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The maths of deforestation estimates

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Calculating deforestation in the Amazon

In Brazil, deforestation figures are provided annually by the Instituto Nacional de Pesquisas Espaciais - (National Institute of Space Research, or INPE). These are estimated from 100 to 220 images taken by Landsat satellite during the dry season in the Amazon.

What is taken into account

The estimate only considers the Amazon rainforest biome; loss of savanna and natural fields within the rainforest is not considered.

The interpretation of the Landsat images reckons only the noticeable gaps in the rainforest canopy, comparing the images taken from one year with those taken the year before. This makes it possible to track the evolution of deforestation.
Estimating the undoing of the Amazon: why figures vary
Despite the quantity of assessments and the use of satellite imagery, estimates of deforestation within the confines of the Amazon Basin vary considerably.

This is partly due to a problem of definitions, including the delimitation of the region itself, the inclusion or exclusion of the cerrado scrub savanna, classification of secondary forests as "forest" or "deforested," and the inclusion of flooding by hydroelectric dams.

Also, differences in the radiation frequencies recorded and in image resolution for the data gathered by different satellites, and in interpretation of these data, also contribute to discrepancies. Operationally, areas are classified as deforested if they are readily recognized as cleared on Landsat satellite imagery.

It is also important not to confuse deforestation with burning: not all forest is affected by deforestation, and many areas are burned that are either not originally forest (especially savanna) or have already been deforested (especially established cattle pastures). Amazon rainforest can sometimes burn without being cleared first, as in the case of the Great Roraima Fire of 1998, but these events leave most trees standing and are not considered deforestation.1
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1Goudie (Ed.) 2001. Encyclopedia of Global Change. Environmental Change and Human Society

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