The ST_Geometry storage type
Release 9.2 Last modified April 3, 2008	Print all topics in : "Feature geometry and raster data storage"

ArcSDE geodatabase storage for Oracle, DB2, and Informix uses a SQL data type—ST_Geometry—which provides full geodatabase support and access as well as SQL access to feature class geometry. This enables you to write SQL applications using your DBMS that can access and use feature operations and queries. Each DBMS is supported as follows:

DBMS	Description
Oracle	The ST_Geometry data type is one of the ArcSDE options for storing feature geometry. It is included with ArcSDE for Oracle and is one of the feature storage options you have for Oracle. It is a high performance storage type provided as part of ArcSDE for Oracle that includes ISO and OGC compliant SQL access to geodatabase features.
IBM DB2	The DB2 Spatial Extender provided by IBM as part of the DB2 product is the only option used to store feature geometry in DB2 geodatabases. It was co-developed by ESRI and IBM, is a high performance storage type that provides ISO and OGC compliant SQL access to geodatabase features, and uses the ST_Geometry data type.
IBM Informix	The Informix Spatial DataBlade provided by IBM as part of the Informix product is the only option used to store feature geometry in Informix geodatabases. It was co-developed by ESRI and IBM, is a high performance storage type that provides ISO and OGC compliant SQL access to geodatabase features, and uses the ST_Geometry data type.

This topic describes how geometry has been implemented within ST_Geometry—as a User-defined Data Type (UDT). UDTs are data structures and functions that are defined based on the operations that can be performed on the data, not in terms of how you implement them. The IBM DB2 Spatial Extender and IBM Informix Spatial Datablade use the ST_Geometry data type, and the ST_Geometry data type is supported in ArcSDE for Oracle geodatabases.

As a data type, ST_Geometry allows you to define columns that store spatial data. The ST_Geometry data type itself is an abstract, non-instantiated superclass, the subclasses (or subtypes) of which may be instantiated. An instantiated data type is one that can be defined as a table column and have values of its type inserted into it. A column can be defined as type ST_Geometry, but ST_Geometry values cannot be inserted into it since they cannot be instantiated. Therefore, only the subtype values can be inserted into this column. The ST_Geometry data type can accept and store any of its subtypes, while its subtypes (subclass data types) can only accept their own values.

The following chart demonstrates the hierarchy of the ST_Geometry data type and its subclasses (or subtypes). ST_Curve, ST_Surface, ST_MultiCurve, and ST_MultiSurface are defined to be noninstantiated types. No constructors are defined for these types.

Subclasses

The ST_Geometry data type is not instantiable but instead must store its instantiable subclasses. The subclasses are divided into two categories: the base geometry subclasses and the homogeneous collection subclasses. The base geometries include ST_Point, ST_LineString, and ST_Polygon, while the homogeneous collections include ST_MultiPoint, ST_MultiLineString, and ST_MultiPolygon. As the names imply, the homogeneous collections are collections of base geometries. In addition to sharing base geometry properties, homogeneous collections have some of their own properties as well.

To discover the subtype of an ST_Geometry, you would use the ST_GeometryType function. The ST_GeometryType function takes an ST_Geometry and returns the instantiated subtype in the form of a character string. To find out how many base geometry elements are contained in a homogenous collection, you would use the ST_NumGeometries function, which takes a homogeneous collection and returns the number of base geometry elements it contains. The ST_GeometryN function takes a homogeneous collection and an index and returns the nth base geometry.

Each subtype stores the type of geometry implied by its name; for instance, ST_MultiPoint stores multipoints. Each subtype has particular functions that can return information about the subtype. A summary of the subtypes, their descriptions, and example functions that can be used to get information about the subtypes are listed in the table below.

