How to convert Landsat 8 DN to Top of Atmosphere reflectance (TOA) using Envi 5.2

Landsat 8 TOA corrected image

I hope that you are doing great and safe from thecoronavirus Pandemic. In this tutorial I will demonstrate how to convert Landsat 8 Digital number values (DN) to Top of Atmosphere Reflectance Using Envi 5.2 software.

The Landsat 8 data product is provided as a compressed file containing quantized and calibrated scaled Digital Numbers (DN) data acquired by both the Operational Land Imager (OLI) and Thermal Infrared Sensor (TIRS). These products are delivered in 16-bit unsigned integer format and can be rescaled to the Top Of Atmosphere (TOA) reflectance using radiometric rescaling coefficients provided in the product metadata file (MTL file).

To convert DN to TOA for a particular band, we use the rescaling coefficients provided in the metadata file in a twostep process.

Step 1: We first convert the DN to uncorrected TOA using the following equation:

UnTOA = MpDN +Ap …………..equation 1

Where:

UnTOA : Un corrected TOA

DN: Digital number Value

 Mp =Band-specific multiplicative rescaling factor from the metadata file

Ap =Band-specific additive rescaling factor from the metadata

Step 2: we then apply the sun angle correction to obtain the TOA Values

TOA = (UnTOA)/SIN(θ)………………………………equation 2

Where:

UnTOA : Un corrected TOA
θ:        Sun elevation angle(degrees)

       

Why should we convert DN to TOA?

In case we want analyze, study and gain insights about our environment and earth through processing satellite imagery.
One of the common example is deriving chlorophyll a concentration from Landsat 8 imagery and also computing NDVI products.

How to covert DN TO TOA using envy

Step 1: Open envy software and load the bands you need to convert to TOA.

Landsat 8 DN image

Step 2: Open the meta data file using word pad. For each band copy out the REFLECTANCE_ADD_BAND constant and the REFLECTANCE_MULTI_BAND constants. Each band has a given constant to enable conversion from Dn to Toa Additionally, Copy also the sun elevation angle.

Band specific parameters necessary to compute toa

Step 3:click basic tools à band math and apply equation:1 as shown in the figure below:

applying the additive and multiplication coefficients in band math

Use the reflectance add and multi constant for that particular band, any unknown variable such as B1,B2,B3 etc. to represent the subject band in the equation.

Step 4: Select add to list to enter the equation, then ok.. Envi will then prompt you to select the unknown band(B1) from the list of available bands. And save the desired file to a given location. After click ok.

specify the unknown band

Step 5: An output file will be added to the software. Open band math again and apply equation 2 on the output file as shown in the figure below.

apply the sun correction

Note that 0.871755876 represents the Sin(θ); where θ is the sun elevation angle in degrees found in the metadata file. This angle varies depending on the imagery. thus always check it in the meta data file.

Proceed to save the output in a specified location and press OK. Visualize the output in a new window

Landsat 8 Toa image

source:https://yceo.yale.edu/how-convert-landsat-dns-top-atmosphere-toa-reflectance

https://www.usgs.gov/land-resources/nli/landsat/using-usgs-landsat-level-1-data-product