Glossary

Popular terms used throughout this website

 


CULTURE

Features constructed by man that are under, on, or above the ground which are delineated on a map. These include roads, trails, buildings, canals, sewer systems, and boundary lines. In a broad sense, the term also applies to all names, other identification, and legends on a map. 

DATA

Refers to remotely sensed images that can be produced as photographic or digital products.

DATA ACCESS SECURITY

Measures taken to control system users ability to view or modify data. these measures can include logical views of data and explicit access rights by group or individual users. 

DATABASE

A logical collection of inter related information, managed and stored as a unit, usually on some form of mass storage system such as magnetic tape or disk. A GIS database includes data about the Spatial locations and shape of geographic features recorded as points, lines, pixels, grid cells or tins as well as their attributes.

Database Design: The formal process of analysing facts about the ral world onto a structured database model. Database design is characterised by the following phases: requiremental analysis, logical design and physical design.

DATUM

Geodetic datums help to define the curvature of the earth to allow for position calculations. A datum is essentially a correction to the chosen ellipsoid utilizing ground control. As with FORMAT, our products support a variety of the most frequently used datums, such as WGS84 and NAD83.

DBMS

Database Management Systems. It is a set of computer programs for organising the information in a database. Typically, a DBMS contains routines for data input, verification.

DECLINATION

In astronomy, the angular distance of a celestial body above (north, plus) or below (south, minus) the celestial Equator. Magnetic declination is the angular difference between magnetic north and true (geographic) north at the point of observation; it is not constant but varies with time because of the "wandering" of the magnetic north pole. 

DEM EXTRACTION

A step in Stereoscopic Modeling process. DEM Extraction uses the resampled epipolar LEFT and RIGHT images created by the Prospective Projection step, and creates an output elevation raster object of the selected model area.

DEPTH CURVE

Line on a map or chart connecting points of equal depth below the datum.

DIGITAL ELEVATION MODEL (DEM)

Digital information that provides a uniform matrix of terrain elevation values. It provides basic quantitative data for deriving terrain elevation, slope, and/or surface roughness information.

DIKE

Bank of earth or stone used to form a barrier, frequently and confusingly interchanged with levee. A dike restrains water within an area that normally is flooded.

DOWNLINKING

Electronically transmitting data from a satellite to the ground.

DPI

Dots Per Inch. A measure of scanner, screen, and printing resolution. The more dots per inch, the more detail a device can process for a given area of page or display. On the other hand, the more dots per inch, the higher the demands on machine storage and processing (files get large and processing slows down).

DTM

Digital Terrain Model. Elevation data in a 3 x 3 arcsecond grid form or a similar rectilinear form created by the Defense Mapping Agency.

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