Historically, aerial film cameras have been calibrated by the United State Geological Survey (USGS) to insure the instrument meets certain specifications. This calibration process ensures imagery taken with the camera is metric in quality and interoperable with traditional photogrammetric equipment. There is no comparable USGS process for digital sensors and sensor vendors typically perform the calibration. Digital sensors thus do not have the same type of endorsement that comes with the USGS calibration.
USGS, in collaboration with NASA, is prototyping an alternate approach to calibration for aerial digital sensors. Instead of calibrating the sensor, the process characterizes the quality of the imagery produced from a digital sensor and post-processing procedures. Emerge was one of the first companies to capture and deliver imagery for characterization, earlier this year.
Emerge brought its Digital Sensor System (DSS) to the Stennis Space Center test range in Mississippi to capture the required imagery. The Stennis test range is a 5 x 5 mile area containing accurately surveyed (less than 6 cm accuracy) targets. Emerge developed a typical 1 ft ground sample distance (GSD) flight plan and captured imagery of the range in January of this year. Emerge’s Andover, MA analysis facility turned the raw data into orthophotos. The final product was returned to the NASA staff at Stennis who compared the locations of the targets on Emerge’s imagery with those determined in the geodetic survey. The results were quite impressive: The imagery produced a Circular Error 90% (CE90) of 0.4775m, meeting the National Map Accuracy Standards requirement for a scale of 1:1,600. The CE95 was 0.5446m, which meets the Federal Geographic Data Committee (FGDC) requirement.
Emerge participated in this prototype characterization under the NASA Space Act Agreement. The work will, in part, form the basis of what may become the USGS characterization process for imagery produced using digital sensor technology. In the meantime, the NASA characterization process provides independent evidence of the image quality and accuracy that can be achieved using the Emerge DSS.
Emerge’s Gerry Kinn who’s been exploring methods to assess digital products and processes, states, “This prototype orthoimagery product characterization, which will hopefully become a standard, offers another validation tool to users of digital imagery. It also helps solidify digital imagery products and encourages market growth.”