Description of the Parallax Correction Tables
Tables exist for all MSG satellite positions, i.e. for 0 deg, for 3.4 deg W and for 9.5 deg E.The files need to be unzipped (use gunzip on Linux or Winzip on Windows) before they can be accessed. Each table is an ASCII file, where each line within the file contains the following numbersMSG line and element (starting at (1,1) in the SE corner) Latitude and longitude of this line/element pair 80 pairs of latitude/longitude describing the “true” location of a cloud, seen in pixel (line,element), for 80 different cloud heights: from 0.5 to 20 km height in 0.5 km stepsLatitudes North and Longitudes east are positive.
Example:Image line 51, element 1853 is at latitude -80.57 and longitude 0.56. A 500 m high cloud seen in (51,1853) is in reality at latitude -80.30 and longitude 0.55, a 1 km cloud is at -80.09 and 0.54, … , and a 20 km high cloud at -76.76 and 0.40.
For the full MSG disk, the tables would have been awfully large, so they are splitted into four quadrants, numbered 01, 02, 03, 04.
- 01 refers to the SE quadrant (lines 1-1856, elements 1-1856),
- 02 refers to the SW quadrant (lines 1-1856, elements 1857-3712),
- 03 refers to the NE quadrant (lines 1857-3712, elements 1-1856), and
- 04 refers to the NW quadrant (lines 1857-3712, elements 1857-3712)
So in total you have 12 files:
- parallax_01_0deg: SE image section for the 0 deg position
- parallax_02_0deg: SW image section for the 0 deg position
- parallax_03_0deg: NE image section for the 0 deg position
- parallax_04_0deg: NW image section for the 0 deg position
- parallax_01_3.4W: SE image section for the 3.4W position
- parallax_02_3.4W: SW image section for the 3.4W position
- parallax_03_3.4W: NE image section for the 3.4W position
- parallax_04_3.4W: NW image section for the 3.4W position
- parallax_01_9.5E: SE image section for the 9.5E position
- parallax_02_9.5E: SW image section for the 9.5E position
- parallax_03_9.5E: NE image section for the 9.5E position
- parallax_04_9.5E: NW image section for the 9.5E position
Please note that the data for the 3.4W position are rectified to 0 deg, for the 9.5 E position the data are rectified to 9.5E.Also included is the Fortran subroutine parallax.f90 which shows the source code for the parallax correction. It is very easy to use and everybody is welcome to use this in his/her own processing scheme.
The parallax files are very big as they cover every MSG pixel – so it takes a long time to even read the files. So, it is best you cut out your area of interest, e.g. Europe. You can download the files, including a Fortran90 program to help you read the files.
More information can be found in this document.
Marianne Koenig, EUMETSAT