Feature class

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Feature classes are homogeneous collections of common features, each having the same spatial representation, such as points, lines, or polygons, and a common set of attribute columns, for example, a line feature class for representing road centerlines. The four most commonly used feature classes in the geodatabase are points, lines, polygons, and annotation (the geodatabase name for map text).

In the illustration below, these are used to represent four datasets for the same area: (1) manhole cover locations as points, (2) sewer lines, (3) parcel polygons, and (4) street name annotation.


In this diagram, you might also have noted the potential requirement to model some advanced feature properties. For example, the sewer lines and manhole locations make up a storm sewer network, a system with which you can model runoff and flows. Also, note how adjacent parcels share common boundaries. Most parcel users want to maintain the integrity of shared feature boundaries in their datasets using a topology.


[edit] Types of feature classes in the geodatabase

Vector features (geographic objects with vector geometry) are versatile and frequently used geographic data types, well suited for representing features with discrete boundaries, such as wells, streets, rivers, states, and parcels. A feature is simply an object that stores its geographic representation, which is typically a point, line, or polygon, as one of its properties (or fields) in the row. In ArcGIS, feature classes are homogeneous collections of features with a common spatial representation and set of attributes stored in a database table. For example, a line feature class for representing road centerlines.

Generally, feature classes are thematic collections of points, lines, or polygons, but there are seven feature class types:

[edit] Feature geometry and feature coordinates

Feature classes contain both the geometric shapes of each feature as well as their descriptive attributes. Each feature geometry is primarily defined by its feature type (point, line, or polygon). But additional geometric properties can also be defined. For example, features can be single part or multipart, can have 3D vertices, can have linear measures (called m-values), and can contain parametrically defined curves. This section provides a short overview of these capabilities.

Single-part and multipart lines and polygons
Line and polygon feature classes in the geodatabase can be composed of single parts or multiple parts. For example, a state can contain multpile parts (Hawaii's islands) but is considered to be a single state feature.

Vertices, segments, elevation, and measurements

Feature geometry is primarily composed of coordinate vertices. Segments in lines and polygon features span vertices. Segments can be straight edges or can be parametrically defined curves. Vertices in features can also include z-values to represent elevation measures and m-values to represent measurements along line features.

Segment types in line and polygon features

Lines and polygons are defined by two key elements: (1) an ordered list of vertices that define the shape of the line or polygon and (2) the types of line segments used between each pair of vertices. Each line and polygon can be thought of as an ordered set of vertices that can be connected to form the geometric shape. Another way to express each line and polygon is as an ordered series of connected segments where each segment has a type: straight line, circular arc, elliptical arc, or Bezier curve.

The default segment type is a straight line between two vertices. However, when you need to define curves or parametric shapes, you have three additional segment types: circular arcs, elliptical arcs, and Bezier curves that can be defined. These shapes are often used for representing built environments such as parcel boundaries and roadways.

Vertical measurements using z-values

Feature coordinates can include x,y and x,y,z vertices. Z-values are most commonly used to represent elevations, but they can represent other measurements such as annual rainfall or measures of air quality.

Linear measurements using m-values

Linear feature vertices can also include m-values. Some GIS applications employ a linear measurement system used to interpolate distances along linear features, such as along roads, streams, and pipelines. You can assign an m-value to each vertex in a feature. A commonly used example is a highway milepost measurement system used by departments of transportation for recording pavement conditions, speed limits, accident locations, and other incidents along highways. Two commonly used units of measure are milepost distance from a set location, such as a county line, and distance from a reference marker.
Vertices for measurements can be either (x,y,m) or (x,y,z,m).

Support for these data types is often referred to as Linear Referencing. The process of geolocating events that occur along these measurement systems is referred to as Dynamic Segmentation.

Measured coordinates form the building blocks for these systems. In the linear referencing implementation in ArcGIS, the term route refers to any linear feature, such as a city street, highway, river, or pipe, that has a unique identifier and a common measurement system along each linear feature. A collection of routes with a common measurement system can be built on a line feature class as follows:

[edit] Feature tolerances

Locational accuracy and support for a high-precision data management framework are critical in GIS data management. A key requirement is the ability to store coordinate information with enough precision. The precision of a coordinate describes the number of digits that are used to record the location. This defines the resolution at which spatial data is collected and managed.

Since geodatabases can record high-precision coordinates, users can build datasets with high accuracy levels and with greater resolution as data capture tools and sensors improve over time (e.g., data entry from survey and civil engineering, cadastral and COGO data capture, increased imagery resolution, LIDAR, building plans from CAD).

The geodatabase records its coordinates using integer numbers and can handle locations with very high precision. In various ArcGIS operations, feature coordinates for the geodatabase are processed and managed using some key geometric properties. These properties are defined during the creation of each feature class or feature dataset.

The following geometric properties help define coordinate resolution and processing tolerances used in various spatial processing and geometric operations:

[edit] Feature class storage in the geodatabase

In the geodatabase, each feature class is managed in a single table. A Shape column in each row is used to hold the geometry or shape of each feature.

In the feature class table:

In ArcSDE geodatabases, relational databases hold feature classes as tables in the DBMS. ArcSDE is supported on five RDBMSs: Oracle, DB2, Informix, SQL Server and PostgreSQL.

[edit] Additional Links and References

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