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What are map projections?

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What are map projections? ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

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.

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: 

Its measurement framework which is either geographic (in which spherical coordinates are measured from the earth's center) or planimetric (in which the earth's coordinates are projected onto a two-dimensional planar surface).

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Unit of measurement (typically feet or meters for projected coordinate systems or decimal degrees for latitude–longitude).

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The definition of the map projection for projected coordinate systems.

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Other measurement system properties such as a spheroid of reference, a datum, and projection parameters like one or more standard parallels, a central meridian, and possible shifts in the x- and y-directions.

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

A global or spherical coordinate system such as latitude–longitude. These are often referred to

What are map projections?

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as geographic coordinate systems. 

A projected coordinate system based on a map projection such as transverse Mercator, Albers equal area, or Robinson, all of which (along with numerous other map projection models) provide various mechanisms to project maps of the earth's spherical surface onto a twodimensional Cartesian coordinate plane. Projected coordinate systems are sometimes referred to as map projections.

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. The spatial reference or coordinate system is metadata. It describes the coordinate framework that the data is already using. Caution: When you define the coordinate system for a dataset using the Define Projection tool or the dataset property page, you are updating the metadata to identify the current coordinate system. The dataset's extent and coordinate values will not change. The dataset must already be using the coordinate system. To change a dataset's coordinate system, including its extent and values, use the Project or Project Raster tools.

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: 

The coordinate system

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The coordinate precision with which coordinates are stored (often referred to as the coordinate resolution)

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Processing tolerances (such as the cluster tolerance)

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The spatial or map extent covered by the dataset (often referred to as the spatial domain)

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

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Concept

Where to go for more information

To understand geographic coordinate systems and latitude–longitude

See What are geographic coordinate systems?

To understand projected coordinate systems

See What are 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

To learn about the geodatabase spatial reference.

See the Geodatabase Spatial Reference.

Map projection and coordinate system tasks

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 or modifying 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

Coordinate system definition and projection

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 georeference unregistered raster data

See Changing the geographic coordinates of a raster dataset: georeferencing

To georeference unregistered CAD data

See Georeferencing a CAD dataset

Data transformation tasks

Working with Vertical Coordinate Systems Common task

Where to go for more information

To learn vertical coordinate system concepts

See What are vertical coordinate systems?

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See Defining_feature_class_properties

To define a vertical coordinate system for a feature class Working with vertical coordinate systems

What existing coordinate systems and transformations are available? Object type

Where to go for more information

Geographic or vertical coordinate systems

See geographic_coordinate_systems.pdf

Projected coordinate systems

See projected_coordinate_systems.pdf

Geographic (datum) transformations

See geographic_transformations.pdf

What existing coordinate systems and transformations are available?

Related Topics About map projections Geographic transformation methods What are geographic coordinate systems? What are projected coordinate systems?

Copyright © 1995-2010 ESRI, Inc. All rights reserved.

Projection basics for GIS professionals Coordinate systems, also known as map projections, are arbitrary designations for spatial data. Their purpose is to provide a common basis for communication about a particular place or area on the earth's surface. The most critical issue in dealing with coordinate systems is knowing what the projection is and having the correct coordinate system information associated with a dataset. There are two types of coordinate systems—geographic and projected. A geographic coordinate system uses a three-dimensional spherical surface to define locations on the earth. It includes an angular unit of measure, a prime meridian, and a datum (based on a spheroid). In a geographic coordinate system, a point is referenced by its longitude and latitude values. Longitude and latitude are angles measured from the earth's center to a point on the earth's surface. The angles often are measured in degrees (or in grads). Learn more about geographic coordinate systems. A projected coordinate system is defined on a flat, two-dimensional surface. Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions. A projected coordinate system is always based on a geographic coordinate system that is based on a sphere or spheroid. In a projected coordinate system, locations are identified by x,y coordinates on a grid, with the origin at the center of the grid. Each position has two values that reference it to that central location. One specifies its horizontal position and the other its vertical position. Learn more about projected coordinate systems. When the first map projections were devised, it was assumed, incorrectly, that the earth was flat. Later the assumption was revised, and the earth was assumed to be a perfect sphere. In the 18th century, people began to realize that the earth was not perfectly round. This was the beginning of the concept of the cartographic spheroid.

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What are map projections?

To more accurately represent locations on the earth's surface, mapmakers studied the shape of the earth (geodesy) and created the concept of the spheroid. A datum links a spheroid to a particular portion of the earth's surface. Recent datums are designed to fit the entire earth's surface well. These are the most commonly used datums in North America: 

North American Datum (NAD) 1927 using the Clarke 1866 spheroid

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NAD 1983 using the Geodetic Reference System (GRS) 1980 spheroid

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World Geodetic System (WGS) 1984 using the WGS 1984 spheroid

Newer spheroids are developed from satellite measurements and are more accurate than those developed in the 19th and early 20th centuries. You will find that the terms "geographic coordinate system" and "datum" are used interchangeably. Learn more about datums. The coordinates for a location will change depending on the datum and spheroid on which those coordinates are based, even if using the same map projection and projection parameters. For example, the geographic coordinates below are for the city of Bellingham, Washington, using three different datums: Datum

Latitude

Longitude

NAD 1927

48.7440490722656

-122.466903686523

NAD 1983

48.7438798543649

-122.46818353793

WGS 1984

48.7438798534299

-122.46818353793

A principle of good data management is to obtain the coordinate system information from the data source providing the data. Do not guess about the coordinate system of data because this will result in an inaccurate GIS database. The necessary parameters are the following:

Geographic coordinate system (Datum) Unit of measure Zone (for UTM or State Plane) Projection Projection parameters

Projection parameters may be required, depending on the map projection. For example, the Albers and Lambert conic projections require the following parameters:

1st standard parallel 2nd standard parallel Central meridian Latitude of origin False easting False northing Unit of measure

You can define a coordinate system for data with the Define Projection tool in the Data Management toolbox. If the data has a coordinate system definition, but it does not match the typical coordinate system

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used by an organization, you can reproject the data. You can reproject data in a geodatabase feature dataset, feature class, shapefile, or raster dataset using the Project tool or Project Raster tool in the Data Management toolbox.

