# GIS Glossary/Z

## GIS Glossary

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### Z

##### z-axis

In a spherical coordinate system, the vertical line that runs parallel to the earth's rotation, passing through 90 degrees north latitude, and perpendicular to the equatorial plane, where it crosses the x- and y-axes at the origin (0,0,0).

##### z-coordinate

The value for a given surface location that represents an attribute other than position. In an elevation or terrain model, the z-value represents elevation; in other kinds of surface models, it represents the density or quantity of a particular attribute.

##### zenith

In astronomy, the point on the celestial sphere directly above an observer. Both the zenith and nadir lie on the observer's meridian; the zenith lies 180 degrees from the nadir, and is observable.

##### zenith angle

In Survey Analyst for field measurements, a vertical angle that is formed by the intersection of two lines in a vertical plane. Zenith angles are observed on the vertical circle of a TPS instrument.

##### zenithal projection

A map projection that transforms points from a spheroid or sphere onto a tangent or secant plane. The azimuthal projection is also known as a planar or zenithal projection.

##### zero length line event

In linear referencing, a line event whose from-measure is equal to its to-measure. A zero length line even may occur, for example, along routes, when a polygon touches a route but does not overlap it.

##### z-factor

A conversion factor used to adjust vertical and horizontal measurements into the same unit of measure. Specifically, the number of vertical units (z-units) in each horizontal unit. For example, if a surface's horizontal units are meters and its elevation (z) is measured in feet, the z-factor is 0.3048 (the number of meters in a foot).

##### ZIP Code

Acronym for zone improvement plan code. A five-digit code, developed by the U.S. Postal Service, that identifies the geographic delivery area served by an individual post office or metropolitan area delivery station.

##### ZIP+4 Code

An enhanced ZIP Code that consists of the five-digit ZIP Code plus four additional digits that identify a specific geographic segment within the five-digit delivery area, such as a city block, office building, or other unit.

##### zonal analysis

The creation of an output raster in which the desired function is computed on the cell values from the input value raster that intersect or fall within each zone of a specified input zone dataset. The input zone dataset is only used to define the size, shape, and location of each zone, while the value raster identifies the values to be used in the evaluations within the zones.

##### zonal functions

The creation of an output raster in which the desired function is computed on the cell values from the input value raster that intersect or fall within each zone of a specified input zone dataset. The input zone dataset is only used to define the size, shape, and location of each zone, while the value raster identifies the values to be used in the evaluations within the zones.

##### zonal statistics

In ArcGIS Spatial Analyst, the calculation of a statistic for each zone of a zone dataset based on values from another dataset, a value raster. A single output value is computed for each cell in each zone defined by the input zone dataset.

##### zone

All cells in a raster with the same value, regardless of whether or not they are contiguous.

##### zone of interpolation

The area in a TIN layer for which values (elevation, slope, and aspect) are calculated. When a TIN layer is clipped to a smaller size to create a more focused study area, the parts that lie outside the study area remain triangulated and are represented as outside lines, but they have no values. These parts are said to be outside the zone of interpolation.

##### zoning

The application of local government regulations that permit certain land uses within geographic areas under the government’s jurisdiction. Zoning regulations typically set a broad category of land use permissible in an area, such as residential, commercial, agricultural, or industrial. Zoning regulations can also set constraints on building construction within areas, which may affect factors such as the maximum height of structures, minimum setbacks from property lines, the amount of parking that must be provided, or the density of housing.

##### zoom

To display a larger or smaller region of an on-screen map or image.

##### z-score

A statistical measure of the spread of values from their mean, expressed in standard deviation units, where the z-score of the mean value is zero and the standard deviation is one. In a normal distribution, 68 percent of the values have a z-score of plus or minus 1, meaning they lie within one standard deviation of the mean. Ninety-five percent of the values have a z-score of plus or minus 1.96, meaning they lie within two standard deviations of the mean; 99 percent of the values have a z-score of plus or minus 2.58. Z-scores are a common scale on which different distributions, with different means and standard deviations, can be compared.

##### z-tolerance

In raster-to-TIN conversion, the maximum allowed difference between the z-value of the input raster cell and the z-value of the output TIN at the location corresponding to the raster cell center.

##### z-value

The value for a given surface location that represents an attribute other than position. In an elevation or terrain model, the z-value represents elevation; in other kinds of surface models, it represents the density or quantity of a particular attribute.