title: Modeling vegetation heights from high resolution stereo aerial photography: an application for broad-scale rangeland monitoring.
abstract: Vertical vegetation structure in rangeland ecosystems can be a valuable indicator for assessing rangeland health and monitoring riparian areas, post-fire recovery, available forage for livestock, and wildlife habitat. Federal land management agencies are directed to monitor and manage rangelands at landscapes scales, but traditional field methods for measuring vegetation heights are often too costly and time consuming to apply at these broad scales. Most emerging remote sensing techniques capable of measuring surface and vegetation height (e.g., LiDAR or synthetic aperture radar) are often too expensive, and require specialized sensors. An alternative remote sensing approach that is potentially more practical for managers is to measure vegetation heights from digital stereo aerial photographs. As aerial photography is already commonly used for rangeland monitoring, acquiring it in stereo enables three-dimensional modeling and estimation of vegetation height. The purpose of this study was to test the feasibility and accuracy of estimating shrub heights from high-resolution (HR, 3-cm ground sampling distance) digital stereo-pair aerial images. Overlapping HR imagery was taken in March 2009 near Lake Mead, Nevada and 5-cm resolution digital surface models (DSMs) were created by photogrammetric methods (aerial triangulation, digital image matching) for twenty-six test plots. We compared the heights of individual shrubs and plot averages derived from the DSMs to field measurements. We found strong positive correlations between field and image measurements for several metrics. Individual shrub heights tended to be underestimated in the imagery, however, accuracy was higher for dense, compact shrubs compared with shrubs with thin branches. Plot averages of shrub height from DSMs were also strongly correlated to field measurements but consistently underestimated. Grasses and forbs were generally too small to be detected with the resolution of the DSMs. Estimates of vertical structure will be more accurate in plots having low herbaceous cover and high amounts of dense shrubs. Through the use of statistically derived correction factors or choosing field methods that better correlate with the imagery, vegetation heights from HR DSMs could be a valuable technique for broad-scale rangeland monitoring needs.
authors: United States Department of Agriculture - Agricultural Research Service (USDS-ARS), Jornada Experimental Range, P.O. Box 30003, MSC 3JER, New Mexico State University, Las Cruces, NM 88003-8003, USA. Electronic address: jgillan@nmsu.edu.
authors: Jeffrey K
authors: JK
authors: Gillan
authors: United States Department of Agriculture - Agricultural Research Service (USDS-ARS), Jornada Experimental Range, P.O. Box 30003, MSC 3JER, New Mexico State University, Las Cruces, NM 88003-8003, USA.
authors: Jason W
authors: JW
authors: Karl
authors: U.S. Geological Survey, Southwest Biological Science Center, 2290 SW Resource Blvd. Moab, UT 84532, USA.
authors: Michael
authors: M
authors: Duniway
authors: New Mexico State University, Department of Engineering Technology and Surveying Engineering, P.O. Box 30001, MSC 3566, Las Cruces, NM 88003-8001, USA.
authors: Ahmed
authors: A
authors: Elaksher
mesh_terms: D000401:Aircraft; D002140:California; D003247:Conservation of Natural Resources / Q000379:methods*; D017753:Ecosystem; D004784:Environmental Monitoring / Q000379:methods*; D008954:Models, Biological; D009505:Nevada; D010781:Photography; D010944:Plants; D058998:Remote Sensing Technology / Q000295:instrumentation*
keywords: Digital terrain model; Photogrammetry; Rangeland monitoring; Remote sensing; Vegetation height