Read more than visible colour

SumbandilaSat’s linescan imager sampled six bands across visible, red-edge and near-infrared wavelengths. A band is a selected range of wavelengths recorded by the instrument. Red-edge measurements sample the transition between red and near-infrared reflectance that can help characterize vegetation. Near-infrared is beyond human vision; displaying it as red on a screen is a visualization choice.

A false-colour composite is useful when its channel assignment is known. Bright red vegetation in one composite does not mean the plants were literally red. A scientific interpretation also depends on illumination, atmospheric effects, sensor calibration and the surfaces mixed within a sample. Colour alone is an invitation to investigate, not a complete diagnosis of plant health or land use.

The manufacturer lists 6.25-metre ground sampling for the linescan imager and a 52-kilometre swath. Sampling describes the spacing of measurements on the ground; the swath describes coverage across the track. Neither number promises cloud-free coverage, perfect geolocation or identification of every object that small. Optical blur, motion and contrast also affect what can be distinguished.

In a deliberately simplified grid, a 25-metre-wide strip aligned with the sample direction spans four 6.25-metre intervals. If the strip is narrow, diagonal or near a boundary, a sample may mix it with neighboring terrain. Enlarging that picture on a display makes its samples bigger on screen without adding measurements. The spacecraft’s separate matrix imager has different specifications; do not attach its scene size to the linescan instrument.

Compare bands at the same place

A linescan image is assembled as the spacecraft moves. Different bands must be aligned so a spectral comparison concerns the same place. Suppose a red sample falls on a field while the nominally matching near-infrared sample falls on an adjacent road. A calculated contrast could then reflect misregistration instead of the field’s properties. Matching array positions is not sufficient unless their ground coordinates agree.

For an invented example using calibrated reflectances, near-infrared is 0.6 and red is 0.2. Their normalized difference, (near-infrared − red) divided by (near-infrared + red), is 0.5. This arithmetic illustrates one comparison, not a SumbandilaSat product or a universal health threshold. Check alignment, calibration and cloud or shadow flags before interpreting such an index. Similar numbers can arise from different physical conditions.