permission; editions from 1931 onwards remain restricted and are not reproduced here. See the Library’s copyright guidance. Editions in this release:

• 1890, Vol. 1 — 86 sheets. Library of Congress item sanborn03573_001.
• 1890, Vol. 2 — 90 sheets. Library of Congress item sanborn03573_002.
• 1901, Vol. 1 — 109 sheets. Library of Congress item sanborn03573_004.

Sheet images are delivered by the Library of Congress IIIF image service and each one links to the official viewer and catalogue record. Coverage counts on this site come from the Library’s public JSON API and were verified on 2026-08-13: 62 Baltimore items in total, 22 of them dated 1880–1930 (query).

USGS historical topographic maps

From the USGS Historical Topographic Map Collection, which holds more than 185,000 georeferenced sheets printed between 1884 and 2006, all public domain. We locate maps through the TNM Access API; 46 quadrangles intersect our central-Baltimore bounding box (query). Maps in this release:

• 268 historical USGS topographic quadrangles (1887–2002), each cropped to its printed neatline and placed on its published corner coordinates. Every map's ScienceBase source record is linked from the overlay tool; the full index is data/topo-maps.json.

How maps are placed, and how wrong that placement can be

Historical topographic quadrangles are supplied by USGS as georeferenced files. For the overlay we crop each sheet to its printed neatline and place it on the corner coordinates published for that quadrangle. Several sources of error remain, and they add up:

• Original survey error. A sheet drawn in 1908 at 1:125,000 was never intended to place a building. At that scale one millimetre on paper is about 125 metres on the ground.
• Paper and printing. Sheets shrink, stretch and were printed in registration that varies.
• Scanning and resampling. Digitisation and reprojection introduce further displacement.
• Datum differences. Older sheets predate modern datums; shifts of tens of metres are normal.

Treat the overlay as indicative at the scale of a street or a block. Do not use it to locate a parcel boundary, a tank, a building corner or anything else that a surveyor would be asked to locate.

Sanborn sheets are presented as images, not as a georeferenced overlay, precisely because we are not willing to imply a positional accuracy we cannot support. You navigate them the way an underwriter did: through the index sheet to a sheet number.

What does not follow from a historical map

• That a use recorded on one edition existed before or after that edition. A 1901 sheet describes 1901.
• That an absence of lettering means an absence of use. Surveyors recorded what mattered to fire underwriting, and they made mistakes.
• Anything at all about subsurface conditions. Maps show surfaces and structures.
• That a tank drawn on a sheet was ever removed, or that one not drawn was never installed.
• Anything about the current condition, value or safety of a property.

Corrections

If we have mis-transcribed a document, mislabelled an edition or placed a map badly, tell us and we will fix it and say that we did. Historical accuracy is the only asset this site has.

Privacy

We do not publish the names of living people. Where a user contributes a photograph or a recollection, we moderate it before publication and we remove personal information about living people on request.