# Atmospheric correction

Before conducting spectral analysis, it is important to perform atmospheric correction on the imagery. 

The energy collected at the sensor has travelled through the atmosphere twice and is therefore impacted by the makeup and composition of the atmosphere through scattering and absorption. 

The solar zenith and azimuth, and the sensor 'look' geometry also impact the degrees to which absorption and scattering have a bearing on the top of atmosphere radiance (the energy arriving at the sensor). 

There are a number of approaches to atmospheric correction. 

![Credit: Mathworks](/files/Xl6kj146f7XrU2GjANNp)

### Top of Atmosphere correction

This type of correction refers to the conversion of top of atmosphere radiance into surface reflectance, without taking account of atmospheric composition (scattering and absorption)

{% hint style="info" %}
Antarctica's atmosphere contains very little particulates which impact scattering and absorption, meaning a top of atmosphere correction is sometimes adequate. Conversely, there is little data available of the composition of the atmosphere in Antarctica, meaning applying full radiative transfer modelling is not always possible or effective. 
{% endhint %}

### Atmospheric correction

There are many options for correcting for atmospheric parameters using algorithms that apply radiative transfer modelling. 

Landsat level-1 products are not atmospherically corrected. Landsat level-2 are corrected using the LEDAPS surface reflectance modelling. More information can be found [here](https://www.usgs.gov/landsat-missions/landsat-collection-2-level-2-science-products).


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://guides.geospatial.bas.ac.uk/spectral-analysis-of-imagery/atmospheric-correction.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.