# A Review of In-Situ and Remote Sensing Technologies to Monitor Water and Sanitation Interventions
## 2 Sensing WASH  
Remote monitoring, via satellite assets and distributed in situ sensors, may help meet many of the challenges of information asymmetry and data gaps in developing countries, including unreliable (and time-consuming) survey data and reliance on spot checks to assess accurate reporting and performance. Remote data can be used to understand programmatic, social, economic, and seasonal changes that may influence the quality of a program. For instance, satellite technology has allowed measurements on a global scale over long time periods, and is now proving useful in coastal waters, estuaries, lakes, and reservoirs, which are relevant to water quality managers. Additionally, the behavioral patterns of users can be studied to better understand how and when water and energy technologies are being used. Further, the information can assist the policy design and implementation of efforts to expand coverage, by providing comprehensive evidence of how best to prioritize and sequence interventions, and by estimating the cost-effectiveness and efficacy of upgrading to \"safely managed\" services.  
In this paper, we review the use of remote sensing and local sensors for water and sanitation monitoring. The term \"remote sensing\" usually describes the collection of data by satellites. In most cases, \"remote\" refers to spectral imagery collected by cameras and other spectral instruments across a broad range of wavelengths. For observing the earth, satellites take spectral data reflecting from the atmosphere and the Earth's surface. Interpretation of these data (often represented as imagery) requires an understanding of the spectral data and physical properties of the Earth and its atmosphere. It also often requires calibration against data collected on the Earth's surface or in the atmosphere directly--data from sensors that are in situ, rather than remote. Modern optical sensors can capture both qualitative imagery and quantitative information, by using a range of different wavebands of light. This means the data is often in a familiar \"photographic\" form, showing the Earth from the perspective of space. However, it can also contain additional information that helps researchers and decision makers better understand the dynamics of a specific area and variables that play a key role in determining certain trends. Different wavelengths of light filtered through the imagery can, for instance, identify land boundaries, land classifications, atmospheric water vapor, industrial soil pollution, and sediments.  
In situ instrumentation technologies, ranging from flowmeters and water quality sensors to motion detectors installed in lattrines to monitor usage or adoption, can complement such information. These sensor technologies can be used both operationally or within a sampling frame to be combined with other dimensions of information. Data can be logged locally for manual retrieval or transmitted over short range to nearby enumerators, or to remote operators and researchers over Wi-Fi, cellular, and satellite networks. Some instrumentation is in common use, while other technologies are emerging. However, given the remote and power-constrained environments involved, and the high degree of variability in the characteristics of fixed infrastructure, including in age, materials, quality, servicing, and functionality, any electronic-sensor-based solution would have to either be bespoke engineering, or compensate for these complexities through analytics. For example, a conventional flowmeter designed for a rural borehole water distribution scheme would have to address pipe diameter, material, pressure, depth, thread type, and other characteristics that require custom engineering and plumbing. Instead, a nonintrusive ultrasonic flowmeter may be more easily adapted for a variety of water schemes.