Subtype	Description	Functions used with the subtype
ST_Point	A zero-dimensional geometry that occupies a single location in coordinate space An ST_Point has a single x,y coordinate value, is always simple, and has a NULL boundary. -Used to define features such as oil wells, landmarks, and elevations	ST_X—Returns a point's x-coordinate value as a double-precision number -ST_Y—Returns a point's y-coordinate value as a double-precision number -ST_Z—Returns a point's z-coordinate value as a double-precision number -ST_M—Returns a point's m-coordinate value as a double-precision number
ST_LineString	A one-dimensional object stored as a sequence of points defining a linear interpolated path ST_LineStrings have length. The ST_LineString is simple if it does not intersect its interior. The endpoints (the boundary) of a closed ST_LineString occupy the same point in space. An ST_LineString is a ring if it is both closed and simple. The endpoints normally form the boundary of an ST_LineString unless the ST_LineString is closed, in which case the boundary is NULL. The interior of an ST_LineString is the connected path that lies between the endpoints, unless it is closed, in which case the interior is continuous. ST_LineStrings are often used to define linear features such as roads, rivers, and power lines.	ST_StartPoint—Returns the first point of a linestring ST_EndPoint—Returns the last point of a linestring ST_PointN—Takes a linestring and an index to an nth point and returns that point ST_Length—Returns a linestring's length as a double precision number ST_NumPoints—Returns the number of points in a linestrings sequence as an integer ST_IsRing—Returns 1 (TRUE) if a linestring is a ring or 0 (FALSE) if it is not ST_IsClosed—Returns 1 (TRUE) if a linestring is closed or 0 (FALSE) if it is not
ST_Polygon	A two-dimensional surface stored as a sequence of points defining its exterior bounding ring and 0 or more interior rings ST_Polygon has area and is always simple The exterior and any interior rings define the boundary of an ST_Polygon, and the space enclosed between the rings defines the ST_Polygon's interior. The rings of an ST_Polygon can intersect at a tangent point but never cross. Defines parcels of land, water bodies, and other features having spatial extent	ST_Area—Returns a polygon's area as a double-precision number ST_ExteriorRing—Returns a polygon's exterior ring as a linestring ST_NumInteriorRing—Returns the number of interior rings a polygon contains ST_InteriorRingN—Takes a polygon and an index and returns the nth interior ring as a linestring ST_Centroid—Returns a point that is the center of the polygon's envelope ST_PointOnSurface—Returns a point that is guaranteed to be on the surface of the polygon
ST_MultiPoint	A collection of ST_Points Has a dimension of 0 An ST_MultiPoint is simple if none of its elements occupy the same coordinate space. The boundary of an ST_MultiPoint is NULL. -Define such things as aerial broadcast patterns and incidents of a disease outbreak
ST_MultiLineString	A collection of ST_LineStrings ST_MultiLineStrings have length. ST_MultiLineStrings are simple if they only intersect at the endpoints of the ST_LineString elements. ST_MultiLineStrings are nonsimple if the interiors of the ST_LineString elements intersect. The boundary of an ST_MultiLineString is the nonintersected endpoints of the ST_LineString elements. The ST_MultiLineString is closed if all its ST_LineString elements are closed. The boundary of an ST_MultiLineString is NULL if all the endpoints of all the elements are intersected. Used to define such entities as streams or road networks	ST_Length—Returns the cumulative length of all the ST_LineString elements of a multilinestring as a double-precision number ST_IsClosed—Returns 1 (TRUE) if the multilinestring is closed and 0 (FALSE) if it is not
ST_MultiPolygon	A collection of polygons ST_MultiPolygons have area. The boundary of an ST_MultiPolygon is the cumulative length of its elements' exterior and interior rings. The interior of an ST_MultiPolygon is defined as the cumulative interiors of its element ST_Polygons. The boundary of an ST_MultiPolygon's elements can only intersect at a tangent point. Define features such as a forest stratum or a noncontiguous parcel of land such as a Pacific island chain.	ST_Area—Returns the cumulative ST_Area of a multipolygon's polygon elements as a double-precision number ST_Centroid—Returns a point that is the center of a multipolygon's envelope ST_PointOnSurface—Returns a point that is guaranteed to be normal to the surface of one of the multipolygon's polygon elements

Throughout the remainder of this topic, the term geometry or geometries collectively refers to the superclass ST_Geometry data type and all of its subclass data types. Whenever it is necessary to specify the geometry superclass directly, it will be referred to as the ST_Geometry superclass or the ST_Geometry data type.

The following sections elaborate on the properties of the geometry, the example functions that can be used to discover these properties, and how to use it with ArcGIS.

Geometry properties

There are many properties that characterize geometry. Some of these properties, along with the functions you can use to determine a geometry's properties, are listed below.

