GIS Glossary/H

GIS Glossary

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hachure
A short line on a map that indicates the direction and steepness of a slope. Hachures that represent steep slopes are short and close together; hachures that represent gentle slopes are longer, lighter, and farther apart. Contours, shading, and hypsometric tints have largely replaced hachuring on modern maps.

hachured contour
On a topographic map, concentric contour lines drawn with hachures to indicate a closed depression or basin. Concentric contour lines drawn without hachure marks indicate a hill.

halftone image
A continuous tone image photographed through a fine screen that converts it into uniformly spaced dots of varying size while maintaining the gradations of highlight and shadow. The size of the dots varies in proportion to the intensity of the light passing through them.

Hamiltonian circuit
A path through a network that visits each junction in the network only once and then returns to its point of origin. Hamiltonian circuits are named after the Irish mathematician, physicist, and astronomer William Rowan Hamilton.

Hamiltonian path
A path through a network that visits each junction in the network only once without returning to its point of origin. Hamiltonian paths are named after the Irish mathematician, physicist, and astronomer William Rowan Hamilton.

hardware key
A small hardware device that provides the unique number used in the generation of a license file. The key is plugged into either the parallel or USB port on the License Manager Server. UNIX users of ArcGIS software do not need a hardware key. Hardware keys are also called dongles.

HARN
Acronym for High Accuracy Reference Network. A regional or statewide resurvey and readjustment of NAD 1983 control points using GPS techniques. The resurvey date is often included as part of the datum name: NAD 1983 (1991) or NAD91.

hatch class
In linear referencing, a group or category of hatch definitions.

hatch definition
In linear referencing, a specification for where hatch marks are drawn on a line feature. Each hatch definition has its own set of properties that include the multiple of the hatch interval at which the hatches in the hatch definition will be placed, the line or marker symbol of the hatches, and whether the hatches will be labeled. The use of multiple hatch definitions allows for the design of complex hatching schemes.

hatch style
In linear referencing, an organized collection of symbols and settings for the hatch definitions that make up a hatch class. Hatch styles are stored in a style file (.style) and created by the user to maintain standards for displaying hatches on multiple maps with multiple data sources.

hatches
In linear referencing, a series of vertical line or marker symbols displayed on top of features at an interval specified in route measure units.

hatching
In linear referencing, a type of labeling that posts and labels hatches or symbols at a regular interval along measured line features.

HDOP
Acronym for horizontal dilution of precision. A measure of the geometric quality of a GPS satellite configuration in the sky. HDOP is a factor in determining the relative accuracy of a horizontal position. The smaller the DOP number, the better the geometry.

heading
The direction of a moving object, expressed as an angle from a known direction, usually north.

heads-up digitizing
Manual digitization by tracing a mouse over features displayed on a computer monitor, used as a method of vectorizing raster data.

heap
In computer programming, a variation on the binary tree data structure in which each node's value is greater than the value of its leaf nodes. Sorting data in a heap allows an element to be located more quickly than it could be found in an ordinary list.

hectare
A metric areal unit of measure equal to 10,000 square meters. One hectare is equal to 100 ares or 2.47 acres.

height
The vertical distance between two points, or above a specified datum.

Helmert transformation
A geometric transformation that scales, rotates, or translates images or coordinates between any two Euclidean spaces. It is commonly used in GIS to transform maps between coordinate systems. In a Helmert transformation, parallel lines remain parallel. The midpoint of a line segment remains a midpoint, and all points on a straight line remain on a straight line. The Helmert transformation is named for the German mathematician and geodesist Friedrich Robert Helmert (1843-1917).

Help Node
In ArcGIS 8.3 and previous versions, a branch in the ArcToolbox tree providing overview help for ArcToolbox toolsets.

hemisphere
Half of a celestial body, such as the earth.

heuristic
In computer science, an algorithm that incorporates a shortcut or simplification for solving a programming problem, such as searching. While a heuristic may run faster than a more rigorous algorithm, there is no guarantee that it will find the best solution.

hexadecimal
A number system using base 16 notation, usually comprised of the digits 0&#8211;9 and the letters A&#8211;F or a&#8211;f.

hierarchical database
A database that stores related information in a tree-like structure, where records can be traced to parent records, which in turn can be traced to a root record.

hierarchy
A type of network attribute for a network element in a network dataset. Hierarchy can be used during network analysis to assign priority to a network element. For example, in a transportation network dataset, a "road class" hierarchy can be assigned to edges to favor major roads instead of local streets.

High Accuracy Reference Network
A regional or statewide resurvey and readjustment of NAD 1983 control points using GPS techniques. The resurvey date is often included as part of the datum name: NAD 1983 (1991) or NAD91.

