Tools for Getting Historical Photos in GIS

There is a fortunate convergence of technologies that makes it easier than ever to get historical photography into our GIS. The most important technologies are the newly available orthocorrection software tools and the online data and camera reports. Also of value are the simple techniques for creating pseudocamera reports.

Although image orthocorrection is not required to detect changes, orthocorrection and subsequent analysis in GIS provides two major advantages:

1. changes are much easier to detect using the built-in image display options in widely used GIS mapping applications, and
2. documenting the magnitude of change is much easier and can usually be objectively quantified.

Orthocorrection Software Tools

Commercially available orthocorrection software makes powerful softcopy photogrammetry tools available to all Forest Service personnel. This software has a significant learning curve, but it allows users to efficiently orthocorrect large blocks (or sets) of overlapping photos.

Online Data (DOQs, DEMs, and Camera Reports)

In addition to the scanned photography, orthocorrection requires horizontal and vertical reference data and information about the camera that acquired the photography. Typically, digital orthophotoquads (DOQs) are used for the horizontal reference, digital elevation models (DEMs) are used for the vertical reference, and a USGS Optical Sciences Lab Camera Report is used for the camera information. All of this data is available online, allowing for far more efficient data preparation for orthocorrection.

Creating Pseudo-Camera Reports

USGS camera reports are not available for any resource photography prior to 1973. When one is needed, it is fairly easy to create your own pseudocamera report. The vital information needed from an official USGS camera report includes: the principal point, the fiducial measurements, the fiducial orientation (with respect to the data strip), and the lens focal length. If we know the scan resolution of the scanned photo, the dimension of one pixel in the image is known, (e.g., if the image was scanned at 600 dpi, each resulting pixel is 1/600 inch) and distances between fiducials on the image can be measured. We have created a simple spreadsheet that converts imaged fiducial file coordinates to millimeters in a camera coordinate system similar to USGS camera reports. For these pseudocamera reports, we have to assume the principal point is at coordinate 0,0 and we initially assume the focal length is 6 inches (152.4 mm). The fiducial orientation can be defined by the user—but then has to be applied consistently.

Obviously, these pseudocamera reports are not as accurate as official camera reports. The spatial accuracy of the resulting orthocorrected photography will likely be less accurate as well. However, for many resource applications, satisfactory accuracies can be obtained.