Popular terms used throughout this website



Science that deals with the measurement and description of the physical features of the oceans, seas, lakes, rivers, and their adjoining coastal areas, with particular reference to their use for navigation.

Hydrographic Survey: Survey of water area, with particular reference to submarine relief, and any adjacent land.


Scientific study of the waters of the Earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of ground water.


Topography referred to the national geodetic vertical datum of 1929. The science or art of describing heights of land surfaces with reference to this datum.


Science or art of determining terrain relief, by any method.


A quantitative method that classifies or identifies objects or patterns on the basis of their multispectral values; the normal output is analogous to an image map requiring no visual interpretation. 


An image that has been processed to be like a map in appearance, scale, geometry, boundary, and projection with a degree of precision that satisfies the user. Measurements made from an image map yield results equal to those made from the corresponding planimetric, topographic, or other map. Similarly, either the image map or the conventional map can be overlaid and matched with the other.


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.


Special data structure used in a database to speed search records in tables or spatial features in geographic data sets.


Instrument that detects infrared radiation and converts the detected energy to an electrical signal for recording on photographic film or magnetic tape.


Chart showing isogonic lines properly labeled with their magnetic declination.


Line joining points on the Earth's surface having equal magnetic declination as of a given date.


A field in a database table identified as of importance is designated a primary key. There can be only one primary key in a table. A primary key field must have unique entries for each record in the table. A primary key field also may not be blank in any of the records in a table. A foreign key is a field with the same name as a primary key that is located in a different table. Foreign keys are identified automatically when primary keys are designated in other tables attached to the same vector/CAD object. Primary and foreign key relationships between tables let you establish computed fields that incorporate values from more than one table in their defining expression.


(window context) The static text part of a component that presents information to the user about that component, such as the action invoked by a push button or the mode selected by a check button.


(raster context) Identifying and grouping the clusters that result from any kind of automated image interpretation. You choose labels (names for types of features) based upon your knowledge of the areas or materials in the image.


The materials that cover a study site, such as vegetation, bare soil, rock, sand, and water.