Working with MrSid

Satellite images from the internet are often saved as MrSid-files (*.sid). MrSid is a raster format, using less memory than other popular formats (e.g. tif, jpg, etc.). Some GPS-Software like TTQV or GlobalMapper could import MrSid.

In order to use sid-files with other programmes, they have to be converted. A nice free tool to view and convert MrSid is the free MrSid-Viewer. It enables you save sid-files complete or in parts to tif-format. The MrSid-Viewer generates the tif-file and a calibration-file containing the coordinates of the corners of the new image. The calibration file is saved with the name as the tif-file but ends with ".tab". The tab-file contains plain text, which could be opened with any editor to read or copy and paste the coordinates.

Excerpt from a tab file
Definition Table
 (540395.625, 5277223.75) (0,0)Label "TopLeft",
 (554246.625, 5277223.75) (972, 0) Label "TopRight",
 (540395.625, 5268075.25) (0, 642) Label "BottomLeft",
 (554246.625, 5268075.25) (972, 642) Label "BottomRight"
The coordinates are placed in the first brackets of each line, in the order of "easting , northing"

Depending on yor GPS-software, the tif-files could be imported or have to be changed to a supported format like gif, png or something else. This could be done with the free IrfanView.

Satellite images from the NASA use UTM coordinates with mapdatum WGS84. The first numbers of the filename (e.g. N-32-50_2000) represents the UTM zone. Other sources may use different systems. Check the homepage to find the appropriate information.

To view satellite images together with GPS-Data GlobalMapper (Freeware Version) is recommended, because it supports many file types. Another interesting software is fGIS (Freeware) which supports many data formats and comes in several languages.

© Thomas Hasse
11.02.05