Example 1. Atmospheric correction of a Landsat ETM+ image

Introduction

In this example we have a Landsat ETM+ image of part of Chichester Harbour, southern England for which we have no simultaneous data on the state of the atmosphere over the site. This is often the case, leaving us with the choice of either leaving the data uncorrected, or applying a radiative transfer model based on generalised atmospheric conditions, or estimating the atmospheric conditions from the image data. Assuming that we wish to make some sort of correction for the atmosphere, the simplest method would be an image-based approach based on 'dark object subtraction' (DOS). This technique successfully accounts for scattering in the shorter wavelength bands (ETM+ bands 1, 2 and 3), but is less accurate in the infra-red region, as it assumes 100% transmittance (i.e. no absorption due to water vapour in the atmosphere) and also ignores the downwelling irradiance from the sky. Moran et al. (1992) found that the DOS method created more error in Landsat TM band 4 than leaving the data uncorrected. For this reason we will use a radiative transfer model instead, and chose a generalised atmosphere which would be reasonable for southern England in Spring. The particular model chosen is the Second Simulation of the Satellite Signal in the Solar Spectrum (6S) described by Vermote et al. (1997). This model is very widely used in remote sensing and is available in a number of modified versions as well as in its original form which may be downloaded from ftp://kratmos.gsfc.nasa.gov/pub/6S/. Even if you are going to use one of the derivatives, it is worth downloading the documentation from this website as this contains an excellent description of the model and important information about its limitations etc. The manual is split into a description of the model (6smanv2.0_P1.pdf), and a description of the 100+ Fortran subroutines (6smanv2.0_P2.pdf and 6smanv2.0_P3.pdf).

Figure 1
ETM+ image of Chichester Harbour, southern England. © NASA, 2002.
(click image for a larger version)

The image above is a colour composite of Landsat ETM+ bands 3, 2, and 1 (RGB) extracted from image ???/??? acquired on 28th March 2002. The image would be difficult to atmospherically correct using the dark object subtraction method because the coastal waters are very shallow and contain a lot of sediment. Neither are there areas of topographic shadow large enough to be used as dark objects.

Preparing to run 6S under Microsoft Windows on a PC

The definitive public version of the 6S model (ver. 4.1) comprises over 100 Fortran subroutines designed to compile under Unix. It is also possible to compile and run this version under the Apple Macintosh operating system without much trouble. MS Windows users face a more challenging task. However, there are two alternative versions of 6S which are particularly suited to the occasional user : the first is Msixs which is a GUI version of 6S which can run under Cygwin/X (a port of the X-Windows system to the PC) and the second is a simplified command line version of the model created by Mauro Antunes. Both of these are better suited to applying to image data than the original version of 6S which was designed to process individual pixels.

The main limitations of the Antunes implementation of 6S are that it is restricted to 8-bit image data and that it does not include the brdf subroutines available in Msixs or the full version of the model. However, for our present purposes, neither of these limitations are too severe so Antunes-6S offers an ideal way to perform an initial atmospheric correction of our ETM+ image.

Antunes-6S can be downloaded from the website listed below and the Landsat ETM+ image subset can be downloaded from the NCAVEO resources page. The Landsat image comprises the six reflective bands from the ETM+, each stored as a flat 8-bit binary file 512 x 512 with an associated ASCII header (ENVI format). This format can be read by the majority of image processing packages (e.g. MultiSpec) as well as Antunes-6S. The NCAVEO site also has a short article describing how to apply the Antunes-6S model to the Chichester harbour ETM+ subset if you would like to try it for yourself (NKB-05-01 : Notes on Antunes 6S model)

Running the command line version of the 6Smodel

Both the original version of 6S and the Antunes version require a 'control file' which is simple an ASCII file containing the values of the parameters needed by the model. An example of a 6S control file for processing ETM+ band 1 of the Chichester Harbour image is given below. Click on the line number to get more explanation of what the values on each line mean. The control file needed for Antunes-6S is similar, but not identical to that given below.

The result of applying the Antunes-6S model to the Landsat ETM+ image is shown below (Figure 2). Visually, the atmospherically corrected image looks very similar to the uncorrected image, although the contrast is better, especially over the sea and the urban area. However, the difference between the two becomes apparent when we look at plots of the digital number values (Figures 3 - 5). The spectra from the atmospherically corrected image show the correct relationship between data in the different ETM+ bands and are more readily interpreted with reference to reflectance spectra from typical surfaces.

Figure 2.
Original colour composite (left) and the atmospherically corrected data (right).

Figure 3.
Spectrum of a grass pixel from the original data compared with the atmospherically corrected data.

Figure 4.
Spectrum of a water pixel from the original data compared with the atmospherically corrected data.

Figure 5.
Spectrum of an asphalt pixel from the original data compared with the atmospherically corrected data.

References

Tanré, D., Deroo, C., Duhaut, P., Perbos, J. and Deschamps, P. Y., 1990. Description of a computer code to simulate the satellite signal in the solar spectrum : the 5S code. International Journal of Remote Sensing 11, 659-668.
Vermote, E.F., Tanré, D., Deuzé, J.L., Herman, M., and Morcette, J.J., (1997), Second simulation of the satellite signal in the solar spectrum, 6S: An overview., IEEE Transactions on Geoscience and Remote Sensing 35(3), 675-686.

URLs (checked 22 July 2005)

Cygwin/X : http://x.cygwin.com/

Mauro Antunes version of 6S : http://www.ltid.inpe.br/dsr/mauro/6s/

Msixs : http://www-loa.univ-lille1.fr/SOFTWARE/Msixs/msixs_gb.html

This work is licenced under a Creative Commons Licence.

© NCAVEO, 2005
Network for Calibration and Validation of Earth Observation data
School of Geography, University of Southampton
Southampton SO17 1BJ, UK