High Precision Geodetic Network
A regional or statewide resurvey and readjustment of NAD 1983 control points using GPS techniques. The resurvey date is often included as part of the datum name: NAD 1983 (1991) or NAD91.

high-level language
A programming language that uses keywords and statements that are similar to expressions in human language or mathematics and is, therefore, easier for people to comprehend and use. A high-level language is named for the high level of abstraction it affords developers over low-level processor functions such as memory access and register storage, meaning such operations do not demand the developer's attention.

high-pass filter
In digital image processing, a spatial filter that blocks low-frequency (long-wave) radiation, resulting in a sharpened image.

hillshading
Shadows drawn on a map to simulate the effect of the sun's rays over the varied terrain of the land.

histogram
A graph showing the distribution of values in a set of data. Individual values are displayed along a horizontal axis, and the frequency of their occurrence is displayed along a vertical axis.

histogram equalization
The redistribution of pixel values in an image so that each range contains approximately the same number of pixels. A histogram showing this distribution of values would be nearly flat.

historic parcel
In Survey Analyst - Cadastral Editor, a parcel that has been replaced with a new parcel by subdivision, merge, or other means due to a change in the survey record.

historical marker
In ArcMap, a user-created reference to a time and date stamp. Historical markers can be used to easily connect to a historical version for a specific date and time.

historical version
In ArcMap, a version that a user connects to by using a historical marker or indicating a particular date and time. Once connected, the version provides a read-only view of the geodatabase for the chosen time.

history model
A model created, dated, and saved when the application is closed to document the tools and parameter values used for each session. The history model is contained within the history toolbox and can be viewed when the application is reopened.

HKCR
HKEY_CLASSES_ROOT registry hive. A Windows registry root key that points to the HKEY_LOCAL_MACHINE\Software\Classes registry key. It displays essential information about OLE and association mappings to support drag-and-drop operations, Windows shortcuts, and core aspects of the Windows user interface.

hole
A small gap in a raster line feature, usually considered to be an error caused by the poor quality of a source document or by the scanning process.

honor
In network datasets, a type of junction connectivity policy in which junctions connect to other junctions based on the edge connectivity policy set by the user; junctions "honor" the existing edge connectivity policy.

horizon
The apparent or visible junction of land and sky.

horizon circle
The circle containing all points equidistant from the center of an azimuthal projection.

horizontal angle
The angle formed by the intersection of two lines in a horizontal plane.

horizontal control
A network of known horizontal geographic positions, referenced to geographic parallels and meridians or to other lines of orientation such as plane coordinate axes.

horizontal control datum
A geodetic datum for any extensive measurement system of positions, usually expressed as latitude-longitude coordinates, on the earth's surface. A horizontal geodetic datum may be local or geocentric. If it is local, it specifies the shape and size of an ellipsoid representing the earth, the location of an origin point on the ellipsoid surface, and the orientation of x- and y-axes relative to the ellipsoid. If it is geocentric, it specifies the shape and size of an ellipsoid, the location of an origin point at the intersection of x-,y-, and z-axes at the center of the ellipsoid, and the orientation of the x-,y-, and z-axes relative to the ellipsoid. Examples of local horizontal geodetic datums include the North American Datum of 1927, the European Datum of 1950, and the Indian datum of 1960; examples of geocentric horizontal geodetic datums include the North American Datum of 1983 and the World Geodetic System of 1984.

horizontal datum
A geodetic datum for any extensive measurement system of positions, usually expressed as latitude-longitude coordinates, on the earth's surface. A horizontal geodetic datum may be local or geocentric. If it is local, it specifies the shape and size of an ellipsoid representing the earth, the location of an origin point on the ellipsoid surface, and the orientation of x- and y-axes relative to the ellipsoid. If it is geocentric, it specifies the shape and size of an ellipsoid, the location of an origin point at the intersection of x-,y-, and z-axes at the center of the ellipsoid, and the orientation of the x-,y-, and z-axes relative to the ellipsoid. Examples of local horizontal geodetic datums include the North American Datum of 1927, the European Datum of 1950, and the Indian datum of 1960; examples of geocentric horizontal geodetic datums include the North American Datum of 1983 and the World Geodetic System of 1984.

horizontal dilution of precision
A measure of the geometric quality of a GPS satellite configuration in the sky. HDOP is a factor in determining the relative accuracy of a horizontal position. The smaller the DOP number, the better the geometry.

