Dataset and Initial Image Processing
One-hundred and thirteen monochromatic aerial images were captured
from August 23rd 1952 to February 14th 1953 (referred to as the 1953
images) by the US Department of Agriculture. They cover an area 700
km2 (270 mi2) that includes all of Baltimore City, and an area
immediately adjacent to the City at a scale of 1:20,000. The 113
images used in this analysis were a subset of a larger set consisting
of 359 aerial images of all of Baltimore City and County between July
12th 1952 and February 14th 1953. The reason for capture is unknown.
Initial digital images were provided by Johns Hopkins University
Sheridan Libraries. Image manipulation software was used to extract
the borders from the raster images to correspond to information
boundaries.
Georectification
Each image was assigned the WGS 1984 Web Mercator (auxiliary sphere)
projected coordinate system. An ESRI basemap of 2015 imagery was used
to georectify the 1927 images in ArcGIS. Five control points were
selected, with one in each of the NW, NE, center, SW, and SE areas of
an image by identifying the same point in 1953 and 2015. Control
points included buildings, intersections and other landmarks known to
exist in the same location in both time periods.
Transformations were applied to finalize the images to the basemap
with the goal of keeping RMS error as low as possible, iteratively for
each image. Of the five transformation types available in ArcGIS, two
consistently yielded the best results: 1st Order Polynomial (Affine)
and Projective Transformation. Each image was transformed using one of
these methods depending on the circumstances for each. Finally, the
set of 113 images were “stitched” together using the Mosaic tool in
ArcGIS to create a single raster layer of the coverage area.
