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Chapter 45. Preprocessing Images
i.rectify16 will use the points you defined with i.points17 to ... rectify your image. In
other words it geocodes your map by linking the data elements contained in your
image to geographical coordinates. The module is pretty straightforward, but if questions remain, consult the man page.
Colors
Use r.colors18 to redefine the color table of your images. For example, you can use the
"color=grey.eq" parameter setting to apply a contrast stretch to an image (useful for
those that appear too dark).
Displaying images
In order to display your image in a recognizable way you have to create a color composite from its different frequency channels. Thus, to get a natural color composite
you would combine the red, green and blue channels. Two options are open to you:
•
Simply display a composite with d.rgb19. This does not create a new map, but displays three layers in a composite form.
•
Create a new composite map. For this purpose you can use i.composite20. This is
also very useful if you need a color composite as base map, as for example for
geocoding or for classification.
Radiometric preprocessing
In order to avoid certain phenomena such as atmospheric and terrain effects, but also
image transformation by the vendor, several corrections need to be applied to your
image data before you can procede to serious analysis. For most of these corrections
you should use r.mapcalc21 (see Chapter 36 for links to general tutorials on this module and see the relevant litterature for more information on radiometric techniques).
Notes
1. http://grass.itc.it/gdp/html_grass5/html/r.in.gdal.html
2. http://grass.itc.it/gdp/html_grass5/html/i.in.erdas.html
3. http://grass.itc.it/gdp/html_grass5/html/r.in.bin.html
4. http://grass.itc.it/gdp/html_grass5/html/i.out.erdas.html
5. http://grass.itc.it/gdp/html_grass5/raster.html
6. http://grass.itc.it/gdp/html_grass5/html/i.group.html
7. http://grass.itc.it/gdp/html_grass5/html/i.group.html
8. http://grass.itc.it/gdp/html_grass5/html/i.target.html
9. http://grass.itc.it/gdp/html_grass5/html/i.points.html
10. http://grass.itc.it/gdp/html_grass5/html/i.rectify.html
11. You can also try
12. http://grass.itc.it/gdp/html_grass5/html/i.points3.html
and
13. http://grass.itc.it/gdp/html_grass5/html/i.rectify3.html
which are more recent, but still unfinished modules.
12. http://grass.itc.it/gdp/html_grass5/html/i.points3.html
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