What is a multispectral imagery index?

There is one more thing we want to introduce you to in the field of multispectral imagery, and that is indices. In this case, an index is a value or score calculated from other values. Multispectral imagery indices are calculated using the numeric value of two different bands in the same pixel. They are a great way of comparing two bands mathematically and visually in ways that can reveal some interesting information about the environment.

The most common type of multispectral imagery index is a normalised difference index. This employs a simple formula:

A normalised difference will yield values of between 1 and -1, depending on the relative values of both bands. If they are similar the index will be around 0, if they are very different it will be closer to 1 or -1.

_{Indicative normalised difference values based on the values of Band A and Band B.}

This formula offers a really useful way to highlight areas of contrast between two bands, and therefore areas where the surface reflectance is quite different in two distinct parts of the electromagnetic spectrum. This will make much more sense with a couple of examples!

NDVI – Normalised Difference Vegetation Index

Normalised Difference Vegetation Index (NDVI) uses two bands to which plants respond very differently. Vegetation absorbs very little energy in the near infrared part of the spectrum, but it absorbs red light very effectively as plants use this energy in photosynthesis. By using these two bands, NDVI can show areas that are likely and unlikely to be vegetated to a high degree of confidence.

For NDVI, anything close to 1 almost certainly contains plenty of vegetation, while anything near or below 0 almost certainly does not. Look at the NDVI plot below, high values are plotted as light grey or white, and low values in dark grey or black.

_{Can you see the two tell settlement mounds in the middle of the fields? Does the NDVI data show any other areas that might be of archaeological interest?}

_{NDVI plot in northern Iraq. Sentinel-2 data, courtesy of ESA.}

NDWI – Normalised Difference Water Index

_{Can you see any dry islands in the marsh which might warrant archaeological investigation?}

_{NDWI plot of part of the southern Iraqi marshes. Sentinel-2 data, courtesy of ESA.}