About the Maps

THE MAPS:  The maps were produced using GIS (Geographic Information Systems) software.  GIS draws upon large amounts of data collected in many ways. For instance, these maps were generated by data collected from satellites. Satellites collect information using a variety of means (radar, light, etc.).  To collect this data, the satellite’s sensors give each piece of the picture or picture element or pixel, a value for brightness or temperature, etc. that is recorded as a number. To convert this digital information to an image, a color can be assigned to all objects of a class or category such as forest types or crops.  Information is collected and integrated over time. (For instance, satellites can tell the difference between deciduous and evergreen by collecting information in both summer and winter.)  Resolution is described by the size of the area represented by a pixel (one piece of data.).  If the area represented by a pixel is 30 X 30 meters, each 30 X 30 meter square is stored as a number.

DIGITAL SHADED-RELIEF MAP: For the Digital Shaded-Relief Map, each pixel represents the dominant elevation even though more than one elevation may actually occur in the pixel.  When a cartographer (a map maker) decides to make a map, he or she selects the data to be used from the data bank and using the GIS software, produces a map.  For instance, to produce the Digital Shaded-Relief Map of Pennsylvania, the cartographer chose elevation data as the information criteria. The cartographer used a coloring system and assigned each pixel (at a 30 X 30 meter resolution) a color according to the elevation.   The advantage of a map such as this is that is has much more detail and is far more accurate than traditional, hand drawn maps.  It is also much faster and ultimately less expensive to produce.

LAND COVER MAP:  For the Land Cover map, each pixel represents the dominant type of land cover. The purpose of this map is to illustrate the varied land cover of the state of Pennsylvania.  This information could be used for watershed management, environmental inventories, transportation modeling, fire risk assessment, and land management. The land cover information is divided into twenty-one different classes of land cover and stored in a data bank. When the data was accessed for this land cover map, not all classes were represented in Pennsylvania (note there is no desert or grassland.)  The remaining classes were compacted into only fifteen classes.  The following is an explanation of these classes:


LOW INTENSITY RESIDENTIAL:  Land includes areas with a mixture of constructed materials and vegetation or other cover. Constructed materials account for 30-80% of the total area.

HIGH INTENSITY RESIDENTIAL: Includes heavily build-up urban centers where people reside. Examples include apartment complexes and row houses. Vegetation occupies less than 20% of the landscape. Constructed material accounts for 80-100% of the total area.

COMMERCIAL/INDUSTRIAL/TRANSPORTATION:  Includes all highly developed lands not classified as High Intensity Residential. A significant land area is covered by concrete, asphalt, or other construction materials. Vegetation occupies less than 20% of the landscape.  Examples of such areas include skyscrapers, shopping centers, factories, industrial complexes, airport runways, and interstate highways.


DECIDUOUS FOREST: Area dominated by trees and shrubs where 75% or greater of the cover present is characterized by individuals that simultaneously shed their foliage in response to an unfavorable season.

EVERGREEN FOREST:  Areas dominated by trees and shrubs where 75% or greater of the cover present is characterized by individuals that maintain their leaves all year. Canopy is never without green foliage here.

MIXED FOREST:  Areas dominated by shrubs and trees where neither deciduous nor evergreen species represent more than 75% of the cover present.


PASTURE/HAY:  Grasses, legumes, or grass-legume mixtures planted or intensely managed for livestock grazing or the production of seed or hay crops.

ROW CROPS: All areas used for the production of crops such as corn, soybeans, vegetables, tobacco, and cotton.

URBAN/RECREATIONAL GRASSES:  Vegetation planted in developed settings for recreation, erosion control, or aesthetic purpose. Examples include parks, lawns, and golf courses.


BARE ROCK/SAND/CLAY: Bare rock, sand, silt, gravel, or other earthen material with little or no vegetation regardless of its inherent ability to support life. Vegetation, if present, is more widely spaced and scrubby than that in the vegetated categories.

QUARRIES/STRIP MINES/GRAVEL PITS:  Areas of extractive mining activities with significant surface expression.

TRANSITIONAL:  Areas dynamically changing from one land cover to another, often because of land use activities.  Examples include transition phase between forest and agricultural land, temporary clearing of woody or herbaceous vegetation.


OPEN WATER:  All areas of open water with less than 25% cover of trees, shrubs, persistent emergent plants, emergent mosses, lichens, or other land cover.

WOODY WETLANDS:  Areas of forested and shrub-land vegetation where the soil or substrate is periodically saturated with or covered with water.

EMERGENT HERBACEOUS WETLANDS:  Non-woody vascular perennial vegetation where the soil or substrate is periodically saturated with or covered with water.