Note that each subtype inherits the properties of the ST_Geometry superclass but also has properties of its own. Functions that operate on the ST_Geometry data type will accept any of the subclass data types (subtypes). However, some functions have been defined at the subtype level and will only accept certain subtypes. For example, the ST_GeometryN function only takes ST_Multi(-linestring,-point,-polygon) subtype values as input.

You can use the following links to navigate to different geometry properties:

• Spatial reference system
  
• Dimensionality
  
• Interior, exterior, or boundary
  
• Simple or nonsimple
  
• Empty or not empty
  
• Envelope
  
• Closed/Ring
  
• The number of simple geometries in a multipart geometry
  
• Number of points
  
• Length and area
  
• Z-coordinates
  
• Measures
  

• Spatial reference system
  
The spatial reference system identifies the coordinate transformation matrix for each geometry. All spatial reference systems known to the database are stored in the spatial_references table.

The ST_SRID function takes an ST_Geometry and returns its spatial reference identifier as an integer.



NOTE: Each spatial reference system has its own spatial reference ID.



You can find more information on spatial references in the topic The properties of a spatial reference. When using an IBM DBMS, see the IBM DB2 Spatial Extender and geodetic Extender User's Guide and Reference or the IBM Informix Spatial DataBlade Module User's Guide.

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• Dimensionality
  
A geometry can have a dimension of 0, 1, or 2.

The dimensions are as follows:


• 0—Has neither length nor area
  


• 1—Has a length
  


• 2—Contains area
  


The point and multipoint subtypes have a dimension of 0. Points represent zero-dimensional features that can be modeled with a single coordinate, while multipoints represent data that must be modeled with a cluster of unconnected coordinates.

The subtypes linestring and multilinestring have a dimension of 1. They store features such as road segments, branching river systems, and any other features that are linear in nature.

Polygon and multipolygon subtypes have a dimension of 2. Forest stands, parcels, water bodies, and other features that have perimeters that enclose a definable area can be rendered by either the polygon or multipolygon data type.

Dimension is important not only as a property of the subtype but also in determining the spatial relationship of two features. The dimension of the resulting feature or features determines whether or not the operation was successful. The dimensions of the features are examined to determine how they should be compared.

To evaluate the dimension of a geometry, use the ST_Dimension function, which takes an ST_Geometry feature and returns its dimension as an integer.

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• Interior, boundary, exterior
  
All geometries occupy a position in space defined by their interiors, boundaries, and exteriors. The exterior of a geometry is all of the space not occupied by the geometry. The interior is the space occupied by the geometry. The boundary of a geometry is the location between its interior and exterior. The subtype inherits the interior and exterior properties directly; however, the boundary property differs for each.

You would use the ST_Boundary function to determine the source ST_Geometry's boundary.

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• Simple or nonsimple
  
Some subtypes of ST_Geometry are always simple, such as ST_Points or ST_Polygons. However, the subtypes ST_LineStrings, ST_MultiPoints, and ST_MultiLineStrings can be either simple or nonsimple. They are simple if they obey all topological rules imposed on them and nonsimple if they do not.

Some topological rules include:


• An ST_LineString is simple if it does not intersect its interior and nonsimple if it does.
  
• An ST_MultiPoint is simple if none of its elements occupies the same coordinate space (have the same x,y coordinates) and nonsimple if they do.
  
• An ST_MultiLineString is simple if none of its elements' interiors are intersected by its own interior and nonsimple if the elements' interiors do intersect.
  


The ST_IsSimple predicate function is used to determine if an ST_LineString, ST_MultiPoint, or ST_MultiLineString is simple or nonsimple. ST_IsSimple takes an ST_Geometry and returns 1 (TRUE) if the ST_Geometry is simple and 0 (FALSE) if it is not.

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• Empty or not empty
  
A geometry is empty if it does not have any points. An empty geometry has a NULL envelope, boundary, interior, and exterior. An empty geometry is always simple. Empty linestrings and multilinestrings have a 0 length. Empty polygons and multipolygons have a 0 area.

The ST_IsEmpty predicate function can be used to determine if a geometry is empty. It takes an ST_Geometry and returns 1 (TRUE) if the ST_Geometry is empty and 0 (FALSE) if it is not.

