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 "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\u2019s Orthoimagery toolbar with a goal to produce a mosaic 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. Three-dimensional ground control points of high accuracy were added to the project to minimize the root mean squared alignment error as checked by 30-40 independent test points. If the user\u2019s 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 March 18th, 2000 and April 25th, 2001 on various dates. The 6,449 photos cover Allegheny, Armstrong, Beaver, Butler, Fayette, Greene, Indiana, Washington, and Westmoreland counties in Pennsylvania. These aerial photos were taken by EarthData International for the Southwestern Pennsylvania Commission (SPC). The photos are at a scale of 1:14,400 with a focal length of 153.0mm. 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.",
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