Terms starting with E

EARTH ELLIPSOIDS

The Earth is not a perfect sphere but a somewhat flattened, pear shape; wider at the equator than at the poles. To describe this shape, cartographers use an idealized geometrical model called an Earth Ellipsoid, with fixed dimensions for the major and minor axes. These dimensions will not agree with measured values for all regions due to the Earth's curvature at different latitudes. For this reason, various Ellipsoid models are needed.

Every map has associated with it the Earth Ellipsoid best suited for that area. If you are working with base maps, consult the map itself to see which Ellipsoid is being used. Some Ellipsoids are commonly used throughout the various regions of the world, such as Clarke 1886 and International 1967. The following is a list of the most commonly used Earth Ellipsoids and their areas of use: 

Name/Date

Area of Use

WGS 84/GRS 80/1980

Newly adopted

WGS 72/1972

US DoD, Oil Industry

WGS 66/1966

NASA

Australian/1965

Australia

Krasovsky/1940

Russia

International/1909

Entire Earth

Hayford Ellipsoid

Entire Earth

Clarke/1880

Africa/France

Clarke/1866

North America, Philippines

Airy/1830

Great Britain

Modified Airy

Great Britain

Bessel/1841

Central Europe, Chile, Indonesia

Modified Everest

India, Burma, Afghanistan, Thailand

Everest/1830

India, Burma, Afghanistan, Thailand

South East Asia

Singapore

Hough

Marshall Islands

EDM (ELECTRONIC DISTANCE MEASURING DEVICE)

Instruments that measure the phase difference between transmitted and reflected or retransmitted electromagnetic waves of known frequency, or that measure the round-trip transit time of a pulsed signal, from which distance is computed.

ELEVATION

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

EPHEMERIS

A list of parameters describing the orbital track of a sensor; used to identify the location of the vehicle and its orbit at any given time.

EROSION

Group of natural processes including weathering, dissolution, abrasion, corrosion, and transportation that remove material from any part of the Earth's surface.

ERRORS

Horizontal accuracy (represented as CE90) is a horizontal measurement on the ground defining the radius of a circle within which an object of known coordinates should be found on an image. The probability of a point in the image meeting the recorded accuracy is 90% for CE90. This parameter is expressed in meters.

Vertical accuracy (represented as LE90) is the accuracy at which elevation of a point in the image can be determined. The probability of a point in the image meeting the recorded accuracy is 90% for LE90. This parameter is expressed in meters.

ESTUARY

That portion of a stream influenced by the tide of the body of water into which it flows; an arm of the sea at a river mouth.