Porto Velho, Brazil, and its surroundings
|  | ERS Radar multitemporal image of Porto Velho (Brasil) | |
This image is a so-called 'multitemporal image', and was acquired by ESA's ERS radar satellite. It includes three overflights at an altitude of 700 km above the Earth. One was taken on 6 May 1993, the second on 25 October 1997, and the third on 11 October 1998.
Download here a larger version of the image of Porto Velho and surroundings (2.6 Mb)
Despite very frequent cloud coverage in that region of the world, all three radar images are cloud-free. Indeed, the microwaves used by the radar can penetrate the cloud coverage.
The image is a composition of three dates, and the colours give some indication about the changes that occured in the period between 1993 and 1998.
Unchanged features appear in black, white or grey. If a feature does not change, the value of its pixels remains the same on all three images. The town of Porto Velho in the Rondônia State of Brazil is one of those. It is depicted in white, and is situated at 8º43' South and 63º54' West on the Madeira River, a triburary of the Amazon.
The prevailing greyish-magenta colour in the image represents, to a large extent, the still untouched forest. But after 1993, many changes occured. The very dark and black areas represent deforestation before 1993. All patches in reddish-magenta are surfaces cut between 1993 and 1997, and all green-yellowish areas were deforested mainly between October 1997 and October 1998. The many straight linear features reveal access roads along the borders. They give us some information about where the forest will be cut next.
| | | Porto Velho and surroundings |
ERS and Envisat continue to monitor the situation. A new data acquisition will shortly be done!
The radar image also represents the topography very well, and indicates a hilly area. These are the zones where soil erosion can be expected as soon as the trees are gone.
But some colours also appear in the river. Brightly coloured single points are ships, but the white feature across the river is a rapid. There the water is very turbulent, strongly scattering the radar signal. The faint colours reveal increased currents, and are influenced by local winds.
Technical detail: the acquisition of 11 october 1998 is displayed in red, data taken on 25 October 1997 is displayed in green, and data of 6 May 1993 is diplayed in blue.
The pixel size in this image is 62.5 m.
Using LEOWorks you can make some meaurements:
- How wide is the river at Porto Velho, and how wide is it at its widest point near the rapids?
- What is the surface of the extended magenta reddish area in the centre of the image, and when was it cleared?
| 
1
