How Were the Landscape Indicators Measured? 
Many kinds of data were used to prepare the indicators 
shown in this atlas. Federal agencies were the primary 
source for data, including maps of elevation, watershed 
boundaries, road and river locations, population, soils, 
and land cover. Sources included the U.S. Geological 
Survey (USGS), the U.S. Environmental Protection 
Agency (EPA), the U.S. Department of Agriculture 
(USDA), the U.S. Census Bureau, the U.S. Fish and 
Wildlife Service, the Louisiana Department of Environ¬ 
mental Quality, The Mississippi Alluvial Plain Project of 
The Nature Conservancy, and the North American 
Landscape Characterization (NALC) Program. 
Data collected by satellites were used to map land 
cover and its change over time. The sensors carried 
on satellites measure the light reflected from the 
Earth’s surface. Because different surfaces reflect 
different amounts of light at various wavelengths, it is 
possible to identify land cover from satellite measure¬ 
ments of reflected light. Figure 1.7 illustrates the 
differential reflectance properties of water, sediments 
suspended in water, and land surfaces for a typical satel¬ 
lite image. Examples of land cover maps derived from 
satellite images appear later in this atlas. 
In a typical digital map, data are stored as a series of 
numbers for each theme. These maps can be thought of 
as checkerboards, where each grid square (or pixel, which 
is an abbreviation of “picture element”) represents a data 
value for a particular landscape attribute (for example soils, 
topography, or land cover type) at a specific location. 
Figure 1.7 
Illustration of differential light 
reflectance properties for 
water, sediments suspended 
in water, and land surfaces 
over a portion of Vancouver, 
British Columbia -These 
images can be manipulated in 
various ways to extract 
information about the Earth’s 
surface. 
Source: North American 
Landscape 
Characterization 
Program 
