## 3 Aerial Remote Sensing  
Long before the launch of Landsat 1, high-resolution aerial photography was the only form of remotely sensed data that was publicly available. After World War I (WWI), many public and private collections of aerial imagery began to be collected and made available [43]. In 1928, former WWI fighter pilot [PERSON] formed Tobin Aerial Surveys (later named Tobin International) and began capturing stereographic aerial imagery of much of the US Gulf Coast states to aid in oil and gas exploration, creating one of the oldest and largest aerial datasets in the world [44; 45; 46]. [PERSON] went on to aid in the allied reconnaissance effort in World War II (WWII), and later created the first computerized database of oil well locations in Texas [44]. In the 1930s, the US government began using fixed wing aerial photography to assist in the implementation of conservation programs under the Agricultural Adjustment Act of the New Deal [47]. After WWII, aerial photography was used extensively to monitor compliance with agricultural incentive programs [47]. Today, much of this imagery is publicly available, and can provide much needed historical perspective to past grazingland and agricultural management decisions [5; 48; 49] that may not be captured in historical ground-based datasets [50]. Though historical images are mostly single-band, they do allow us to perform geospatial and geostatistical analyses through nearly 90 years of imagery [5; 48]. The invention of laser technology in the late 1950s to the early 1960s eventually led to the development of the modern Light Detection and Ranging (LiDAR) platform [51; 52]. Early LiDAR was used in bathymetric and topographic measurements [53], but soon was used in forestry [54; 55; 52], and later in the identification and management of grazing-lands [56; 57; 58; 59]. The ability to add LiDAR sensors to unmanned aerial systems (UAS), has greatly expanded its applicability [60; 61; 62] by lowering the barrier to acquire high resolution data over smaller parcels. The use of UAS have increased substantially in recent years across many industries, including research, construction, law enforcement/rescue, real estate, cinematography, and personal use to name a few [63]. Subsequently, UASs have become a commonly used tool for solving geospatial problems in grazingland science and management [64]. Tasks tackled with UAS range from basic such as animal inventory, checking of fences and water troughs [65], to complex measures such as aboveground biomass, NDVI and other spectral indices, land cover classification, plant identification, and digital surface models [26; 27; 65; 66; 67]. Spatial data collected from UASs have high spatial resolution [66]. Digital surface models and orthomosaics with 1 cm resolution can be obtained with a consumer-grade UAS when combined with state of the art 3-D processing software that discerns structure from motion [66; 67; 68]. UAS applications can be used to supplement field-based measurements to reduce overall time spent in the field, allowing grazingland managers and scientists to gather information more efficiently and with less subjectivity [60; 68; 69]. Once considered impossible [4], plant heights and biomass may now be attainted through remote sensing with the use of UASs when combined with on-the-ground measurements [66; 70]. UAS data are advantageous for determining stocking rates and grazing utilization in hard-to-reach locations [71], defining areas of woody plant encroachment [69], managing wildlife habitat [72], and documenting anthropogenic effects on natural areas [73]. UAS systems are not without their limitations. In the US, several agencies stipulate when and where one can operate UASs. Federal and state agencies have regulations aimed at creating a safe environment for both manned and unmanned aircraft. Flight paths are restricted to line of sight of the operator and visual observers, and payload cannot exceed 25 kg (55 lbs.) for the total weight [65].