Description: Historic aerial photography seamless mosaics were generated by staff of the Donald W. Hamer Center for Maps and Geospatial Information (https://libraries.psu.edu/maps) using a structure-from-motion photogrammetry workflow as implemented in either Agisoft Metashape Professional or ESRI ArcGIS Pro’s Orthoimagery toolbar with a goal to produce a mosaic of at least 10-meter horizontal positional accuracy that was free of seamline errors and other common image distortions. For collections that were originally flown with calibrated metric cameras with known camera calibration information, this was used in the interior orientation or alignment step of the photogrammetry workflow and as an initial estimate for self-calibrated camera parameters. Where camera calibration information was unknown or never existed (non-metric cameras), camera parameters were estimated by the software with an approximate focal length starting estimate of 152mm. To achieve the horizontal positional accuracy goal, three-dimensional ground control points of high accuracy were added to the project until the root mean squared alignment error as checked by 30-40 independent test points measured less than 10-meters as described by ASPRS Positional Accuracy Standards for Digital Geospatial Data 2024. If the user’s project requires a specific horizontal or vertical positional accuracy, we encourage you to perform an independent accuracy assessment of the orthomosaics and/or contact us about producing an orthomosaic of higher positional accuracy. Also note that horizontal positional accuracy will always decrease near the edges of an orthomosaic. We chose to include edge areas of orthomosaics rather than crop them because the image edges may contain important information for a given project. Note that the original images will cover a somewhat larger area than the orthomosaics as a result of the photogrammetry workflow. For more information on the methods used to generate orthomosaics and/or to cite this work please see the below reference: Nathan B. Piekielek, Grace Brittany Waltemate and Karen Schuckman (2022) Evaluating Software Tools to Orthorectify Archival Aerial Photographs, Journal of Map and Geography Libraries, 18:3, 209-229, DOI: 10.1080/15420353.2023.2219257. This collection of historic aerial photographs was captured between November 1st, 1990 and May 11th, 1991 on various dates. The 835 photos in this collection cover areas in the following Pennsylvania counties: Allegheny, Armstrong, Beaver, Butler, Indiana, Washington, and Westmoreland. These photos were taken by Kucera International for the Southwestern Pennsylvania Regional Planning Commission (SPRPC). This collection was given to the Donald W. Hamer Center for Maps and Geospatial Information by the PA Department of Conservation and Natural Resources, Bureau of Geological Survey Library. The photos were scanned at 800 dpi on an Epson Expression 11000 scanner by the Donald W. Hamer Center for Maps and Geospatial Information.