EARTHTRENDS DATA TABLES
TECHNICAL NOTES: Land Use and Human Settlement Patterns
For more information, please consult http://earthtrends.wri.org


DEFINITIONS AND METHODOLOGY
Total Land Area is measured in thousand hectares and excludes the area under inland water bodies. Inland water bodies generally include major rivers and lakes. Data on land area were provided to the Food and Agriculture Organization (FAO) by the United Nations Statistical Division.

Forested Area is calculated by WRI as a percentage of total land area using data from MODIS satellite imagery analyzed by the Global Land Cover Facility (GLCF) at the University of Maryland and from FAO’s Global Forest Resources Assessment 2000 (FRA 2000).  

MODIS Satellite Imagery identifies the percent of tree crown cover for each 500-meter pixel image of land area based on one year of MODIS photography. Data were aggregated to country-level by the GLCF at the request of WRI.  The values presented here show the percentage of total land area with more than 10 percent or 50 percent of the ground covered by tree crowns.

The Food and Agriculture Organization (FAO) Estimates are drawn from FRA 2000. Forest area includes both natural forests, composed primarily of native tree species, and plantations, forest stands that are established artificially. If no other land use (such as agro-forestry) predominates, any area larger than 0.5 hectares with tree crowns covering more than 10 percent of the ground is classified as a forest.  Forest statistics are based primarily on forest inventory information provided by national governments; national gathering methodologies can be found at http://www.fao.org/forestry/fo/fra/index.jsp. FAO harmonized these national assessments with the 10-percent forest definition mentioned above. In tropical regions, national inventories are supplemented with high resolution Landsat satellite data from a number of sample sites covering a total of 10 percent of the tropical forest zone. Where only limited or outdated inventory data were available, FAO used linear projections and expert opinion to fill in data gaps. If no forest statistics existed for 1990 and 2000, FAO projected forward or backward in time to estimate forest area in the two reference years. 

Arable and Permanent Cropland is calculated by WRI as a percent of total land area.  Arable land is land under temporary crops (double-cropped areas are counted only once), temporary meadows for mowing or pasture, land under market and kitchen gardens, and land temporarily fallow (less than five years).  Abandoned land resulting from shifting cultivation is not included in this category.  Permanent cropland is land cultivated with crops that occupy the land for long periods and need not be replanted after each harvest, such as cocoa, coffee, and rubber; this category includes land under trees grown for wood or timber.  Wherever possible, data on agricultural land use are reported by country governments in questionnaires distributed by FAO.  However, a significant portion of the data is based on both official and unofficial estimates. 

Permanent Pasture is calculated by WRI as a percent of total land area.  Permanent pasture is land used long-term (five years or more) for herbaceous forage crops, either cultivated or growing wild. Shrublands and savannas may be classified in some cases as both forested land and permanent pasture.

Drylands is calculated by WRI as the percent of total land area that falls within three of the world’s six aridity zones—the arid, semi-arid, and dry sub-humid zones. The United Nations Convention to Combat Desertification (UNCCD) adopted this definition of drylands in order to identify areas where efforts combating land degradation should be focused and methods for attaining sustainable development should be promoted. The world is divided into six aridity zones based on the aridity index—the ratio of mean annual precipitation (PPT) to mean annual potential evapotranspiration (PET). Drylands of concern to the UNCCD include those lands with an aridity index between .05 and .65 (excluding polar and sub-polar regions). Ratios of less than .05 indicate hyper-arid zones, or true deserts. Ratios of 0.65 or greater identify humid zones. The areas with an aridity index between .05 and .65 encompass the arid, semi-arid, and dry sub-humid areas. See the UNCCD's website at http://www.unccd.int/main.php for more information. Climatic data from 1950 to 1981 were used to define aridity zone boundaries for the globe with a resolution of about 50 km. 

Population Density is calculated by WRI as the number of persons per square kilometer of land area using FAO land-area data shown in the first column. Population data are from the United Nations Population Division.

