Robin's Blog

Rules of thumb in Remote Sensing

Many fields have a collection of rules of thumb – tacit knowledge that isn’t often talked about, but is often used in the field. Some of these rules have come from published information (papers, books, presentations etc) and some has just grown up over the years.

This post isn’t anywhere near done yet – I’m going to try and keep this post up-to-date as I learn more of the rules of thumb used in Remote Sensing.

• Green vegetation reflects at about 5% in the Red and 40% in the Near Infrared. This is useful for getting an approximate reflectance for vegetation from surface irradiance spectra, or for checking reflectance values gathered in the field. (Source: personal communication, E. J. Milton)
• Around 40% of the radiance from a pixel comes from outside of the pixel area. This obviously varies based on the sensor and the viewing characteristics, but this seems to be a fairly good approximation for many purposes. (Sources: Various papers, for example Ruiz and Lopez (2002) for the SPOT sensor).
• Calibration sites should be at least 3×3 pixels. This is also sometimes stated as the calibration site area should be at least eight times the pixel area, which is roughly equivalent. This ensures that boundary effects are not too large, and that errors in geometric correction do not affect the location of the site too much.
• A spectral resolution of around 20nm is sufficient for identification of most materials found on Earth. This was discussed in a seminal article by Goetz and Calvin (1987), and means that any instrument with a spectral resolution greater than this is literally hyper-spectral – as in it gives too much data. However, this extra data can still be useful for various purposes (for example, accurately locating certain atmospheric absorption bands)
• 10-100 training pixels are required per class for accurate Maximum Likelihood classification. This was discussed in Swain and Davis (1978) and is generally taken to be a useful approximate guess.
• 3 arc-second resolution is approximately 90m resolution. Not much more to say really – although of course remember that the size of an arc-second in metres varies across the globe.

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This post originally appeared on Robin's Blog.

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Categorised as: Academic, Remote Sensing