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A country-by-country evaluation of source data is given in Table 2 of Annex 1. Only an overview is presented here.

Developed countries: It has been possible to compile detailed data for most countries with accuracy which generally exceeds by far that achieved for developing countries. Data on growing stock, woody biomass, growth rates and harvest yields are included. However, the quality and quantity of the data vary considerably between the countries, and there are many gaps in the information supplied. Therefore, regional totals cannot be made for all variables studied. Moreover, core definitions have been interpreted and applied quite differently in the various country groups due to differences in basic concepts and schools of thought.

Developing countries: The situation can be described as follows:

- Out of 143 countries assessed, all except seven possessed one estimate of forest cover carried out sometime between 1970 and 1990, mostly based on remote sensing.

- The variations in the date of the collection of the country data still present a major constraint. It has been found that on average the country data made available for the 1990 assessment were, in fact, close to ten years old, i.e. relating to around 1980;

- Twenty-five countries had carried out more than one forest cover assessment, which formed the basis for developing the adjustment function.

- There had been substantial developments in technological capability, in particular the application of geographic information systems (GIS), remote sensing and modelling techniques.

Global synthesis: This component was not designed in the planning stage, but was carried out as a final operation after the completion of the assessments for the developed and developing countries.

Results obtained regarding forest area change are incomplete and not conclusive. In estimating area changes, there are two problems. The first concerns the concept of "gross" and "net" change. The positive and negative changes in the forest cover of a country may cancel each other out and produce a net change in forest cover close to zero. Purely for forest area accounting this does not matter; however for the assessment of the "environmental functions" of the forest, it is preferable to have separate estimations for both positive and negative changes in forest cover.

The second question concerns changes in the definitions and mensurational standards used in a country over time. This may result in estimates which include "real change" as well as "spurious change" due to improvements in measurement techniques. In such a case, it is necessary toreappraise the baseline results according to the improved definitions and only then compare values at two different points in time. Only in this way can valid estimates of change be obtained.

On a global scale further complications arise because definitions and/or measurement techniques differ among countries. These may produce estimates at global level of unknown reliability. In the 1990 assessment, harmonization of country data on a global basis was undertaken for the developing countries, but not for the developed ones. This, however, should not distort the results as the magnitude of change in the developed countries is rather small.

In the context of change assessment, the pan-tropical survey based on remote sensing implemented as a part of Forest Resources Assessment 1990 Project, deserves a special mention. It provides statistically reliable and consistent estimates of forest cover over space and time. Of particular interest are the "change matrices" which show the extent as well as direction of land cover changes and can thus help to explain the process of deforestation and forest degradation and, in particular, the transfer of forest land to agriculture and other purposes.

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