Terms starting with A

ABSOLUTE COORDINATES

Coordinates that are referenced to the origin of a given coordinate system.

ABSTRACTION

A way of viewing a real world object, usually a simplification. For example, a road may be represented as a centerline in one application and an area bounded by kerblines in another.

ACCURACY

Refers to the reference placement of the image during registration in relation to its actual ground location. Accuracy is often represented as a statistical "error" such as CE90. See "Error" for more details.

ACTIVE REMOTE SENSING

A remote-sensing system, such as radar, that produces electromagnetic radiation and measures its reflection back from a surface.

ACTIVE SENSOR

A sensor that generates its own electromagnetic energy, usually within the microwave wavebands. RADAR is an example of such a system.

ADJUSTMENT

Process designed to remove inconsistencies in measured or computed quantities by applying derived corrections to compensate for random or accidental errors.

  • Adjustment, Land- line:  Positioning land lines on a map to indicate their true, theoretical, or approximate location relative to the adjacent terrain and culture, by reconciling the information shown on Bureau of Land Management plats and field records with the ground evidence of the location of the lines.
  • Adjustment, standard accuracy: Adjustment of a survey resulting in values for positions and (or) elevations that comply with the National Map Accuracy Standards.

AEROTRIANGULATION

The process of developing a network of horizontal and or vertical positions from a group of known positions using direct or indirect measurements from aerial photographs and mathematical computations. 

ALTITUDE

The height or vertical elevation of a point above a reference surface. Altitude measurements are usually based on a given reference datum, such as mean sea level.

The height above the horizon, measured in degrees, from which a light source illuminates a surface. Altitude is used when calculating a hillshade, or for controlling the position of a light source in a scene.

ALTITUDE MATRIX

A rectangular grid containing elevation data. An altitude matrix can be obtained from the stereoscopic study of overlapping aerial photographs on analytical stereoscopic plotters, or from interpolation of irregularly spaced elevation data points.

APPLICATION WEB SERVICE

A Web service that solves a particular problem; for example, a Web service that finds all of the hospitals within a certain distance of an address. An application Web service can be implemented using the native Web service framework of a Web server; for example, an ASP.NET Web service (WebMethod) or Java Web service (Axis).

ASCII

American Standard Code for Information Interchange (pronounced "askee"). The 7-bit (128 characters) used as a computer's alphabet. The Latin alphabet character set encoded into digital values between 0 and 127 includes lowercase and uppercase letters, the numerals 0-9, English punctuation marks, special symbols (such as @#$%^&*) and non-displaying characters often used as printer control codes. The eighth bit, giving values from 128 to 255, is used in a nonstandard fashion and is not part of the standard ASCII code. PCs normally have the "extended" character set in their system font for digital values from 128 to 255. 
The term "ASCII file" is often used to mean a text-only file. Documents in most word processors are not text-only files, since they include header information and formatting characters. However most word processors have an export or print-to-file utility that will convert a document into a text-only ASCII format.

ASCII FILE

A text-only file. Documents in most word processors are not text-only files, since they include binary header information and formatting characters. However most word processors import so-called ASCII files and have an Export, Save/Text-only, or print-to-file utility that converts a document into an "ASCII" format. However, these files are not true ASCII files because they may include the characters from 128-255. The characters in this range are different between platforms such as PCs and Macintoshes and even from font to font with a platform, and are not part of the standard 7-bit ASCII code. (See also: ASCII.)

AT-NADIR

A point on the ground directly beneath the satellite, at a 90-degree angle above the Earth’s surface.

ATMOSPHERIC ABSORPTION

The process where electromagnetic energy, as it passes through the earth's atmosphere, interacts with gas molecules and energy is converted into the internal energy of the molecule. The energy is usually considered to be 'lost', particularly for remote sensing applications.

ATMOSPHERIC CORRECTION

A set of techniques used in digital image processing to compensate for the atmospheric effects present in remotely sensed imagery.