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• Envelope
  
Every geometry has an envelope. The envelope of a geometry is the bounding geometry formed by the minimum and maximum x,y coordinates. For point geometries, since the minimum and maximum x,y coordinates are the same, a rectangle is created around these coordinates to create an envelope. For line geometries, the line's endpoints represent two sides of the envelope and the other two sides are created just above and just below the line.

The ST_Envelope function takes an ST_Geometry and returns an ST_Geometry that represents the source ST_Geometry's envelope.

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• IsClosed and IsRing
  
Linestring geometries can be closed and/or be rings. Linestrings can be closed without being rings. You can determine if a linestring is closed by using the ST_IsClosed predicate function; it returns TRUE if the start point and endpoint of the linestring intersect. Rings are linestrings that are closed and simple. The ST_IsRing predicate function can be used to test if a linestring is truly a ring; it returns TRUE if the linestring is closed and is simple.

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• Number of simple geometries in a multipart geometry
  
As stated earlier in this topic, multipart geometries are made up of individual geometries. You might want to determine how many individual geometries are in a multipart geometry, such as an ST_MultiPoint, ST_MultiLineString, and ST_MultiPolygon. To do this, use the ST_NumGeometries predicate function. This function returns a count of the individual elements in a collection of geometries.

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• Point (vertex) collection and number of points
  
A geometry can have zero or more points. A geometry is considered empty if it has zero points. The point subtype is the only geometry that is restricted to zero or one point; all other subtypes can have zero or more.

ST_Point
An ST_Point is a zero-dimensional geometry that occupies a single location in coordinate space. An ST_Point has a single x,y coordinate value, is always simple, and has a NULL boundary. ST_Point can be used to define features such as oil wells, landmarks, and water sample collection sites.

Functions that operate solely on the ST_Point data type include the following:


• ST_X—Returns a point data type's x-coordinate value as a double-precision number
  
• ST_Y—Returns a point data type's y-coordinate value as a double-precision number
  
• ST_Z—Returns a point data type's z-coordinate value as a double-precision number
  
• ST_M—Returns a point data type's m-coordinate value as a double-precision number
  


ST_MultiPoint
An ST_MultiPoint is a collection of ST_Points and, like its elements, has a dimension of 0. An ST_MultiPoint is simple if none of its elements occupy the same coordinate space. The boundary of an ST_MultiPoint is NULL. ST_MultiPoints can define such things as aerial broadcast patterns and incidents of a disease outbreak.

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• Length and area
  
Length and area are measurable characteristics of geometries. Linestrings and the elements of multilinestrings are one-dimensional and possess the characteristic of length. Polygons and the elements of multipolygons are two-dimensional surfaces and, therefore, have an area you can measure. Units of measurement vary depending on how the data is stored.

ST_LineString
An ST_LineString is a one-dimensional object stored as a sequence of points defining a linear interpolated path. The ST_LineString is simple if it does not intersect its interior. The endpoints (the boundary) of a closed ST_LineString occupy the same point in space. An ST_LineString is a ring if it is both closed and simple. Like the other properties inherited from the superclass ST_Geometry, ST_LineStrings have length. ST_LineStrings are often used to define linear features such as roads, rivers, and power lines.

The endpoints normally form the boundary of an ST_LineString unless the ST_LineString is closed, in which case the boundary is NULL. The interior of an ST_LineString is the connected path that lies between the endpoints unless it is closed, in which case the interior is continuous.

Functions that operate on ST_LineStrings include:


• ST_StartPoint—Returns the first point of the specified ST_LineString
  
• ST_EndPoint—Returns the last point of an ST_LineString
  
• ST_PointN—Takes an ST_LineString and an index to nth point and returns that point
  
• ST_Length—Returns the length of an ST_LineString as a double-precision number
  
• ST_NumPoints—Evaluates an ST_LineString and returns the number of points in its sequence as an integer
  
• ST_IsRing—A predicate function that returns 1 (TRUE) if the specified ST_LineString is a ring and 0 (FALSE) otherwise
  
• ST_IsClosed—A predicate function that returns 1 (TRUE) if the specified ST_LineString is closed and 0 (FALSE) if it is not
  


The graphic below shows examples of ST_LineString objects: (1) is a simple, nonclosed ST_LineString; (2) is a nonsimple, nonclosed ST_LineString; (3) is a closed, simple ST_LineString and, therefore, a ring; and (4) is a closed, nonsimple ST_LineString-it is not a ring.



