GRASS-Wiki - User contributions [en]

Atmospheric correction

2009-04-11T09:19:02Z

⚠️Annekatrien: Tutorial on atmospheric correction of Landsat images in the North Carolina dataset

'''Aim of this tutorial:'''

* Transforming a Landsat ETM+ image from DN values to radiance values
* Conducting an atmospheric correction of a Landsat image using the i.atcorr module in Grass GIS 6.3.0

'''Required elements:'''

* Grass 6.3.0 or higher
* North Carolina sample dataset: http://grass.itc.it/download/data.php

After downloading of the North Carolina dataset, it must be unzipped into your GIS database. When starting Grass GIS, select this GIS database, open the ‘nc_spm_08’ dataset as LOCATION, and ‘PERMANENT’ as MAPSET.

= Calculating radiance values =

This mapset contains, amongst others, a Landsat ETM+ image of 24th of May 2002. Every pixel of this image contains a DN or grey value. In order to be able to make calculations with satellite imagery, or compare values amongst different sensors, these values have to be converted to radiances or reflectances. The formulas to do this conversion are described here for Landsat images.
The conversion from DN values to top-of-atmosphere radiances is done as follows:

'''Lλ = ((LMAXλ - LMINλ)/(QCALMAX-QCALMIN)) * (QCAL-QCALMIN) + LMINλ'''

Where:

* Lλ = Spectral Radiance at the sensor's aperture in watts/(meter squared * ster * μm)
* QCAL = the quantized calibrated pixel value in DN
* LMINλ = the spectral radiance that is scaled to QCALMIN in watts/(meter squared * ster * μm)
* LMAXλ = the spectral radiance that is scaled to QCALMAX in watts/(meter squared * ster * μm)
* QCALMIN = the minimum quantized calibrated pixel value (corresponding to LMINλ) in DN
* QCALMAX = the maximum quantized calibrated pixel value (corresponding to LMAXλ) in DN =255
* Lλ is the apparent radiance as seen by the satellite sensor.

LMINλ and LMAXλ are the radiances related to the minimal and maximal DN value, and are reported in the metadata file for each image, or in the table 1. High gain or low gain is also reported in the metadata file of each Landsat image. The minimal DN value (QCALMIN) is 1 for Landsat ETM+ images (1), and the maximal DN value (QCALMAX) is 255. QCAL is the DN value for every separate pixel in the Landsat image.

Accessing the metadata:

$ r.info lsat7_2002_xx

Under ‘comments’, the maximal and minimal radiance (LMAX and LMIN) for each band are given. Conversion to radiance (this calculation is done for band 1, for the other bands, the numbers in italics need to be replaced with their correct values):

$ r.mapcalc “lsat7_2002_10_rad=((''191.6'' - (''-6.2'')) / (255 - 1)) * (lsat7_2002_10 - 1) + (''-6.2'')”

= Atmospheric correction =

This radiance image can be used for the atmospheric correction with the 6S algorithm. The algorithm will transform the top-of-the-atmosphere radiance values to top-of-canopy reflectance values using predefined information on the aerosol content and atmospheric composition of the time the image was taken. What follows describes the method to use this algorithm in Grass GIS. Again, only the calculations for band 1 are shown, and the numbers to be changed for the other bands are indicated in red.The icnd file consists of the following parameters, and can be made in Notepad:

* 8 ''- indicates that it is an ETM+ image''
* 05 24 14.30 -78.691 35.749 ''- image taken on the 24th of May, at 14.30; the center of the image lies at 78.691°W and 35.749°N ''
* 2 ''- the midlatitude summer atmospheric model''
* 1 ''- the continental aerosol model''
* 50 ''- the visibility for the aerosol model''
* -0.600 ''- the area lies 600 meters above sea level''
* -1000 ''- image taken of a satellite sensor''
* 61 ''- spectral band''

This file is than used in the i.atcorr module:

$ i.atcorr –a iimg=lsat7_2002_10_rad ialt=elevation icnd=icdn.txt oimg=lsat7_2002_10_atcorr

Where:
* -a refers to a Landsat image taken after July 2000
* iimg is the image to be corrected
* ialt is the altitude map
* icnd is the path to the icnd.txt file
* oimg is the name of the output image

More information on the use of i.atcorr for other images can be found at http://grass.itc.it/grass64/manuals/html64_user/i.atcorr.html .

