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22 City Trees throwlines in large trees or palms. Fixed-wing UAVs are best used for large-scale projects of many acres/hectares, missions with multiple sensors, or long stretches of flight over road centerlines. It is essential to know what kind of imagery you require. Why? Visual imagery can be processed by staff. Any other sensor data collected is measured in tens of gigabytes and must be processed by specialized software run on powerful computers. Such analysis is likely beyond the capabilities of most munic- ipalities but is easily performed by any number of Big Data service companies. Data service companies are proliferating rapidly as the market for data analysis grows. There is likely a suitable company within an easy drive of many readers, but most business can be transacted over the Internet so proxim- ity is not the most important factor in choosing a company. Consider choosing a data company that can deliver flexible product output based on your needs, rather than a seeming- ly lower-cost mass-market data service company offering a focused, limited set of products ("data in a box"). Visual sensors can be used for risk assessment, inventories, disaster response, construction and permit monitoring, creating Digital Elevation Models, for crew safety assessment and moni- toring, social media images, and education, among other tasks. Thermal IR sensors can be used to monitor turf irrigation effi- ciency and to locate irrigation leaks, assess plant water stress, and assist in public safety and building efficiency analyses (for example, of heat loss from buildings and cooling from tree shade). There is some evidence thermal IR sensors can detect cavities in individual trees as well 7 . Multi- and hyperspectral sensors can be used for assessing plant and forest health, early detection and monitoring of pest outbreaks, supplementing inventories, and monitoring seasonal progress of pests such as elm leaf beetle. Identifying tree spe- cies and locating invasive plant species or species of concern are also possible with these sensors. LiDAR can be used to precisely measure tree canopies to monitor growth and change over time and to precisely measure any urban object such as signage or street lights—often at millimeter scale. UAVs will soon be used to supplement inventories because individual tree canopy delineation is now easily performed by software algorithms using data collected in the IR spectrum. Some arborists are already using rotor-wing UAVs to rapidly set throwlines, especially for very large trees and palms. Perhaps most importantly, early detection and monitoring of urban forest outbreaks of pests such as emerald ash borer, oak wilt, ash decline, and others will soon be performed using UAVs carrying multi- or hyperspectral sensors. Municipal foresters will rapidly and flexibly collect data with UAVs to set baselines of public forest health and to detect change over time. UAVs will aid in timely and flexible management action on pest outbreaks, pruning cycles, and drought stress, among other concerns. Missions can be flown manually or be fully automated. Standardized, replicable flight plans will be easy to set, as a mission plotted over road centerlines is all that is needed to monitor public trees in the right of way. Several brands of soft- ware exist today that can automate any conceivable municipal forest flight plan, and any flight plan can be saved to re-fly in the future to replicate past missions for consistency. Lastly, flights at an elevation of 100 meters (328 feet) AGL will not only capture all public trees in rights of way but also vegetation extending to about 45 meters (148 feet) from a road centerline, providing urban forest managers additional data for management purposes if desired. Legal Considerations Because of the wide disparity between civil aviation authorities in different countries, space restrictions mean that this article can- not discuss UAV laws in detail. Readers should research both the national laws of their country as well as any local laws that may prevent flights for municipal forestry operations. Several coun- tries do allow commercial UAV operations within their borders, but it is important to realize most countries' civil aviation authorities are struggling to maintain safe airspaces while incorporating UAV technology. The speed of technological change is running far ahead of any government's ability to react appropriately. Privacy may or may not be addressed by civil aviation author- ities. For example, in the United States, the Federal Aviation Administration has remained silent regarding privacy and avia- tion laws for UAVs and instead left decisions to the civil courts. Regardless of where you live, all commercial UAV pilots must adhere to existing law as well as make operational judgments about flying if neighbors express concern about their privacy. Always err on the side of caution where privacy is concerned, and even if the law is on your side, have a plan to cease oper- ation if privacy concerns arise. It is better in the long run to be safe and respectful than to cause a conflict. 1 Greenwood, F. 2015. How Vermont Used Drones After a Train Derailment. Slate October 9, 2015. 2 For example, see Mailey, C. 2013. Are UAS More Cost-Effective Than Manned Flights? Association of Unmanned Vehicle Systems International Hampton Roads Chapter Newsletter, October 24, 2013. 3 Moore, G.M. 2016. Drones Will Be Arboricultural Busy Bees. Arborist News 25:1 pp. 32-35. 4 See, for example, a Switzerland-based drone analytics case study, Price Wars: Counting the Costs of Drones, Planes, and Satellites. 5 For example, a comparative analysis of remote sensing platforms in citrus orchards found UAVs had much better resolution than piloted aircraft: Sankaran et al. 2013. Huanglongbing (Citrus Greening) Detection Using Visible, Near Infrared and Thermal Imaging Techniques. Sensors (Basel). 13:2 pp. 2117–2130. 6 A region in the electromagnetic spectrum occurring at about 705 nanometers (nm)—the region where there is a sudden change in the reflectance of vegetation, due to the fact that plants absorb and use visible light for metabolism, but use very little infrared light and so it is reflected, unused. 7 Catena, A. and Catena, G. 2008. Overview of Thermal Imaging for Tree Assessment. Arboricultural Journal 30, pp 259-270.