ST_MultiLineString
An ST_MultiLineString is a collection of ST_LineStrings. ST_MultiLineStrings are simple if they only intersect at the endpoints of the ST_LineString elements. ST_MultiLineStrings are nonsimple if the interiors of the ST_LineString elements intersect.

The boundary of an ST_MultiLineString is the nonintersected endpoints of the ST_LineString elements. The ST_MultiLineString is closed if all its ST_LineString elements are closed. The boundary of an ST_MultiLineString is NULL if all the endpoints of all the elements are intersected. In addition to the other properties inherited from the superclass ST_Geometry, ST_MultiLineStrings have length. ST_MultiLineStrings are used to define noncontiguous linear features, such as streams or road networks.

The following graphic provides examples of ST_MultiLineStrings: (1) is a simple ST_MultiLineString for which the boundary is the four endpoints of its two ST_LineString elements. (2) is a simple ST_MultiLineString because only the endpoints of the ST_LineString elements intersect. The boundary is two nonintersected endpoints. (3) is a nonsimple ST_MultiLineString, because the interior of one of its ST_LineString elements is intersected. The boundary of this ST_MultiLineString is the three nonintersected endpoints. (4) is a simple nonclosed ST_MultiLineString. It is not closed because its element ST_LineStrings are not closed. It is simple because none of the interiors of any of the element ST_LineStrings intersect. (5) is a single, simple, closed ST_MultiLineString. It is closed because all its elements are closed. It is simple because none of its elements intersect at the interiors.



Functions that operate on ST_MultiLineStrings include ST_Length and ST_IsClosed.

The ST_Length function evaluates an ST_MultiLineString and returns the cumulative length of all its ST_LineString elements as a double-precision number.

The ST_IsClosed predicate function returns 1 (TRUE) if the specified ST_MultiLineString is closed and 0 (FALSE) if it is not closed.

ST_Polygon
An ST_Polygon is a two-dimensional surface stored as a sequence of points defining its exterior bounding ring and 0 or more interior rings. ST_Polygon is always simple. ST_Polygon defines features that have spatial extent, such as parcels of land, water bodies, and areas of jurisdiction.

This graphic shows examples of ST_Polygon objects: (1) is an ST_Polygon for which the boundary is defined by an exterior ring. (2) is an ST_Polygon with a boundary defined by an exterior ring and two interior rings. The area inside the interior rings is part of the ST_Polygon's exterior. (3) is a legal ST_Polygon, because the rings intersect at a single tangent point.



The exterior and any interior rings define the boundary of an ST_Polygon, and the space enclosed between the rings defines the ST_Polygon's interior. The rings of an ST_Polygon can intersect at a tangent point but never cross. In addition to the other properties inherited from the superclass ST_Geometry, ST_Polygon has area.

Functions that operate on ST_Polygon include:


• ST_Area—Returns the area of an ST_Polygon as a double-precision number
  
• ST_ExteriorRing—Returns the exterior ring of an ST_Polygon as an ST_LineString
  
• ST_NumInteriorRing-Returns the number of interior rings that an ST_Polygon contains
  
• ST_InteriorRingN—Evaluates an ST_Polygon and an index and returns the nth interior ring as an ST_LineString
  
• ST_Centroid—Returns an ST_Point, which represents the center of the ST_Polygon's envelope
  
• ST_PointOnSurface—Returns an ST_Point that is guaranteed to be on the surface of the specified ST_Polygon
  


ST_MultiPolygon
The boundary of an ST_MultiPolygon is the cumulative length of its elements' exterior and interior rings. The interior of an ST_MultiPolygon is defined as the cumulative interiors of its element ST_Polygons. The boundary of an ST_MultiPolygon's elements can only intersect at a tangent point. In addition to the other properties inherited from the superclass ST_Geometry, ST_MultiPolygons have area. ST_MultiPolygons define features such as a forest stratum or a noncontiguous parcel of land such as a Pacific island chain.

