Index USIGS Glossary

-- Z --
Term Definition/Description Source
z-axis In a three-dimensional rectangular coordinate system, the axis of reference that is perpendicular to both the x- and y-axes at their point of intersection. HDBK-850
zenith That point of the celestial sphere vertically overhead. See also geocentric zenith; geodetic zenith. HDBK-850
zenith angle See zenith distance. HDBK-850
zenith camera A special camera so designed that its optical axis may be pointed accurately toward the zenith. It is used for the determination of astronomic positions by photographing the position of the stars. See also photographic zenith tube. HDBK-850
zenith distance The vertical angle between the zenith and the object which is observed or defined. Zenith distance is the complement of the altitude. Also called zenith angle. See also coaltitude. HDBK-850
zenith telescope A portable instrument adapted for the measurement of small differences of zenith distance, and used in the determination of astronomic latitude. HDBK-850
zenithal chart See azimuthal chart. HDBK-850
zenithal map projection See azimuthal map projection. HDBK-850
zero meridian plane; zero meridian The chosen meridian plane from which longitude is measured. The zero meridian plane is actually only a half-plane, on one side of the polar axis. Often abbreviated to zero meridian although strictly speaking a meridian is a line of intersection of the plane along a reference surface, for example an ellipsoid. CEN/TC 287
z-motion Movement of a stereoplotting projector in a vertical direction. HDBK-850
zonal harmonics The set of spherical harmonics that are functions of latitude only and therefore do not affect the rotational symmetry of the surface about its polar axis. HDBK-850
zone time The local mean time of a reference zone. See also time zone. HDBK-850
zoom system See pancratic system. HDBK-850
z-scale (JCS) On an oblique photograph, the scale used in calculating the height of an object. Also, the name given to this method of height determination. HDBK-850
ZULU time See Universal Time (UT1). JPUB 1-02
z-value data points Data points providing a value perpendicular or normal to a specified surface (elevation). HDBK-850

Last Updated by Mark Owens 17 February 1999.