Spatial Data Types Overview
There are two types of spatial data. The geometry data type supports planar, or Euclidean (flat-earth), data. The geometry data type both conforms to the Open Geospatial Consortium (OGC) Simple Features for SQL Specification version 1.1.0 and is compliant with SQL MM (ISO standard). SQL Server also supports the geography data type, which stores ellipsoidal (round-earth) data, such as GPS latitude and longitude coordinates.
SQL Server spatial tools is a Microsoft sponsored open-source collection of tools for use with the spatial types in SQL Server. This project provides a set of reusable functions which applications can make use of. These functions may include data conversion routines, new transformations, aggregates, etc. See Microsoft/SQLServerSpatialTools in GitHub for more details.
Spatial data objects
The geometry and geography data types support 16 types of spatial data objects, or instance types. However, only 11 of these instance types are instantiable; you can create and work with these instances (or instantiate them) in a database. These instances derive certain properties from their parent data types.
The figure below shows the geometry hierarchy upon which the geometry and geography data types are based. The instantiable types of geometry and geography are indicated in blue.
There's an additional instantiable type for the geography data type: FullGlobe. The geometry and geography types can recognize a specific instance as long as it's a well-formed instance, even if the instance isn't defined explicitly. For example, if you define a Point instance explicitly using the STPointFromText() method, geometry and geography recognize the instance as a Point, as long as the method input is well formed. If you define the same instance using the
STGeomFromText() method, both the geometry and geography data types recognize the instance as a Point.
The subtypes for geometry and geography types are divided into simple and collection types. Some methods like
STNumCurves() work only with simple types.
Simple types are:
Collection types are:
Geometry and geography data type differences
The two types of spatial data often behave similarly. There are some key differences in how the data is stored and manipulated.
How connecting edges are defined
The defining data for LineString and Polygon types are vertices only. The connecting edge between two vertices in a geometry type is a straight line. However, the connecting edge between two vertices in a geography type is a short great elliptic arc between the two vertices. A great ellipse is the intersection of the ellipsoid with a plane through its center. A great elliptic arc is an arc segment on the great ellipse.
How circular arc segments are defined
Circular arc segments for geometry types are defined on the XY Cartesian coordinate plane (Z values are ignored). Circular arc segments for geography types are defined by curve segments on a reference sphere. Any parallel on the reference sphere can be defined by two complementary circular arcs where the points for both arcs have a constant latitude angle.
Measurements in spatial data types
In the planar (flat-earth) system, measurements of distances and areas are given in the same unit of measurement as coordinates. Using the geometry data type, the distance between (2, 2) and (5, 6) is five units, regardless of the units used.
In an ellipsoidal, or round-earth system, coordinates are given in degrees of latitude and longitude. However, lengths and areas are typically measured in meters and square meters, though the measurement may depend on the spatial reference identifier of the geography instance. The most common unit of measurement for the geography data type is meters.
Orientation of spatial data
The ring orientation of a polygon is not an important factor in the planar system. The OGC Simple Features for SQL Specification doesn't dictate a ring ordering, and SQL Server doesn't enforce ring ordering.
In an ellipsoidal system, a polygon without an orientation has no meaning, or is ambiguous. For example, does a ring around the equator describe the northern or southern hemisphere? If we use the geography data type to store the spatial instance, we must specify the orientation of the ring and accurately describe the location of the instance.
The interior of the polygon in an ellipsoidal system is defined by the "left-hand rule": if you imagine yourself walking along the ring of a geography Polygon, following the points in the order in which they are listed, the area on the left is being treated as the interior of the Polygon, and the area on the right as the exterior of the Polygon.
When the compatibility level is 100 or below in SQL Server then the geography data type has the following restrictions:
Each geography instance must fit inside a single hemisphere. No spatial objects larger than a hemisphere can be stored.
Any geography instance from an Open Geospatial Consortium (OGC) Well-Known Text (WKT) or Well-Known Binary (WKB) representation that produces an object larger than a hemisphere throws an ArgumentException.
The geography data type methods that require the input of two geography instances, such as STIntersection(), STUnion(), STDifference(), and STSymDifference(), will return null if the results from the methods do not fit inside a single hemisphere. STBuffer() will also return null if the output exceeds a single hemisphere.
In SQL Server, FullGlobe is a special type of Polygon that covers the entire globe. It has an area, but no borders or vertices.
Outer and inner rings in
geography data type
The OGC Simple Features for SQL Specification discusses outer rings and inner rings, but this distinction makes little sense for the SQL Server geography data type; any ring of a polygon can be taken to be the outer ring.
For more information on OGC specifications, see the following documents:
- OGC Specifications, Simple Feature Access Part 1 - Common Architecture
- OGC Specifications, Simple Feature Access Part 2 - SQL Options
Circular arc segments
Three instantiable types can take circular arc segments: CircularString, CompoundCurve, and CurvePolygon. A circular arc segment is defined by three points in a two-dimensional plane and the third point cannot be the same as the first point. Few examples of circular arc segments:
First two examples show typical circular arc segments. Note how each of the three points lies on the perimeter of a circle.