Urban Population as a Percent of Total is the proportion of a country's total population that resides in areas defined as urban in each of the countries of the world. These definitions vary slightly from country to country. Many countries define an urban area by the total number of inhabitants in a population agglomeration. Typically the threshold for considering a region urban is between 1,000 and 10,000 inhabitants. Other countries specify several of their cities or provinces as urban, and the remaining population is defined as rural. Estimates of the proportion of the population living in urban areas are obtained from national sources. Censuses and population registers are the most common sources of those counts. Once values of the urban proportion at the national level are established, they are applied to estimates and projections of the total national population from World Population Prospects: The 2002 Revision.

Percent of Population Living in Cities with More Than 100,000 and 1 Million People indicate population distribution and levels of urbanization within a country. WRI calculated percentages from the Urban Population in World Bank Regions by City Size data set and total population figures from the U.N. Population Division. Urban population data were primarily collected from national statistical offices, international organizations such as the United Nations, and the World Gazetteer web site.  Data from national census bureaus in several OECD countries (Canada, United States) were added to complement this data set.

Percent of Urban Population Living in Slum Conditions is the proportion of a country's urban population that is living in households classified as slum dwellings. A slum household is defined by the United Nations Human Settlements Program (UN-HABITAT) as a group of individuals living under the same roof that lack one or more of the following conditions: “secure tenure status, adequate access to improved water, adequate access to improved sanitation and other infrastructure, structural quality of housing, and sufficient living area.”
While the same methodology was used to determine the slum population in all countries, data sources vary. Where available, household surveys, such as Demographic and Health Surveys (DHS) and Multiple Indicator Cluster Surveys (MICS), were the common sources of data. An effort was made to ensure that households were not counted twice, in the event that they lacked more than one of the indicators. In the absence of household surveys, or when household surveys did not provide answers for the desired indicators, the slum populations were estimated. Estimates were derived from a statistical model using available country data and the Human Development Index (HDI) of theUnited Nations Development Programme (UNDP). 

FREQUENCY OF UPDATE BY DATA PROVIDERS

Total Land Area, Arable and Permanent Cropland Area, and Permanent Pasture data are updated annually by the FAO.  Population data are updated every two years by the United Nations Population Division.  Forested Land Area based on Modis Satellite Imagery was released by the GLCF in 2002.  The FRA is published by the FAO every 5 years; data in this table are from the 2000 release. Drylands Area data were prepared in 1991; no update is planned.  Data on urban population by city size are updated continually by the World Bank. Urban Population Living in Slum Conditions is the first global compilation of such data.  

DATA RELIABILITY AND CAUTIONARY NOTES

Land-area data are intended for broad estimations only and not for strict comparisons. Land-area classification is inherently subjective; experts often express different opinions on the criteria for categorizing ecosystem and use types, and the resolution of the underlying satellite and survey information can vary widely among data sets.  In addition, the information on land-area types shown here is from different sources and represents different time periods.  They are not intended to represent exclusive land-cover types; some degree of overlap is present.  

Forest Cover:  As shown in the table, forest cover estimates differ widely based on collection methodology and classification used.  FAO uses a more complex definition of forests than is used in the MODIS data set, requiring that there be 10 percent tree cover and that forestry be the predominant land use in the survey area. Thus some areas with tree cover of more than 10 percent may not be counted as forest if the predominant land use is determined to be agriculture, urban settlement, or some other nonforestry use. Because the MODIS tree-cover data set makes no such distinction, the tree cover in the “10 percent and above” categories will sum to a larger area than the FAO forest area for most countries.   

Modis Satellite Imagery: Following publication of the Global Land Cover Characteristics (GLCC) database by GLCF, a number of scientific teams assessed the accuracy of the GLCC's approach by comparing the results with higher-resolution satellite imagery. These teams found that the accuracy of the GLCF's approach was, depending on the assessment approach, in a range from 60 to nearly 80 percent, meaning that the assessment teams' classification of a given area agreed with the GLCF's classification between 60 and 80 percent of the time.  

