README.md

Normalized difference vegetation index with uncertainty

Show script or download{:target="_blank"} it.

{% highlight javascript %} {% include_relative script.js %} {% endhighlight %}

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Basic information

• Bands used to calculate NDVI index: B4, B8
• Bands used by the script: B2, B3, B4, B8

General description

This script allows you to visually interpret how the normalized density vegetation index (NDVI) [1] is affected by the uncertainties in detector reflectances of the L1C products.

Since NDVI is defined as a ratio of difference over sum of bands 8 and 4 (near infrared and red):
$$NDVI := \mathtt{Index}(B8,B4) = \frac{B8-B4}{B8+B4}.$$

the uncertainty propagation [3] gives us the uncertainty of the index itself as

$$\Delta_{NDVI} := \frac{\sqrt{B8^2 \Delta_{B4}^2 + B4^2 \Delta_{B8}^2 - 2B4 B8 \Delta_{B4B8}}}{(B8+B4)^2}$$.

where $\Delta_{B4}$ and $\Delta_{B8}$ are uncertainties of red and near infrared bands respectively (reported by ESA to be 0.02 and 0.03). We left out the mixed part $\Delta_{B4B8}$ as if the two uncertainties were not correlated.

The script encodes the uncertainty with darkness, as can be seen in following figure [2]

Description of representative images

NDVI with uncertainty of Madrid. Acquired on 10.26.2019.

References

[1] Wikipedia, Normalized Difference Vegetation Index . Accessed on October 4th 2017.
[2] Sentinel-Hub, Ad hoc testing of algorithms globally. Accessed October 10th 2017.
[3] Wikipedia, Propagation of uncertainty. Accessed October 10th 2017.