API, or application programming interface (API) is an interface implemented by a software program which enables it to interact with other software. It facilitates interaction between different software programs similar to the way the user interface facilitates interaction between humans and computers.

Attribute data is descriptive information about features or elements of a database. For a database feature like census tract, attributes might include many demographic facts including total population, average income, and age. In statistical parlance, an attribute is a “variable,” whereas the database feature represents an “observation” of the variable.

Base Maps provide the background upon which thematic data is overlayed and analyzed. As inputs into a GIS, the term base map is usually applied to those sources of information about relatively permanent, sometimes timeless, features including topography, pedology, geology, cadastral divisions, and political divisions. Within a GIS database, such information may become part of a land base to which other information is indexed and referenced.

Cadastral data is the means by which private and public land is defined, divided, traced, and recorded. The term derives from the French cadastre, a register of the survey of lands and is, in effect, the public record of the extent, value, and ownership of land for purposes of taxation.

Centroid is the term given to the center of an area, region, or polygon. In the case of irregularly shaped polygons, the centroid is derived mathematically and is weighted to approximate a sort of “center of gravity.” Centroids are important in GIS because these discrete X-Y locations are often used to index or reference the polygon within which they are located. Sometimes attribute information is “attached,” “hung,” or “hooked” to the centroid location.

GeoJSON is an open format for encoding a variety of geographic data structures. It is so named because it is based on JSON (JavaScript Object Notation). In fact, every GeoJSON data structure is also a JSON object, and thus JSON tools can also be used for processing GeoJSON data. GeoJSON allows geographic data to be stored in a human-readable way that is generally more compact than XML. Spatial data format types supported in GeoJSON include points, polygons, multipolygons, features, geometry collections, and bounding boxes, which are stored along with feature information and attributes. The geometries and their properties will have a parent object, such as a feature collection.

GIS is the abbreviation for geographic information system. GIS are special-purpose digital databases in which a common spatial coordinate system is the primary means of reference. GIS contain subsystems for: 1) data input; 2) data storage, retrieval, and representation; 3) data management, transformation, and analysis; and 4) data reporting and product generation. It is useful to view GIS as a process rather than a thing. A GIS supports data collection, analysis, and decision making and is far more than a software or hardware product. Other terms for GIS, and special-purpose GIS, include: Land-Base Information System, Land Record System, Land Information System, Land Management System, Multipurpose Cadastre, and AM/FM System.

GPS is an acronym for global positioning system. Developed for the military for navigation and surveying, the GPS relies on satellites (and ground stations) for precise determination of location. Although GPS can be used to determine location very precisely (within centimeters given the correct controls and proper use), it does not solve all the problems of locational determination in GIS databases.

JSON (an acronym for JavaScript Object Notation) is a lightweight text-based open standard designed for human-readable data interchange. It is derived from the JavaScript programming language for representing simple data structures and associative arrays, called objects. Despite its relationship to JavaScript, it is language-independent, with parsers available for virtually every programming language.

KML or Keyhole Markup Language (KML) is an XML-based language schema for expressing geographic annotation and visualization on existing or future Internet-based, two-dimensional maps and three-dimensional Earth browsers. KML was developed for use with Google Earth, which was originally named Keyhole Earth Viewer. It was created by Keyhole, Inc, which was acquired by Google in 2004. The name “Keyhole” is an homage to the KH reconnaissance satellites, the original eye-in-the-sky military reconnaissance system first launched in 1976. KML is an international standard of the Open Geospatial Consortium. Google Earth was the first program able to view and graphically edit KML files. Other projects such as Marble have also started to develop KML support.

Landsat is a system of satellites which scan the earth at a variety of wavelengths. The satellites return information that can be used to inventory and analyze a variety of natural and human resources.

Map Projection is any method of representing the surface of a sphere or other shape on a plane. Map projections are necessary for creating maps. All map projections distort the surface in some fashion. Depending on the purpose of the map, some distortions are acceptable and others are not; therefore different map projections exist in order to preserve some properties of the sphere-like body at the expense of other properties.

Map Scale is the relationship between distance on a map and the corresponding distance on the earth’s surface. Map scale is often recorded as a representative fraction such as 1:1,000,000 (1 unit on the map represents a million units on the earth’s surface) or 1:24,000 (1 unit on the map represents 24,000 units on the earth’s surface). The terms “large” and “small” refer to the relative magnitude of the representative fraction. Since 1/1,000,000 is a smaller fraction than 1/24,000, the former is said to be a smaller scale. Small scales are often used to map large areas because each map unit covers a larger earth distance. Large-scale maps are employed for detailed maps of smaller areas.

Parcel is a fundamental cadastral unit: a piece of land which can be owned, sold, and developed. Parcels have legal descriptions which not only describe their boundaries but also contain information concerning rights and interests.

Photogrammetry uses aerial photographs to produce planimetric and topographic maps of the earth’s surface and of features of the built environment. Effective photogrammetry makes use of ground control by which aerial photographs are carefully compared and registered to the locations and characteristics of features identified in ground-level surveys.

Raster displays and databases build all geographic features from grid cells in a matrix. A raster display builds an image from pixels, pels, or elements of coarse or fine resolution. A raster database maintains a similar “picture” of reality in which each cell records some sort of information averaged over the cell’s area. The size of the cell may again be coarse or fine, ranging from centimeters to kilometers. Many satellites, like Landsat and SPOT, transmit raster images of the earth’s surface. Reflectance at a certain wavelength is measured for each cell in an image. The cells may cover areas on the earth’s surface several hundreds of meters square, the area covered being a function of a particular satellite’s resolution.

Vector displays and databases build all geographic features from points, that is from discrete X-Y locations. Lines are constructed from strings of points, and polygons (regions) are built from lines which close. Vector methods are sometimes contrasted with raster techniques which record geographic features within a matrix of grid cells. The choice between vector and raster GIS has much to do with the application being considered since both methods have strengths and weaknesses. Many current GIS permit transformation between vector and raster input and output.

WMS, or a A Web Map Service (WMS) is a standard protocol for serving georeferenced map images over the Internet that are generated by a map server using data from a GIS database. The specification was developed and first published by the Open Geospatial Consortium in 1999.