FAO Estimates:  FAO acknowledges that the quality of primary data available remains poor, particularly for tropical countries, open woodland areas, and non-production forests. In most tropical countries, forests are not monitored comprehensively or frequently enough to map their extent accurately or to track their rate of change. In the absence of inventory data for specific dates (1990 and 2000), FAO’s latest estimates of forest area and change over time are often based on projections and expert opinion and thus remain educated guesses. Just one or two satellite images appear to have been the prime source of new information for some countries with poor inventory data. Open woodlands are difficult to monitor by remote sensing techniques, and government forestry agencies tend not to survey them as part of normal forest inventories. Non-production forests are not included in these totals, even though many appear to meet the FAO definition of forests. While the quality of data from developed countries is generally better than from developing countries, problems still arise with estimates because of differences in national forestry definitions and systems of measurement, and the use of different reference periods. In northern countries, the boundary between forest and tundra is vague. For a discussion of some data reliability issues associated with FRA 2000, see http://pdf.wri.org/fra2000.pdf.

Drylands: The accuracy of land-area totals is limited by the 50-kilometer resolution of the data set. The climate data set was derived from a limited number of field observations. Actual boundaries between aridity zones are neither abrupt nor static, making delineated borders somewhat artificial. The data should therefore be considered useful as a general indicator of the extent of drylands within each country, rather than as an exact depiction of the climatic situation on the ground. Alternative methods for measuring extent of drylands area include use of soil moisture and agricultural production systems, although these methods may also be subject to similar problems such as low-resolution data, limited field observations, and subjectivity when delineating exact boundaries on the ground. 

Percent of Urban Population Living in Slum Conditions: UN-HABITAT’s definition of slum conditions, described above, may not always measure living conditions with sufficient precision. Sub-national coverage for the household surveys varies as does the international coverage for the different indicators.  Despite these drawbacks, this is the most reliable global data set available on this complex issue.  

SOURCES
Total Land Area and Cropland Area: Food and Agriculture Organization of the United Nations (FAO). 2004. FAOSTAT on-line statistical service. Rome: FAO. Available at http://apps.fao.org.
Forested Area, Modis Satellite Imagery: University of Maryland Global Land Cover Facility (GLCF).  2002.  MODIS 500m Vegetation Continuous Fields Percent Tree Cover. Available at http://glcf.umiacs.umd.edu/data/.
Forested Area, FAO Estimates: Food and Agriculture Organization of the United Nations (FAO). 2001. Global Forest Resources Assessment 2000--Main Report. Rome: FAO. Available at http://www.fao.org/forestry/fo/fra/index.jsp.
Dryland Area: U. Deichmann and L. Eklundh. 1991. Global Digital Data Sets for Land Degradation Studies: A GIS Approach. GRID Case Study Series No. 4. Nairobi, Kenya: United Nations Environment Program/Global Resource Information Database (UNEP/GRID).
Population Density: United Nations Population Division. 2003. World Population Prospects: The 2002 Revision. Dataset on CD-ROM. New York: United Nations. Available at http://www.un.org/esa/population/ordering.htm.
Urban Population: United Nations Population Division. 2004. World Urbanization Prospects: The 2003 Revision. Urban and Rural Areas Dataset (POP/DB/WUP/Rev.2003/Table A.7). Data set in digital form. Available at http://www.un.org/esa/population/ordering.htm. New York: United Nations.
Population by City Size: The World Bank Group. 2004. Urban Population in World Bank Regions by City Size. Washington, DC: World Bank. Available at http://www.worldbank.org/urban/env/population-regions.htm. 
Population Living in Slum Conditions: United Nations Human Settlements Program (UN-HABITAT). 2003. Slums of the World: The Face of Urban Poverty in the New Millennium? Nairobi: UN-HABITAT. Available at http://www.unhabitat.org/publication/slumreport.pdf.