Pixels, bands and clouds

The established CBERS Earth-resources missions use optical imaging, with camera combinations that offer different bands, coverage and detail. CBERS-4A includes WPM, MUX and WFI instruments. These are distinct instruments rather than three interchangeable names for one photograph.

A spectral band samples a range of wavelengths. Comparing bands can help distinguish surface properties, while a finer spatial sampling can help locate smaller features. Neither improvement automatically supplies every other kind of information an application needs.

Consider two hypothetical square-cell products, one with 20-meter sides and another with 10-meter sides. Their cell areas are 400 and 100 square meters. Halving the side length creates four times as many cells over the same area, not twice as many. These numbers are an exercise, not a specification for a CBERS camera.

A feature smaller than a cell can influence its value without being separately resolved. A larger display can make boundaries easier to inspect but does not create new optical detail. Compare products using their actual metadata rather than the size of a screenshot.

Three questions for an image

Clouds can obscure the land in reflected-light imagery. A masked cloudy area means the usable surface observation is missing there; it does not mean the land stayed unchanged. A comparison should distinguish no detected change from no valid observation.

Suppose half a study region is obscured on the second date. A map based only on the clear half cannot support an unqualified statement about the entire region. The missing area belongs in the report, along with the dates and the rules used to identify valid pixels.