The graphic below provides examples of ST_MultiPolygon: (1) is ST_MultiPolygon with two ST_Polygon elements. The boundary is defined by the two exterior rings and the three interior rings. (2) is an ST_MultiPolygon with two ST_Polygon elements. The boundary is defined by the two exterior rings and the two interior rings. The two ST_Polygon elements intersect at a tangent point.

Functions that operate on ST_MultiPolygons include ST_Area, ST_Centroid, and ST_PointOnSurface.

The ST_Area function returns a double-precision number that represents the cumulative ST_Area of an ST_MultiPolygon's ST_Polygon elements.

The ST_Centroid function returns an ST_Point that is the center of an ST_MultiPolygon's envelope.

The ST_PointOnSurface function evaluates an ST_MultiPolygon and returns an ST_Point that is guaranteed to be normal to the surface of one of its ST_Polygon elements.

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• Z-coordinates
  
Some geometries have an associated altitude or depth-a third dimension. Each of the points that form the geometry of a feature can include an optional z-coordinate that represents an altitude or depth normal to the earth's surface.

The ST_Is3D predicate function takes an ST_Geometry and returns 1 (TRUE) if the function has z-coordinates and 0 (FALSE) if it does not.

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• Measures
  
Measures are values assigned to each coordinate. They are used for linear referencing and dynamic segmentation applications. For example, milepost locations along a highway can contain measures that indicate their position. The value represents anything that can be stored as a double-precision number.

The ST_IsMeasured predicate function takes a geometry and returns 1 (TRUE) if it contains measures and 0 (FALSE) if it does not.

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Using ST_Geometry with ArcGIS

ArcGIS is designed to work with the ST_Geometry spatial type through ArcSDE. ST_Geometry is the spatial storage type when using ArcSDE for DB2 or ArcSDE for Informix. ArcSDE for Oracle supports a number of different geometry storage schemas such as ST_Geometry and SDEBINARY. These different schemas can all be used together in the same database. While there can only be one default geometry schema, individual feature classes can be created using different geometry schemas. To learn about setting your Oracle database to use ST_Geometry storage by default, see the section 'Creating feature classes in Oracle with ST_Geometry storage' in the topic A spatial type for Oracle.

ESRI recommends creating feature classes in spatial type format using ArcGIS versus using SQL. This recommendation is based on the following:

• Data that you create or add to your ArcSDE Enterprise geodatabase using ArcGIS Desktop software is automatically registered with ArcSDE and the geodatabase. All required components are created for you by the software. It can be used immediately by ArcGIS. It can also be used immediately via SQL and other applications.
  
(An additional registration step is required to enable the feature class for versioning.)

• By default, the spatial index is generated for you. An additional step is not required to create the spatial index.
  
• Complete tools are available to you for determining and specifying the spatial reference for your data. Many spatial references have already been defined and are available for use. If none of these spatial references represent the projection that is desired, you are able to modify the current projections or create your own. If you want to use the same spatial reference of an existing dataset, you can import that exact spatial reference as well.
  

Creating a new feature class with ArcGIS

There are four primary ways to create a new feature class in your geodatabase:

• Using ArcCatalog
  
• Using a geoprocessing tool named 'Create Feature Class'
  
• Saving the contents of a map layer in ArcMap
  
• Converting an external data source into a geodatabase feature class (e.g., converting a shapefile or a CAD file). To convert an external datasource to a geodatabase feature class, you use the import tools either in ArcCatalog or ArcToolbox. There are two ways you can import a data: using the context sensitive menu in ArcCatalog or using the To Geodatabase toolset in ArcToolbox. When you import data that is in a format not used by the geodatabase, ArcGIS automatically converts it into data types used by the geodatabase.
  

For instructions on how to create a new feature class using ArcGIS, see Creating feature classes.

When you create new feature classes in a geodatabase in DB2 or Informix, it will automatically use ST_Geometry storage. To create a new feature class using ST_Geometry storage in Oracle, you either need to alter your DBTUNE table DEFAULTS GEOMETRY_STORAGE setting, or you need to specify a configuration keyword during feature class creation that instructs ArcGIS to store that particular feature class using ST_Geometry storage. See Geometry storage parameters and keywords for details.

At this stage, you now have a feature class stored in ST_GEOMETRY format. In addition to being ready for use by ArcGIS, the feature class is also available for use via spatial type SQL operators and functions.

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