• ABAC- A nomogram for obtaining the conversion angle to apply when plotting great-circle bearings on a Mercator
• abnormal magnetic variation- Any anomalous deflection, whose cause is unknown, of the compass needle from the magnetic meridian.
• abscissa- The horizontal coordinate of a set of rectangular coordinates. Also used in a similar sense in connection
with oblique coordinates. Also called total departures; xcoordinate.
•absolute accuracy- Absolute accuracy is defined as the statistic which gives the uncertainty of a point with respect to the datum required by a product specification. This definition implies that the effects of all error sources, both random and systematic, must be considered. Absolute accuracy is stated in terms of two components, a horizontal component and a vertical component. The horizontal absolute accuracy associated with a product is stated as circular error, CE, such that 90 percent of all positions depicted by that product have a horizontal error with magnitude less than CE. Likewise, the absolute vertical accuracy associated with a product is stated as a linear error, LE, such that 90 percent of all elevations depicted by the product have an error with magnitude less than LE
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This document was prepared by the National Imagery and Mapping Agency (NIMA), Systems Engineering and Integration Division (SOS). The intent of this document is to provide a common set of definitions and acronyms for the development and evolution of the United States Imagery and Geospatial Information Service (USIGS). The items contained in this glossary have been collected from multiple sources which are listed in Appendix A. Acronyms and abbreviations are given in Section 1. Definitions and descriptions are given in Section 2. In some cases the sources define the same term in different ways. It is expected that these multiple definitions will be addressed as this glossary evolves, but in many cases multiple definitions of a term may ultimately remain in the USIGS Glossary as a reflection of different uses or contexts of the term.
In addition to the many general terms contained in the USIGS Glossary, several terms have been collected from the USIGS Conceptual Data Model (USIGS/CDM) and asssociated logical data mode (LDM) (e.g., Archive & Dissemination Logical Data Model (A&D LDM)) data dictionaries. While the intent of the USIGS Glossary is not to duplicate these data dictionaries in their entireties, many of the entity-level data dictionary terms are general enough in nature to warrant inclusion in the USIGS Glossary.