Digitizing Large Format Aerial Photography Transparencies: Part II
In Part I of this post, I summarized the workflow steps employed in digitizing 9x9 large format film from a 2002 aerial survey of Connecticut. In Part II, I'd like to take a deeper dive into the resulting image and compare it with similar aerials taken through time that used different film stocks and digital technologies.
The 2002 image that I converted in our lab was taken above Watertown, CT. In addition, the USGS hosts Watertown aerial photos from 2008 and 2012. Here's a look at these three surveys in succession:
Though they all have the same 1:1 aspect ratio, only the 2008 and 2012 images were shot at the same scale. So, let's zoom in to a particular area of common interest among the three, resize them all to a comparable resolution and do a more precise and balanced visual examination. On the 2002 survey, I've outlined part of the playing fields on The Taft School's campus that appear in all of the aerials:
While digitizing the 2002 aerial photo, I also included the film's edge information which like many types of film stocks can contain a multitude of coded technical information. In this case, "Wild 15/4" can be found along the film's upper border.
This indicates a Leica aerial camera was employed in the original photography. Additionally, the dark manner in which water is rendered throughout the town and the clear separation of conifer and deciduous foliage suggest the use of black and white infrared (B&WIR) film. Digitized at 14,000px along the long edge, the film's grain is clearly discernible. But so is Taft's football field end-zone lettering and pole vault area:
For the 2008 survey, color film stock was the choice of the day's flight which occurred on April 3rd of that year. Though the film's edge information is cropped out of this USGS-sourced image, the file downloads from the USGS' site as part of a zip-archived data set. Within this bundle, an invaluable XML file exists that contains interesting technical and process metadata on the image. There you can learn that the USGS outsourced the original shoot to AeroMetric, Inc. who used a Zeiss Intergraph aerial photography film system. Interestingly enough, in the metadata's <procdesc> field it is also noted that the image was dodged. Perhaps this may have contributed to some of the grain in the final product. Here's Taft's football field once again. Though the sun's angle is different between shots, observe how the track's lane markers and bleacher seats can act almost like comparative resolution targets in the 2002 and 2008 images:
By 2012, the Watertown survey was a born digital asset. According to the metadata in the shoot's bundled data set, USGS contracted with Kentucky's Photo Science, Inc. for the work. As an interesting aside, Photo Science and AeroMetric, along with Watershed Sciences, merged the following year to form Quantum Spatial. Once again, a Zeiss/Intergraph imaging system was employed in the 2012 flight. However this time the device includes the "Digital Mapping Camera (DMC)" designation in its name. As a result, the view from 10,000' was not projected onto large format film but was instead passed onto a mosaic of digital sensors, most likely DALSA CCD chips. The solely digital workflow resulted in a very clean signal:
Today's current state of the art aerial camera rigs include the 250MP Leica Geosystems’ DMC III which can capture up to 25,000+ pixels across a single CMOS chip, and Vexcel Imaging's UltraCam Eagle II, a system originally co-developed with Microsoft that can use a variety of interchangeable lenses on its multiple CCD sensor arrays.
Finally, if this tale hasn't gotten you fired up yet about aerial imaging, then try Leica's DMC III marketing video below. It's like a great pre-game locker room speech, not necessarily for Taft's football finest, but for those looking to further hone Occam's razor in the name of supreme image-making geekery!