This project is a step-by-step process that involves collecting data from various sources including, The Library of Congress, U.S. Geological Survey, and commercial websites. The process to create the georeferenced images begins with downloading the image(s) from the source website and giving the image a spatial reference for viewing in ArcMap. Next the image is opened in ArcMap and georeferenced with ArcGIS tools over a parcel layer. The image is then cropped and edited to a reasonable file size and then saved in a defined geographic projection. Once all this is complete the image will have gone from a simple scanned picture to an accurate, spatially referenced map that can be viewed and explored. “So far I have georeferenced over a dozen maps and historic aerial images with dozens more to go,” said Sean Emerson.
While the process seems to be clear cut, each map and image presents its own challenge. Some historic maps are easy to georeference because of the grid-like nature of Old Town Alexandria's streets. Meanwhile others are more difficult, such as maps that show a larger area or aerial images with no labels at all. The aerials present a unique challenge when georeferencing because there is no street label. Therefore, these can only be georefernced by geographic features alone.
“I have found that the easiest way to do this is to look for unique geographic features such as cemeteries, diagonal intersections, bridges, and even certain large buildings that stand out on an aerial image,” says Emerson. “I have also found that shorelines and railroad tracks are not useful in georeferencing because rivers and creeks around the city have changed drastically over the years due to erosion and sedimentation and many rail yards in, and around, Alexandria have been torn up and replaced with high rise condominiums and other developments.”
The enormous changes in Alexandria are very evident in these maps. For example, parts of the city are identified as "Alexandria, D.C." on many older maps because the city was once part of the District of Columbia. The aerial images from the 1940s-1960s, when compared to current images, show how much development has occurred in the city. Many images show the Capital Beltway under construction along Cameron Run, and much of the interstate is built on reclaimed land that used to be part of the creek. Comparing images of the beltway reveals how the road has been supersized to meet traffic demands as a result of the development in the area, going from a six lane highway with basic interchanges in 1964, to a 14 lane highway with spaghetti looking interchanges at every exit in 2012.
|Photo Courtesy of: MapStory.org|
“It is fascinating to see the city change and grow over time,” said Emerson. “Working on this project has taught me a lot about the history of Alexandria, from its beginnings as an industrial port city, to a fairly wealthy city and suburb adjacent to Washington, D.C. I hope that this project shows people that each community has a rich history, which often goes untold and unnoticed. This project has taught me that a unique way to tell these stories is through old maps and aerial photos, which can best show the widespread change that occurs when a city grows. The story of Alexandria's history is just one of thousands that can, and should be told for the benefit of future generations.”