# An Overview of Infrared Remote Sensing of Volcanic Activity
## 3 Specific Considerations for the Infrared Remote Sensing of Volcanic Activity  
There are a number of considerations that must be taken into account when acquiring and analysing thermal imagery of volcanic activity, and this section will examine these. Spatial resolution: This term relates to the smallest spatial unit of data that a sensor can collect, and in terms of remote sensing, it relates to the area on the Earth's surface that is imaged by one pixel. It is to some extent a function of the altitude of the sensor, with the area imaged by one pixel being larger the further a sensor is from the surface. Another influence on the pixel size is the type of data being collected, with technological constraints limiting the spatial resolution of longer wavelength sensors. Readers will be familiar with high-resolution Google Earth imagery, which allows small features on Earth's surface to be identified (sometimes features smaller than 1 m in size). Unlike such visual band remote sensing, however, the spatial resolution of infrared imagery is relatively coarse, with the highest resolution that is used for volcanic observations being 30 m (for example, the SWIR bands of NASA's ASTER and Landsat-8 sensors) and with an even coarser resolution for longer wavelength TIR bands (e.g., 1000 m for MODIS). Additionally, even the highest spatial resolution for an infrared sensor currently in orbit is only 5.5 m (the Korea Aerospace Research Institute's Korea Multi-Purpose Satellite, KOMPSAT-3A, [21]). The cause of the disparity in spatial resolution between visual and infrared bands is the higher complexity in optical equipment required for the latter, with cooling of the longer wavelength photon detectors, for example, being necessary, which hinders the development of smaller detector elements [22; 23]. The significance of the requirement for larger infrared-imaging pixels is that the area imaged by one MODIS TIR pixel, for example, is rather large (1000 m \(\times\) 1000 m), and when imaging an active lava-flow, such a pixel will detect radiance from various surface components (i.e., hot-incandescent and cooling lava-flows and even non-volcanic surfaces). As the radiance detected within one pixel is an average of the whole area imaged, with signals from sub-pixel components combined into a one 'pixel-integrated' value [24], the higher the spatial resolution of a sensor (i.e., the smaller the pixel size), the greater the level of accuracy and detail that will be discernible (see Figure 2). In providing a greater level of detail, smaller pixels image a smaller area of Earth's surface, but this has a downside: that any hot surface imaged will constitute a greater proportion of a small pixel's field of view, therefore increasing the possibility of saturation. As such, despite the added detail provided by higher resolution infrared sensors, those of coarser resolution (e.g., 1000 m) remain useful in the remote monitoring of the most radiant volcanic activity. Temporal resolution: The temporal resolution of remotely-sensed data refers to the regularity with which a point on the Earth's surface can be imaged by a sensor, and this, as well as the spatial and temporal resolution of spaceborne remote sensing devices are, in practice, inversely related [25]. The reason for this is that the higher the spatial resolution of a sensor, the smaller its pixels and, therefore, the narrower its swath, where the swath is the width of the area on the Earth's surface that a sensor scans on overpass (Figure 3). As a narrower swath views less of the Earth's surface at any one time, it will image the entire surface of the Earth with less regularity, hence reducing the temporal resolution.