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An overview of map projections

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Last modified November 9, 2006
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Within ArcGIS, every dataset has a coordinate system, which is used to integrate it with other geographic data layers within a common coordinate framework such as a map. Coordinate systems enable you to integrate datasets within maps as well as to perform various integrated analytical operations such as overlaying data layers from disparate sources and coordinate systems.

Layers can be spatially and analytically integrated by ArcGIS if their coordinate systems are known.

What is a coordinate system?

Coordinate systems enable geographic datasets to use common locations for integration. A coordinate system is a reference system used to represent the locations of geographic features, imagery, and observations such as GPS locations within a common geographic framework.

Each coordinate system is defined by:

Types of coordinate systems

There are two common types of coordinate systems used in GIS:

For a conceptual overview, see Georeferencing and coordinate systems.

Coordinate systems (either geographic or projected) provide a framework for defining real-world locations. In ArcGIS, the coordinate system is used as the method to automatically integrate the geographic locations from different datasets into a common coordinate framework for display and analysis.

ArcGIS automatically integrates datasets whose coordinate systems are known

All geographic datasets used in ArcGIS are assumed to have a well-defined coordinate system that enables them to be located in relation to the earth's surface.

If your datasets have a well-defined coordinate system, then ArcGIS can automatically integrate your datasets with others by projecting your data on the fly into the appropriate framework—for mapping, 3D visualization, analysis, and so forth.

If your datasets do not have a spatial reference, they cannot be easily integrated. You need to define one before you can use your data effectively in ArcGIS.

What is a spatial reference in ArcGIS?

A spatial reference in ArcGIS is a series of parameters that define the coordinate system and other spatial properties for each dataset in the geodatabase. It is typical that all datasets for the same area (and in the same geodatabase) use a common spatial reference definition.

An ArcGIS spatial reference includes settings for:

Learning more about coordinate systems

Here is a series of links to help you learn more about applying map projections and coordinate systems in your work.

Learning more about map projection and coordinate system concepts
Concept Where to go for more information
To understand geographic coordinate systems and latitude-longitude See About geographic coordinate systems
To understand projected coordinate systems See About projected coordinate systems
To learn which map projections are supported See List of supported map projections
To learn about datums See Datums
To learn about spheroids and spheres See Spheroids and spheres
To choose a map projection See Choosing a map projection
or About coordinate systems and map projections
To learn about the geodatabase spatial reference.

See the Geodatabase Spatial Reference.

Common coordinate system and map projection tasks in ArcGIS

Here is a series of links to guidance on how to perform a number of common coordinate system tasks in ArcGIS.

Defining the coordinate systems, re-projecting, and transforming datasets
Common task Where to go for more information
To define the spatial reference for a new dataset in the geodatabase See An overview of spatial references in the geodatabase
To record the coordinate system of an existing dataset See the "Define Projection" tool in An overview of the Projections and Transformations toolset
To define the coordinate system for external raster and image files See Defining a raster's coordinate system
To project feature, rasters, and image data layers See An overview of the Projections and Transformations toolset
To identify an unknown coordinate system See Identifying an unknown coordinate system

Datum transformation and rubber-sheeting
Common task Where to go for more information
To learn transformation concepts See Geographic transformation methods
To transform and rubber-sheet data layers See Performing spatial adjustment
To geo-reference unregistered raster data See Changing the geographic coordinates of a raster dataset: georeferencing
To geo-reference unregistered CAD data See Transforming CAD datasets
To perform least-squares adjustment using the Cadastral Editor (which is part of the Survey Analyst Extension) See About least squares adjustment

Working with Vertical Coordinate Systems
Common task Where to go for more information
To learn vertical coordinate system concepts See About vertical coordinate systems
To define a vertical coordinate system for a feature class See Defining feature class properties

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