= References =

* (1) http://landsathandbook.gsfc.nasa.gov/handbook/handbook_htmls/chapter11/chapter11.html
* (2) http://landsat.usgs.gov/documents/L5TM_postcal_v11.pdf

File:Etm minmaxradiance.JPG

2009-04-11T08:57:07Z

⚠️Annekatrien: Table containing the minimum and maximum radiance values for Landsat ETM+ images

Table containing the minimum and maximum radiance values for Landsat ETM+ images

GRASS Education (Free GIS education)

2009-04-11T08:33:26Z

⚠️Annekatrien: /* Tutorials */

__TOC__

Please modify current topics and content and/or add your own ideas and contributions

== Promotional material ==
* See [[Promotional material]]

== Teaching Concepts ==

* [[Gis_Concepts|Basic GIS concepts]]
* [[GRASS_Help#First_Day_Documentation | GRASS First-day documentation ]]

== Teaching Materials ==

=== Seminars & Presentations ===

* GRASS Presentations on [http://www.slideshare.net/search/slideshow?q=grass+gis Slideshare]
* GIS seminar: The GRASS GIS software at Politecnico di Milano, Polo Regionale di Como, 30 May 2006 (6h). Introduction to GRASS 6 and QGIS ([http://www.slideshare.net/markusN/the-grass-gis-software-with-qgis-gis-seminar-presentation Slides PDF/ODP/HTML])

* [http://wiki.osgeo.org/index.php/Library OSGeo Library]: Presentations and educational material

=== Tutorials ===

* [[Import NoaaEnc]] Mini-tutorial

* Tutorials by Yann Chemin: One document of small tutorials to get started in GRASS GIS and GRASS integration with QGIS.
** [http://trac.osgeo.org/grass/browser/grass-addons/tutorials/grass_starter] (LaTeX, Updated 20May07), covering:
*** QGIS introduction
*** QGIS GRASS plugin
*** GRASS GIS introduction
*** GRASS GIS DEM manipulations
*** GRASS GIS habitat analysis exercise
*** GRASS GIS scripting example for habitat exercise is provided in Appendix B

The documents are in http://trac.osgeo.org/grass/browser/grass-addons/tutorials/grass_starter

* [[Country Data Processing]] Tutorial
* [[Grass Vector Layers]] Tutorial
* [[Atmospheric correction]] Tutorial

* [http://casoilresource.lawr.ucdavis.edu/drupal/node/95 Several Tutorials and Examples] mostly related toward soil science and hydrology

=== Workshops ===

* [[GRASS_related_workshops_and_presentations]] held at Lausanne, Switzerland, September 12-15th 2006

=== Training Videos ===

* GRASS 6.3 feature tour:<BR>http://www-pool.math.tu-berlin.de/~soeren/grass/modules/screenshots/grass63feature_tour.html
* The GRASS startup screen (select or create location, mapset,..) '''[TODO]'''
* Using the gis.m GUI '''[TODO]'''
* [http://crs.itb.ac.id/media/flash/ Several flash videos on using GRASS in Ubuntu]

==== How to create them ====

* A short vnc2swf usage tutorial is available here:<BR>http://www-pool.math.tu-berlin.de/~soeren/grass/modules/screenshots/vnc2swf_usage.html

Soeren used [http://www.unixuser.org/~euske/vnc2swf/ vnc2swf] to record that flash movie.

The installation is easy if you have a Linux dist which provides all
needed python packages (pyGames and others I don't remember).

You would need a VNC-Server installed (not too tricky normally). The installation of vnc is easy. There are a lot of free vncserver and
clients out there in the internet.

'''[list some here]'''

To record a vnc session is not very complicated:
* First you have to run a vnc server and the client.
* The next step is to start vnc2swf.py with the address of the vncserver and a file name.

== See Also ==

* [http://wiki.osgeo.org/index.php/Education_Committee_Work_Program OSGeo.org Education Committee Work Program]

== Technical Aspects ==

== Current Restrictions ==

People want to upload their course material including PDFs, slide presentations, Flash tutorial movies, course material, lecture notes,
etc.. Because this wiki only supports text and simple graphics files, we have to find a solution to enable password controlled upload of large files. For this reason it is necessary to provide a dedicated general Free-GIS education server to support GRASS and Free GIS teaching and education efforts. For now contact the grass-dev mailing list if you want something uploaded.

From our point of view a natural home for Free-GIS-Edu documentation, video tutorials, presentations, PDFs, etc. could be:

* [http://trac.osgeo.org/grass/browser/grass-addons/tutorials grass-addons SVN]
* [http://download.osgeo.org/grass OSGeo Download Server]
* [http://trac.osgeo.org/grass/ GRASS bug/wish tracker]
* [http://freegis.org Intevation's FreeGIS.org]

[[Category:Documentation]]