Related Topics Identifying an unknown coordinate system The geoid, ellipsoid, spheroid and datum What are map projections?

Copyright © 1995-2010 ESRI, Inc. All rights reserved.

The geoid, ellipsoid, spheroid and datum, and how they are related The geoid is defined as the surface of the earth's gravity field, which is approximately the same as mean sea level. It is perpendicular to the direction of gravity pull. Since the mass of the earth is not uniform at all points, and the direction of gravity changes, the shape of the geoid is irregular. Click on the link below to access a website maintained by the National Oceanographic & Atmospheric Administration (NOAA). The website has links to images showing interpretations of the geoid under North America. http://www.ngs.noaa.gov/GEOID/GEOID96/geo-indx.html To simplify the model, various spheroids or ellipsoids have been devised. These terms are used interchangeably. For the remainder of this article, the term spheroid will be used. A spheroid is a three-dimensional shape created from a two-dimensional ellipse. The ellipse is an oval, with a major axis (the longer axis), and a minor axis (the shorter axis). If you rotate the ellipse, the shape of the rotated figure is the spheroid. The semimajor axis is half the length of the major axis. The semiminor axis is half the length of the minor axis. For the earth, the semimajor axis is the radius from the center of the earth to the equator, while the semiminor axis is the radius from the center of the earth to the pole. One particular spheroid is distinguished from another by the lengths of the semimajor and semiminor axes. For example, compare the Clarke 1866 spheroid with the GRS 1980 and the WGS 1984 spheroids, based on the measurements (in meters) below. Spheroid

Semimajor axis (m)

Semiminor axis (m)

Clarke 1866

6378206.4

6356583.8

GRS80 1980

6378137

6356752.31414

WGS84 1984

6378137

6356752.31424518

Spheroid comparison

A particular spheroid can be selected for use in a specific geographic area, because that particular spheroid does an exceptionally good job of mimicking the geoid for that part of the world. For North America, the spheroid of choice is GRS 1980, on which the North American Datum 1983 (NAD83) is based. A datum is built on top of the selected spheroid, and can incorporate local variations in elevation. With the spheroid, the rotation of the ellipse creates a totally smooth surface across the world. Because this doesn't reflect reality very well, a local datum can incorporate local variations in elevation.

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The underlying datum and spheroid to which coordinates for a dataset are referenced can change the coordinate values. An illustrative example using the city of Bellingham, Washington follows. Compare the coordinates in decimal degrees for Bellingham using NAD27, NAD83 and WGS84. It is apparent that while NAD83 and WGS84 express coordinates that are nearly identical, NAD27 is quite different, because the underlying shape of the earth is expressed differently by the datums and spheroids used. Datum

Longitude

Latitude

NAD 1927

-122.46690368652

48.7440490722656

NAD 1983

-122.46818353793

48.7438798543649

WGS 1984

-122.46818353793

48.7438798534299

Geographic coordinates below are for the city of Bellingham, Washington using 3 different datums

The longitude is the measurement of the angle from the prime meridian at Greenwich, England, to the center of the earth, then west to the longitude of Bellingham, Washington. The latitude is the measurement of the angle formed from the equator to the center of the earth, then north to the latitude of Bellingham, Washington. If the surface of the earth at Bellingham is bulged out, the angular measurements in decimal degrees from Greenwich and the equator will become slightly larger. If the surface at Bellingham is lowered, the angles will become slightly smaller. These are two examples of how the coordinates change based on the datum.

Related Topics Datums Projection basics for GIS professionals Spheroids and spheres

Copyright © 1995-2010 ESRI, Inc. All rights reserved.

Identifying an unknown coordinate system Coordinate system information is usually obtained from the data source, but not always, as with legacy data. The technique described below helps identify the correct coordinate system. If the coordinate system is unknown, you will receive this warning message when trying to add the layer to ArcMap: The following data sources you added are missing spatial reference information. This data can be drawn in ArcMap, but cannot be projected. The term coordinate system can refer to data expressed in decimal degrees, or a projected coordinate system expressed in meters or feet. The term projection, or PRJ, is an older term that is also used, but it is not as precise. If a data source has a defined coordinate system, ArcMap can project it on the fly to a different coordinate system. If data does not have a defined coordinate system, ArcMap cannot project it on the fly. ArcMap simply will draw it. If you change the data frame's coordinate system, all layers that have coordinate systems will be projected on the fly to the new coordinate system. If you set the data frame's coordinate system and the data with a known coordinate system lines up with the unknown data, the data frame's coordinate system is that of the unknown data's. Steps: 1.

Start ArcMap with a new empty map and add the data with the unknown coordinate

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What are map projections?

system. The data must not have a defined coordin...