Acquisition, Or thorectification, and Object-based Classification of Unmanned Aerial Vehicle (UA V) Imager y for Rangeland Monitoring

The use of unmanned aerial vehicles (UAVs) for natural resource applications has increased considerably in recent years due to their greater availability, the miniaturization of sensors, and the ability to deploy a UAV relatively quickly and repeatedly at low altitudes. We examine in this paper the potential of using a small UAV for rangeland inventory, assessment and monitoring. Imagery with a ground resolved distance of 8 cm was acquired over a 290 ha site in southwestern Idaho. We developed a semiautomated orthorectification procedure suitable for handling large numbers of small-footprint UAV images. The geometric accuracy of the orthorectified image mosaics ranged from 1.5 m to 2 m. We used object-based hierarchical image analysis to classify imagery of plots measured concurrently on the ground using standard rangeland monitoring procedures. Correlations between imageand ground-based estimates of percent cover resulted in r-squared values ranging from 0.86 to 0.98. Time estimates indicated a greater efficiency for the image-based method compared to ground measurements. The overall classification accuracies for the two image mosaics were 83 percent and 88 percent. Even under the current limitations of operating a UAV in the National Airspace, the results of this study show that UAVs can be used successfully to obtain imagery for rangeland monitoring, and that the remote sensing approach can either complement or replace some ground-based measurements. We discuss details of the UAV mission, image processing and analysis, and accuracy assessment. Introduction Civilian applications of unmanned aerial vehicles (UAVs) have increased considerably in recent years due to their greater availability and the miniaturization of sensors, GPS, inertial measurement units, and other hardware (Lambers et al., 2007; Zhou and Zang, 2007; Patterson and Brescia, 2008; Rango et al., 2008; Nagai et al., 2009). The advantages of PHOTOGRAMMETRIC ENGINEER ING & REMOTE SENS ING J u n e 2010 661 Andrea S. Laliberte and Craig Winters are with the Jornada Experimental Range, New Mexico State University, 2995 Knox St., Las Cruces, NM 88003 ([email protected]). Jeffrey E. Herrick and Albert Rango are with the USDA-Agricultural Research Service, Jornada Experimental Range, 2995 Knox St., Las Cruces, NM 88003. Photogrammetric Engineering & Remote Sensing Vol. 76, No. 6, June 2010, pp. 661–672. 0099-1112/10/7606–0661/$3.00/0 © 2010 American Society for Photogrammetry and Remote Sensing Acquisition, Or thorectification, and Object-based Classification of Unmanned Aerial Vehicle (UA V) Imager y for Rangeland Monitoring Andrea S. Laliberte, Jeffrey E. Herrick, Albert Rango, and Craig Winters using a UAV over a piloted aircraft include lower image acquisition costs than from manned aircraft, the ability to deploy the aircraft relatively quickly and repeatedly for change detection, and the ability to fly at low altitudes (Rango et al., 2006). UAVs are well suited for rangeland remote sensing applications. Due to the remoteness and low population density of rangelands, it is easier to obtain permission to fly over those areas compared to more populated areas. Land management agencies such as the U.S. Bureau of Land Management (BLM) and the U.S. Natural Resources Conservation Service (NRCS) have a requirement to monitor and assess millions of acres of rangelands, a task that is not feasible with ground monitoring techniques alone. UAVs have the potential to reduce the number of ground-based measurements required for rangeland inventory, assessment, and monitoring by providing low-cost sub-decimeter resolution digital imagery. UAVs have been used successfully, for estimating shrub utilization (Quilter and Anderson, 2001), for mapping invasive species (Hardin and Jackson, 2005), for measuring plant biomass and nitrogen (Hunt et al., 2005), for documenting water stress in crops (Berni et al., 2009), and for mapping rangeland vegetation (Laliberte and Rango, 2009). UAVs also allow for a landscape-level monitoring approach that offers potential for calculating landscape metrics that reflect changes in landscape processes and dynamics more accurately than plot-scale measurements alone (Bestelmeyer et al., 2006). Despite the significant potential applications of UAVs to rangeland monitoring, and recent improvements in getting from the initial image acquisition to mapping products (Laliberte et al., 2007; Laliberte and Rango, 2009), more formal tests are required for future implementation of this technology. We require a better understanding of relating ground measurements in rangelands to remotely sensed information from UAV imagery, and we need to obtain data in topographically diverse environments. One factor that has limited testing and use of unmanned aircraft for civilian applications in the United States is that the operator has to comply with the regulations of the Federal 661-672_09-033.qxd 5/14/10 2:43 PM Page 661