Location: Range Management Research
Title: Field validation of 1930s aerial photography: What are we missing? Authors
|Archer, Steven -|
|Byrne, Andrew -|
Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 3, 2009
Publication Date: June 1, 2009
Citation: Browning, D.M., Archer, S.R., Byrne, A.T. 2009. Field validation of 1930s aerial photography: What are we missing? Journal of Arid Environments. 73:844-853. Interpretive Summary: Changes in land cover and land use play a pivotal role in driving global change. Early aerial photography (circa mid 1930s) serves as the baseline for the longest time series of imagery aviailable for monitoring changes in land cover; however, there are no published records documenting the detection limits and accuracy of vegetation cover estimates derived from early aerial photography due to scarcity of spatially explicit field data. We translated historic field measurements of shrubs and cacti (from 1932) to a spatially explicit format to quantify detection limitations of 1936 aerial photography for mapping mesquite cover and biomass. Our results benefit members of the remote sensing science community as well as State and Federal agencies (e.g., National Resource Conservation Service, Bureau of Land Management, Bureau of Reclamation, and U.S. National Forest Service) that use historic aerial photography by documenting limitations of base-line assessments using 1930s aerial photography. In addition, members of the academic community involved in modeling ecosystem responses to climate change will benefit from our validation of historic depictions of plant biomass.
Technical Abstract: Aerial photography from the 1930s serves as the earliest synoptic depiction of vegetation cover. We generated a spatially explicit database of shrub (Prosopis velutina) stand structure within two 1.8 ha field plots established in 1932 to address two questions: (1) What are the detection limits of panchromatic 1936 aerial photography?, and (2) How do these influence P. velutina biomass estimates? Shrub polygons were manually digitized on 1936 imagery and linked to 1932 field measurements of P. velutina canopy area. Aboveground 1932 P. velutina biomass was estimated using a site-specific allometric relationship for field-measured canopy area. Shrub canopy detection limits on the 1936 imagery were comparable to those reported for contemporary imagery. Based on a conservative shrub size detection threshold of 3.8 m2, 5.8% of P. velutina biomass was missed. Spatial resolution (0.6 vs. 1.0 m) did not influence detection limits, but the overall accuracy of shrub cover estimates was greater on 1.0 m images. Presence of the sub-shrub Isocoma tenuisecta may also have significantly influenced estimates of P. velutina canopy area. These analyses illustrate the importance of standardizing aerial photo interpretation protocols, accounting for uncertainty estimating shrub biomass, and caution species-specific interpretations for historic aerial photography.