horizontal geodetic datum
A geodetic datum for any extensive measurement system of positions, usually expressed as latitude-longitude coordinates, on the earth's surface. A horizontal geodetic datum may be local or geocentric. If it is local, it specifies the shape and size of an ellipsoid representing the earth, the location of an origin point on the ellipsoid surface, and the orientation of x- and y-axes relative to the ellipsoid. If it is geocentric, it specifies the shape and size of an ellipsoid, the location of an origin point at the intersection of x-,y-, and z-axes at the center of the ellipsoid, and the orientation of the x-,y-, and z-axes relative to the ellipsoid. Examples of local horizontal geodetic datums include the North American Datum of 1927, the European Datum of 1950, and the Indian datum of 1960; examples of geocentric horizontal geodetic datums include the North American Datum of 1983 and the World Geodetic System of 1984.

host
In a computer network, the computer that contains data being accessed by other computers.

hot link
In ArcView 3.x, a tool for associating external files with a feature in a view. When a user clicks a feature in the view with the Hot Link tool, an image, text file, or ArcView document or project displays on screen.

HPGN
Acronym for High Accuracy Reference Network. A regional or statewide resurvey and readjustment of NAD 1983 control points using GPS techniques. The resurvey date is often included as part of the datum name: NAD 1983 (1991) or NAD91.

HRESULT
A 32-bit integer returned from any member of a COM interface indicating success or failure, often written in hexadecimal notation. An HRESULT can also give information about the error that occurred when calling a member of a COM interface. Visual Basic translates HRESULTS into errors; Visual C++ developers work directly with HRESULT values.

HTML
Acronym for Hypertext Markup Language. A markup language used to create Web pages for publication on the Internet. HTML is a system of tags that define the function of text, graphics, sound, and video within a document, and is now an Internet standard maintained by the World Wide Web Consortium.

HTML document
A computer file formatted with HTML tags so that it may be viewed in a Web browser and published on the World Wide Web. An HTML document may incorporate text, images, sound, video, and other media components. Characteristically, it also has embedded references, called hypertext links, to other HTML documents. These links enable a person viewing a document in a Web browser to open other documents&#8212;which may be stored on other computers anywhere in the world&#8212;by clicking on the link using a mouse.

HTML viewer
An ArcIMS viewer that uses a single ArcMap image or standard image service. The HTML viewer does not require a Java plug-in.

HTTP
Acronym for Hypertext Transfer Protocol. The protocol maintained by the World Wide Web Consortium for communicating between servers and clients to exchange HTML documents across the Internet.

HTTPS
Acronym for Hypertext Transfer Protocol (Secure). A variant of HTTP enhanced by a security mechanism. It allows transactions such as e-commerce and data sharing to take place on the World Wide Web in a protected way.

hub
A central node in a network for routing goods to their destinations.

hue
The dominant wavelength of a color, by which it can be distinguished as red, green, yellow, blue, and so forth.

human geography
The field of geography concerning a range of social, cultural, and political aspects of human life as related to their distribution through physical space.

hydrographic datum
A plane of reference for depths, depth contours, and elevations of foreshore and offshore features.

hydrographic survey
A survey of a water body, particularly of its currents, depth, submarine relief, and adjacent land.

hydrography
The measurement and description of water features and their related land areas for the purposes of safe marine navigation.

hydrologic cycle
The circulation of water from the earth through the atmosphere and back again. Its major stages are evaporation, condensation, precipitation, run-off, transpiration, infiltration, and percolation.

hydrology
The study of water, its behavior, and its movements across and below the surface of the earth, and through the atmosphere.

hyperlink
A reference (link) from one point in an electronic document to another document or another location in the same document (the target). Activating the link, usually by clicking it with the mouse, causes the browser to display the target of the link.

Hypertext Markup Language
A markup language used to create Web pages for publication on the Internet. HTML is a system of tags that define the function of text, graphics, sound, and video within a document, and is now an Internet standard maintained by the World Wide Web Consortium.

Hypertext Transfer Protocol
The protocol maintained by the World Wide Web Consortium for communicating between servers and clients to exchange HTML documents across the Internet.

hypsography
The study and representation of elevation and the earth's topography.

hypsometric curve
A curve showing the relationship of area to elevation for specified terrain. A hypsometric curve is plotted on a graph on which the x-axis represents surface area and the y-axis represents elevation above or below a datum (normally sea level). The curve shows how much area lies above and below marked elevation intervals.

hypsometric map
A map showing relief, whether by contours, hachures, shading, or tinting.

hypsometric tinting
Relief or depth depicted by a gradation of colors, usually between contour lines. Each color represents a different range of elevation.

hypsometry
The science that determines the spatial distribution of elevations above an established datum, usually sea level.