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Glossary of terms


A scheme of approximately rectangular tiles covering eastern Pima County developed by Pima County Public Works and currently used only by Regional Wastewater Reclamation and Development Services. Basemap numbers start at 1 in downtown Tucson and are incremented in a clockwise spiral manner with the largest number being 3343. Generally, the larger the number, the further from basemap 1. There are significant gaps in numbering as there are approximately 900 basemaps. There is no direct correlation between basemap and PLSS sections.

Central GIS Database

The collection of major GIS datasets produced by Pima County and the City of Tucson, and residing on a single shared server.


A digital version of a map forming the basic unit of vector data storage in ARC/INFO. A coverage stores geographic features as primary features (such as arcs, nodes, polygons, and label points) and secondary features (such as tics, map extent, links, and annotation). Associated feature attribute tables describe and store attributes of the geographic features.


Generally refers to a spatial data file, or collection of files, that represents a specific geographic theme or layer. Datasets also include stand-alone database tables.


Geographic Information System. An organized collection of computer hardware, software, geographic data, and personnel designed to efficiently capture, store, update, manipulate, analyze, and display all forms of geographically referenced information. GIS technology integrates common database operations such as query and statistical analysis with the unique visualization and geographic analysis benefits offered by maps. These abilities distinguish GIS from other information systems and make it valuable to a wide range of public and private enterprises for explaining events, predicting outcomes, and planning strategies. More information.


Global Positioning System. A satellite based navigational system allowing the determination of any point on the earth's surface with a high degree of accuracy given a suitable GPS receiver. The network of satellites is owned by the US Department of Defence, and as such, the accuracy of the signal is intentionally degraded for non-US military users. The error introduced into the signal is known as selective availability. Error in the accuracy of GPS derived positions can also be introduced through the nature of local conditions, for example multipath. These errors can be greatly reduced using a technique known as differential GPS.


North American Datum of 1927. The North American Datum of 1927 uses the Clarke spheroid of 1866 to represent the shape of the Earth. The origin of this datum is a point on the Earth referred to as Meades Ranch in Kansas. Many NAD27 control points were calculated from observations taken in the 1800s. These calculations were done manually and in sections over many years. Therefore, errors varied from station to station.

Many technological advances in surveying and geodesy since the establishment of NAD27 - electronic theodolites, GPS satellites, Very Long Baseline Interferometry, and Doppler systems - revealed weaknesses in the existing network of control points. Differences became particularly noticeable when linking existing control with newly established surveys. The establishment of a new datum would allow for a single datum to cover consistently North America and surrounding areas.


North American Datum of 1983. The North American Datum of 1983 is based upon both Earth and satellite observations, using the GRS80 spheroid. The origin for this datum is the Earth's center of mass. This affects the surface location of all latitude- longitude values enough to cause locations of previous control points in North America to shift, sometimes as much as 500 feet. A ten-year multinational effort tied together a network of control points for the United States, Canada, Mexico, Greenland, Central America, and the Caribbean.


Public Land Survey System. All lands in the public domain are subject to subdivision by a rectangular system of surveys called the Public Land Survey System (PLSS), established and regulated by the Bureau of Land Management. The original public domain includes the land ceded to the Federal Government by the Thirteen Original States, supplemented with acquisitions from native Indians and foreign powers. It encompasses major portions of the land area of 30 western States.

Under Congressional mandate, cadastral surveys of public lands were undertaken to create parcels suitable for disposal by the Government. The PLSS was developed for this purpose. The PLSS is a rectangular survey system that typically divides the land into 6-mile square townships, which are further subdivided into 1-mile square sections. The extension of the rectangular system of surveys over the public domain has been in progress since 1785. These surveys form the basis of patents issued when public lands pass out of Federal ownership.

Certain lands were excluded from the public domain and not subject to survey and disposal. These lands include the beds of navigable bodies of water, national installations such as military reservations and national parks, and areas such as land grants that had already passed to private ownership prior to subdivision by the Government. More information.

Relational Database Management System (RDBMS)

A database system that directly accesses and manages data organized in a relational fashion.

Spatial Database Engine (SDE)

A high- performance, object-based spatial data access and processing engine implemented in several commercial database management systems using open standards and true client/server architecture.

State Plane

The State Plane Coordinate System (SPCS) is a coordinate system designed for mapping the United States. It was developed in the 1930s by the U.S. Coast and Geodetic Survey to provide a common reference system to surveyors and mappers. The goal was to design a conformal mapping system for the country with a maximum scale distortion of 1 part in 10,000, then considered the limit of surveying accuracy.

Three conformal projections were chosen: the Lambert Conformal Conic for states that are longer in the east-west direction, such as Tennessee and Kentucky, the Transverse Mercator projection for states that are longer in the north-south direction, such as Illinois and Vermont, and the Oblique Mercator projection for the panhandle of Alaska, because it is neither predominantly north nor south, but at an angle.

To maintain an accuracy of 1 part in 10,000, it was necessary to divide many states into zones. Each zone has its own central meridian or standard parallels to maintain the desired level of accuracy. The boundaries of these zones follow county boundaries. Smaller states such as Connecticut require only one zone, whereas Alaska is composed of ten zones and uses all three projections.

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Geographic Information Systems

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Tucson, AZ 85701

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