Abstract
Observing changes in Earth surface topography is crucial for many Earth science disciplines. Documenting these changes over several decades at regional to global scale remains a challenge due to the limited availability of suitable satellite data before the year 2000. Declassified analog satellite images from the American reconnaissance program Hexagon (KH-9), which surveyed nearly all land surfaces from 1972 to 1986 at metric resolution, provide a unique opportunity to fill the gap in observations. However, large-scale processing of analog imagery remains challenging. We developed an automated workflow to generate Digital Elevation Models (DEMs) and orthophotos from scanned KH-9 mapping camera stereo images. The workflow includes a preprocessing step to correct for film and scanning distortions and crop the scanned images, and a stereo reconstruction step using the open-source NASA Ames Stereo Pipeline. The processing of several hundreds of image pairs enabled us to estimate reliable camera parameters for each KH-9 mission, thereby correcting elevation biases of several tens of meters. The resulting DEMs were validated against various reference elevation data, including snow-covered glaciers with limited image texture. Pixel-scale elevation uncertainty was estimated as 5 m at the 68% confidence level, and less than 15 m at the 95% level. We evaluated the uncertainty of spatially averaged elevation change and volume change, both from an empirical and analytical approach, and we raise particular attention to large-scale correlated biases that may impact volume change estimates from such DEMs. Finally, we present a case study of long-term glacier elevation change in the European Alps. Our results show the suitability of these historical images to quantitatively study global surface change over the past 40-50 years.
Highlights
The Earth’s surface has evolved dramatically over the last century as a consequence of anthropogenic activities and climate change (IPCC, 2019; IPCC SROC, 2019)
We presented ASPy, an automated workflow to generate Digital Elevation Models (DEMs) from analog Hexagon (KH-9) mapping camera images, which covered most of Earth’s land surface between 1973 and 1980
The processing uses the crude image geolocation provided by the USGS and an external DEM to avoid the labor-intensive process of manually identifying Ground Control Points (GCPs)
Summary
The Earth’s surface has evolved dramatically over the last century as a consequence of anthropogenic activities and climate change (IPCC, 2019; IPCC SROC, 2019). Documenting these changes at regional scales over such a time span remains a challenge (James et al, 2012). Prior to the Shuttle Radar Topography Mission (SRTM) in 2000, no elevation data are available at a global scale with the decameter resolution required to study Earth surface deformation. A potential source of data to fill in the temporal gap is declassified American intelligence satellite imagery, such as from the Corona and Hexagon programs (Galiatsatos et al, 2007; Burnett, 2012; Fowler, 2013).
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.