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Maps, Atlases, Aerial Images, and Cartographic Resources

Cartographic Vocabulary

Azimuth: Horizontal direction reckoned clockwise from the meridian plane.

Contour: Imaginary line on ground, all points of which are at the same elevation above or below a specific datum.

Contour interval: Difference in elevation between two adjacent contours.

Coordinates: Linear and (or) angular quantities that designate the position of a point in relation to a given reference frame.

Datum (pl. datums): In surveying, a reference system for computing or correlating the results of surveys. There are tow principal types of datums: vertical and horizontal. A vertical datum is a level surface to which heights are referred. In the United States, the generally adopted vertical datum for leveling operations is the National Geodetic Vertical Datum of 1929. The horizontal datum is used as a reference for position. The North American Datum of 1927 is defined by the latitude and longitude of an initial point (Meade's Ranch in Kansas), the direction of a line between this point and a specified second point, and two dimensions that define the spheroid. The new North American Datum of 1983 is based on a newly defined spheroid (GRS80); it is an Earth-centered datum having no initial point or initial direction.

Elevation: Vertical distance of a point above or below a reference surface or datum.

Grid: Network of uniformly spaced parallel lines intersecting at right angles. When superimposed on a map, it usually carries the name of the projection used for the map- that is, Lambert grid, transverse Mercator grid, universal transverse Mercator grid.

Imagery: Visible representation of objects and (or) phenomena as sensed or detected by cameras, infrared and multispectral scanners, radar, and photometers. Recording may be on photographic emulsion (directly as in a camera or indirectly after being first recorded on magnetic tape as an electrical signal) or on magnetic tape for subsequent conversion and display on a cathode ray tube.

Latitude: Angular distance, in degrees, minutes, and seconds of a point north or south of the Equator.

Longitude: Angular distance, in degrees, minutes, and seconds, of a point east or west of the Greenwich meridian.

Map projection: Orderly system of lines on a plane representing a corresponding system of imaginary lines on an adopted terrestrial or celestial datum surface. Also, the mathematical concept for such a system. For maps of the Earth, a projection consists of 1) a graticule of lines representing parallels of latitude and meridians of longitude or 2) a grid.

Map series: Family of maps conforming generally to the same specifications and designed to cover an area or a country in systematic pattern.

Meridian: Great circle on the surface of the Earth passing through the geographical poles and any given point on the Earth's surface. All points on a given meridian have the same longitude.

Origin of coordinates: Point in a system of coordinates that serves as a zero point in computing the system's elements or in prescribing its use.

Prime meridian: Meridian of longitude 0 degrees, used as the origin for measurements of longitude. The meridian of Greenwich, England, is the internationally accepted prime meridian on most charts. However, local or national prime meridians are occasionally used.

Quadrangle: Four-sided area, bounded by parallels of latitude and meridians of longitude used as an area unit in mapping (dimensions are not necessarily the same in both directions). Also, a geometric figure of significance in geodetic surveying.

Scale: Relationship existing between a distance on a map, chart, or photograph and the corresponding distance on the Earth.

Survey: Orderly process of determining data relating to any physical or chemical characteristics of the Earth. The associated data obtained in a survey. An organization engaged in making a survey.

Topography: Configuration (relief) of the land surface; the graphic delineation or portrayal of that configuration in map form, as by contour lines; in oceanography the term is applied to a surface such as the sea bottom or surface of given characteristics within the water mass.

Universal Transverse Mercator (UTM) grid: Military grid system based on the transverse Mercator projection, applied to maps of the Earth's surface extending from the Equator to 84 Degrees north and 80 degrees south latitudes

from WorldAtlas

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Understanding Map Scales

Scale is the relationship between distance on the map and distance on the ground. A map scale usually is given as a fraction or a ratio—1/10,000 or 1:10,000.These "representative fraction" scales mean that 1 unit of measurement on the map—1 inch or 1 centimeter—represents 10,000 of the same units on the ground. If the scale were 1:63,360, for instance, then 1 inch on the map would represent 63,360 inches, or 1 mile, on the ground (63,360 inches divided by 12 inchesequals 5,280 feet, or 1 mile).

The first number (map distance) is always 1. The second number (ground distance) is different for each scale; the larger the second number is, the smaller the scale of the map. "The larger the number, the smaller the scale" sounds confusing, but it is easy to understand. A map of an area 100 miles long by 100 miles wide drawn at a scale of 1:63,360 would be more than 8 feet square. To make the map a more convenient size, either the scale used or the area covered must be reduced.

Many areas have been mapped at different scales. The most important consideration in choosing a map is its intended use. A town engineer, for instance, may need a very detailed map to locate precise sewers, power and water lines, and streets. A commonly used scale for this purpose is 1:600 (1 inch on the map represents 50 feet on the ground). This scale is so large that many features—such as buildings, roads, and railroad tracks—can be drawn to scale instead of being represented by symbols.

United States Geological Survey


What Is a Map Projection?

A map projection is a way to represent the curved surface of the Earth on the flat surface of a map. Map projections allow us to represent some or all of the Earth's surface, at a wide variety of scales, on a flat, easily transportable surface, such as a sheet of paper.

There are hundreds of different map projections. The process of transferring information from the Earth to a map causes every projection to distort at least one aspect of the real world—either shape, area, distance, or direction.

The appropriate projection for a map depends on the scale of the map, and on the purposes for which it will be used. For example, a projection may have unacceptable distortions if used to map the entire country, but may be an excellent choice for a large-scale (detailed) map of a county.

- National Atlas of the United States

Projections defined by distortion characteristics

An equal area, or equivalent projection maintains accurate relative sizes.

A conformal projection such as the Mercator maintains angular relationships and accurate shapes over small areas. These projections are used where angular relationships are important, such as for navigational or meteorological charts.

An equidistant projection maintains accurate distances from the center of the projection or along given lines. These projections are used for radio and seismic mapping, and for navigation.

A zenithal projection maintains accurate directions (and therefore angular relationships) from a given central point. These projections are used for aeronautical charts and other maps where directional relationships are important.

Projections defined by developable surface

A cylindrical projection projects information from the spherical Earth to a cylinder. The cylinder may be either tangent to the Earth along a selected line, or may be secant (intersect the Earth) along two lines. Imagine that once the Earth's surface is projected, the cylinder is unwrapped to form a flat surface. The lines where the cylinder is tangent or secant are the places with the least distortion.

A conic projection projects information from the spherical Earth to a cone that is either tangent to the Earth at a single parallel, or that is secant at two standard parallels. Once the projection is complete, the cone is unwrapped to form a flat surface. The lines where the cone is tangent or secant are the places with the least distortion. A polyconic projection uses a series of cones to reduce distortion.

A planar projection projects information to a plane. The plane may be either tangent or secant.