Other two examples show how a line segment can be defined as a circular arc segment. Three points are still needed to define the circular arc segment unlike a regular line segment, which can be defined by just two points.
Methods operating on circular arc segment types use straight-line segments to approximate the circular arc. The number of line segments used to approximate the arc will depend on the length and curvature of the arc. Z values can be stored for each of the circular arc segment types, but will not be used in the calculations.
If Z values are given for circular arc segments then they must be the same for all points in the circular arc segment for it to be accepted for input. For example:
CIRCULARSTRING(0 0 1, 2 2 1, 4 0 1) is accepted, but
CIRCULARSTRING(0 0 1, 2 2 2, 4 0 1) is not accepted.
LineString and CircularString comparison
This example shows how to store identical isosceles triangles using both a LineString instance and CircularString instance:
DECLARE @g1 geometry; DECLARE @g2 geometry; SET @g1 = geometry::STGeomFromText('LINESTRING(1 1, 5 1, 3 5, 1 1)', 0); SET @g2 = geometry::STGeomFromText('CIRCULARSTRING(1 1, 3 1, 5 1, 4 3, 3 5, 2 3, 1 1)', 0); IF @g1.STIsValid() = 1 AND @g2.STIsValid() = 1 BEGIN SELECT @g1.ToString(), @g2.ToString() SELECT @g1.STLength() AS [LS Length], @g2.STLength() AS [CS Length] END
Notice that a CircularString instance requires seven points to define the triangle. LineString instance requires only four points to define the triangle. The reason for this is that a CircularString instance stores circular arc segments and not line segments. The sides of the triangle stored in the CircularString instance are ABC, CDE, and EFA. The sides of the triangle stored in the LineString instance are AC, CE, and EA.
Consider the following example:
SET @g1 = geometry::STGeomFromText('LINESTRING(0 0, 2 2, 4 0)', 0); SET @g2 = geometry::STGeomFromText('CIRCULARSTRING(0 0, 2 2, 4 0)', 0); SELECT @g1.STLength() AS [LS Length], @g2.STLength() AS [CS Length];
Here is the result set.
LS Length CS Length 5.65685... 6.28318...
CircularString instances use fewer points to store curve boundaries with greater precision than LineString instances. CircularString instances are useful for storing circular boundaries like a 20-mile search radius from a specific point. LineString instances are good for storing boundaries that are linear like a square city block.
LineString and CompoundCurve comparison
The following code examples show how to store the same figure using LineString and CompoundCurve instances:
SET @g = geometry::Parse('LINESTRING(2 2, 4 2, 4 4, 2 4, 2 2)'); SET @g = geometry::Parse('COMPOUNDCURVE((2 2, 4 2), (4 2, 4 4), (4 4, 2 4), (2 4, 2 2))'); SET @g = geometry::Parse('COMPOUNDCURVE((2 2, 4 2, 4 4, 2 4, 2 2))');
In the examples above, either a LineString instance or a CompoundCurve instance could store the figure. This next example uses a CompoundCurve to store a pie slice:
SET @g = geometry::Parse('COMPOUNDCURVE(CIRCULARSTRING(2 2, 1 3, 0 2),(0 2, 1 0, 2 2))');
A CompoundCurve instance can store the circular arc segment (2 2, 1 3, 0 2) directly, but a LineString instance would have to convert the curve into several smaller line segments.
CircularString and CompoundCurve comparison
The following code example shows how the pie slice can be stored in a CircularString instance:
DECLARE @g geometry; SET @g = geometry::Parse('CIRCULARSTRING( 0 0, 1 2.1082, 3 6.3246, 0 7, -3 6.3246, -1 2.1082, 0 0)'); SELECT @g.ToString(), @g.STLength();
Storing the pie slice using a CircularString instance requires that three points be used for each line segment. If an intermediate point isn't known, it either has to be calculated, or the endpoint of the line segment has to be doubled as the following snippet shows:
SET @g = geometry::Parse('CIRCULARSTRING( 0 0, 3 6.3246, 3 6.3246, 0 7, -3 6.3246, 0 0, 0 0)');
CompoundCurve instances allow both LineString and CircularString components so that only two points to the line segments of the pie slice need to be known. This code example shows how to use a CompoundCurve to store the same figure:
DECLARE @g geometry; SET @g = geometry::Parse('COMPOUNDCURVE(CIRCULARSTRING( 3 6.3246, 0 7, -3 6.3246), (-3 6.3246, 0 0, 3 6.3246))'); SELECT @g.ToString(), @g.STLength();
Polygon and CurvePolygon comparison
CurvePolygon instances can use CircularString and CompoundCurve instances when defining their exterior and interior rings. Polygon instances can't.