Astronaut-acquired Orbital Photographs as Digital Data for Remote Sensing:
Spatial Resolution
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6. Summary and Conclusions
Astronaut
photographs can be an excellent source of data for remote sensing applications.
Best-case resolutions are similar to that for Landsat or SPOT with pixels
as small as < 10 m. We estimated that of images taken to date, 50.4
% had had lens and obliquity characteristics that make them potential
candidates for remote sensing information. Digitised astronaut photographs
can be overlaid with other satellite data using GIS (Eckardt et al. 2000),
or used to fill in gaps in time series when other imagery is not available.
As a source of public-domain information, they can be very useful for
scientists who do not have access to satellite imagery either because
of the expense of image acquisition (to the end user), or the computer
systems needed for processing satellite images. We summarise these differences
in image acquisition costs (to the user) in table 6.
Searching of the complete database of NASA astronaut photography, including low-spatial-resolution browse images, is available via the Web (Office of Earth Sciences 2000). This provides nearly global access for identifying images that will contribute to a specific scientific project. For the most detailed studies, digital products posted to the web will not be of sufficient quality. To date, we have made it a practice of digitising small numbers of images when requested by scientists at no charge. Such requests (including a description of the project involved) can be made through the authors or using the contact information listed on the website. Investigators with needs for larger numbers of digital images have also been served through collaborative agreements.
In our experience, scientists that find the data most valuable are those in developing countries, those studying areas that have not usually been targets for the major satellites, those having difficulty in finding low-cloud images, those interested in constructing time series, or those interested in using a large number of images.
Because use of astronaut
photography data is unfamiliar to most scientists and remote sensing experts,
we have tried to provide a general synopsis of its major characteristics.
Once common source of confusion about the data arises from its variable
spatial resolution. We have treated the issue of spatial resolution of
astronaut photographs to a level of detail that has not been previously
published. In addition, we provided a primer of equations that can be
used for calculating spatial resolution of a given photograph, and have
incorporated user-friendly methods for non-specialists to estimate spatial
resolution of specific photographs (using tools on our Web interface or
by downloading a spreadsheet). Although more variable than other types
of satellite data, the information in the images can be extracted using
familiar remote sensing techniques such as georeferencing and image classification.
This makes the data source valuable for remote sensing applications in
ecology and conservation biology, geography, geology and other related
fields.
Acknowledgements
We thank the astronauts, cataloguers, and scientists whose efforts over the last 25 years have developed and preserved the remote sensing resource described in this paper. Barbara Boland served as a primary beta-tester for the Photo Footprint Calculator, and helped to improve its user interface. James Heydorn did database searching to help in compilation of summary statistics and figure 5; he also did the programming for our web display of photo footprints. Joe Caruana researched older film types. James C. Maida helped to produce the three dimensional visualisation in figure 9. Karen Scott provided comments on this manuscript and input into the section on window effects. Serge Andréfouët, Kamlesh P. Lulla, Edward L. Webb, and anonymous reviewers made constructive comments that greatly improved the manuscript.
