Basic Principles Coordinate Systems

All spatial data has a coordinate system. A coordinate system locates the data in either two or three-dimensional space. The data's coordinate system is very important if you want to combine data from different sources. Most spatial data are in a geographic or projected (planar) coordinate system. The model for coordinate reference systems described here is based upon the OpenGIS specification and the POSC Epicentre Model V 2.1, which share a common set of basic principles. Please refer to these references for the full description of the concepts and definitions for components of a coordinate reference system.

Applications that use maps as analytical tools need the coordinate system and its properties for all spatial data. ArcSDE uses a projection management system to store projection and coordinate information in a format that is accessible to applications. In addition to x, y planar coordinates, one can also store z coordinates or measures.

ArcSDE assumes planar data when performing calculations. It stores projected or geographic data but measurements and calculations on geographic coordinates may not be accurate. The ArcSDE Projection Engine does the actual creation and maintenance of the coordinate system. It supports the projected and geographic coordinate systems with the PE_PROJCS and PE_GEOGCS structures. A PE_COORDSYS superclass also exists to manipulate active PE_PROJCS or PE_GEOGCS objects.

A geographic coordinate system object contains the following information:

• Name
• Datum
• Angular unit of measure
• Prime meridian

A projected coordinate system contains the geographic coordinate system information and the following:

• Linear unit of measure
• Map projection
• Projection parameters

The ArcSDE Projection Engine has many predefined coordinate systems listed in pedef.h that are accessible to the ArcSDE software by integer code. If you want to define a new coordinate system, you must use the ArcSDE Projection Engine to create it. See the coordinate systems section for more information. Layers actually store a text representation of the coordinate system. Because ArcSDE data is sometimes stored internally and is often manipulated as integers, some additional information is required to define the data. The entire coordinate system information is stored as an opaque pointer to SE_COORDREF in the SE_SPATIALREF structure. The additional information converts between floating point world units and the integer system units.

Each spatial column (layer) has a spatial reference associated with it. Shapes inherit the spatial reference from their spatial column of origin. Shapes defined by an application have a coordinate reference object associated with them at the time they are created. A group of functions of the format SE_coordref_* work with the spatial reference system of a layer. You can retrieve or set the pieces of the SE_COORDREF structure.