In this topic
- About coordinate system concepts
- IDs, factory codes, and WKIDs
- Using coordinate systems in ArcGIS Explorer
About coordinate system concepts
Coordinate systems are mathematical models that describe how geometries are georeferenced to the surface of the earth. A coordinate system includes information such as the unit of measure and the earth model used. Depending on the type of coordinate system, it may also include how the data was projected from a spheroidal shape to a flat plane. A coordinate system also defines the range of valid X and Y properties of geometries using that system.
The following are the two types of coordinate systems used in ArcGIS Explorer:
- Geographic—Defines a spheroidal or spherical model of the earth, and uses angular units defining coordinates in terms of latitude and longitude, which are angles calculated from the center of the earth describing a position on its surface. Coordinates described this way are sometimes referred to as polar coordinates. Calculations based on a spheroidal model of the earth are sometimes known as geodesic or geodetic. The line on the earth chosen to represent zero degrees of longitude is known as the prime meridian.
- Projected—Uses linear units defining positions on the earth's surface as projected to a flat plane. A projection is a specific mathematical model for converting the spheroidal data to a flat plane. A projected coordinate system is always based on an underlying geographical coordinate system with a projection applied to it.
A shape can be converted to use another coordinate system by projecting it. Projecting data can be made more accurate by using specific geographic transformations—more mathematical models defining how coordinates are converted between two specific systems.
ArcGIS Explorer uses the ESRI Projection Engine to define coordinate system information. This supports many different geographical and projected coordinate systems, each defined for a specific use and having a unique identifier. For example, some systems are more accurate in specific areas of the world; others preserve area, distances, or headings. As there are many factors in defining a coordinate system, this has resulted in hundreds of different coordinate systems being defined. As each coordinate system is suited for a different geographical area and type of use, the choice of coordinate system for your data is an important one. A full discussion of these issues is outside of the scope of this topic; therefore, for more information, see What are map projections?
The Projection Engine uses definitions of coordinate systems, units of measurement, and geographic transformations, which are defined by the European Petroleum Survey Group (EPSG)—see the EPSG Web site for more information. For more information on the supported coordinate systems, see the ArcObjects API Library Reference on the Resource Center.
IDs, factory codes, and WKIDs
Unique number codes identify each specific coordinate system. You might see this number referred to in some ArcGIS documentation as a factory code, projection engine code, or a Well Known Identifier (WKID). Identifiers are also defined for each component of a coordinate system, that is, units of measurement, geographic transformation, projection model, prime meridian, and so on.
For example, some ArcGIS Server Web service application programming interfaces (APIs) have WKID parameters that allow you to specify a particular unit of measurement. These same numbers are applicable when developing for ArcGIS Desktop or ArcGIS Engine using ArcObjects where the numbers are defined in enumeration sets that begin with esriSR. For example, the enumerations, esriSRUnitType and esriSRUnit2Type, define identifiers for types of unit of measurement.
These identifier numbers are based on industry standards defined by EPSG and occasionally the code value for which an enumeration stands might change. For this reason, use the enumeration member rather than the numeric value in code. Changes will be noted in the change reports for this API. For more information about the defined units and coordinate systems, see the previously mentioned EPSG Web site.
Using coordinate systems in ArcGIS Explorer
The units of a Geometry can be specified in linear or angular units, depending on the coordinate system used by the shape.
The unique numbers identifying a coordinate system or unit of measurement can be used in ArcGIS Explorer to create instances of CoordinateSystem and Unit classes. For more information on the use of these classes, see the Geometry namespace overview.
Individual geometries can be projected from one coordinate system to another using the GeometryOperations.Project method. ArcGIS Explorer performs projection on-the-fly for each Layer, Note, and Graphic if required (if the Layer, Note, or Graphic has a different coordinate system to the MapDisplay in which it is displayed). As this requires additional calculations, the most efficient way to display your data is to ensure all data has the same coordinate system; however, on-the-fly projection allows you to display multiple disparate data sources in one map without needing to change original datasets.
Geographical transformations can also be specified for projection operations to increase accuracy. For more information, see the GeographicTransformation class and the previously mentioned Geometry namespace overview.
See Also:
CoordinateSystem classCoordinateSystem.Id property
Unit class
Unit.Id property
Geometry namespace overview
GeographicTransformation
What are map projections?