- Types of Entity Classes
- Feature geometry and feature coordinates
- Resource Tolerances
- Resource class memory
- Extend entity classes
Feature classes are homogeneous sets of common features, each with the same spatial representation, such as B. points, lines, or polygons, and a common set of attribute columns, for example B. a line feature class to represent centerlines of streets. The four most commonly used feature classes are points, lines, polygons, and annotation (the name of the geodatabase for the map text).
In the following figure, they are used to represent four sets of data for the same area: (1) manhole cover locations as points, (2) sewer lines, (3) plot polygons, and (4) annotation of street names.
In this diagram, you may have also noticed the potential need to model some advanced resource properties. For example, sewers and culverts form a storm sewer network, a system that allows you to model drainage and flow. Also note that adjacent plots share common boundaries. Most batch users want to preserve the integrity of shared resource boundaries in their datasets by using aTopology.
As mentioned above, users often need to model such spatial relationships and behaviors in their geographic datasets. In these cases, you can extend these basic resource classes by adding a variety of advanced geodatabase items, such as topologies, network datasets, terrains, and address locators.
For more information on how to add these advanced behaviors to your geodatabases, seeExtend entity classes.
Types of Entity Classes
Vector features (spatial objects with vector geometry) are versatile and commonly used geographic data types suitable for representing features with discrete boundaries, such as B. roads, states, and parcels. A feature is an object that stores its geographic representation, which is usually a point, line, or polygon, as one of its properties (or fields) on the line. In ArcGIS, feature classes are homogeneous collections of features with a common spatial representation and a set of attributes stored in a database table, such as a line feature class to represent centerlines of highways.
Typically, resource classes are thematic collections of points, lines, or polygons, but there are seven types of resource classes. The first three support databases and geodatabases. The last four are only supported in geodatabases.
- Points:Features that are too small to be represented as lines or polygons and point locations (such as GPS observations).
- Lines:Represents the shape and location of geographic objects, e.g. B. Centerlines of roads and streams that are too narrow to plot as areas. Lines are also used to represent features that have length but no area, such as B. Contour lines and borders.
- polygons:A set of versatile area features that represent the shape and location of homogeneous feature types such as states, counties, parcels, terrain types, and land use zones.
- Annotation:Map text, including properties for how the text is rendered. For example, in addition to the text string for each annotation, other properties are included, such as: B. the shape points to place the text, its font and point size, and other display properties. The annotation can also be linked to entities and contain subclasses.
- Dimensions:A special type of annotation that displays specific lengths or distances, for example, to indicate the length of one side of a building or property line, or the distance between two features. Dimensions are commonly used in GIS design, engineering, and furniture applications.
- Multipoint:Resources consisting of more than one point. Multipoints are often used to manage very large sets of point collections, e.g. B. Clusters of Lidar points that can contain literally billions of points. Using a single line for such point geometry is impractical. Grouping into multiple point series allows the geodatabase to handle large sets of points.
- Multiparque:A 3D geometry used to represent the outer surface or shell of features that occupy a discrete area or volume in three-dimensional space. Multipatches consist of flat 3D triangles and donuts used in combination to model a three dimensional layer. Multipatches can be used to render anything from simple objects like spheres and cubes to complex objects like isosurfaces and buildings.
Feature geometry and feature coordinates
Feature classes contain the geometric shape of each feature and descriptive attributes. Each feature geometry is primarily defined by its feature type (point, line, or polygon). However, additional geometric properties can also be defined. For example, features can be single or multipart, have 3D vertices, have linear measurements (known as m-values), and contain parametrically defined curves. This section provides a brief overview of these functions.
One-part or multi-part lines and polygons
Line and polygon feature classes can consist of single parts or multiple parts. For example, a state may contain multiple parts (the islands of Hawaii), but it is considered a single governmental entity.
Vertices, Segments, Elevation and Measures
Feature geometry mainly consists of coordinate vertices. Line segments and polygon features span vertices. Segments can be straight edges or parametrically defined curves. Feature vertices can also contain z values to represent elevation measurements and m values to represent measurements along line features.
Segment types in polygon and line features
Lines and polygons are defined by two main elements: an ordered list of vertices that define the shape of the line or polygon, and the types of line segments used between each pair of vertices. Each line and polygon can be seen 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, circle arc, ellipse arc, or Bézier curve.
The default segment type is a straight line between two vertices. However, if you need to define parametric shapes or curves, you can define three additional segment types: circular arcs, elliptical arcs, and Bézier curves. These shapes are commonly used to represent built environments such as plot boundaries and roads.
Vertical measurements with z values
Feature coordinates can include x,y and x,y,z vertices. Z values are most commonly used to represent heights, but they can also represent other measurements such as height. B. annual precipitation or air quality.
Features can be added in x, y coordinates, and optionally z elevation values.
Linear measurements with m values
Linear feature vertices can also contain m-values. Some GIS applications use a linear measurement system, which is used to interpolate distances along linear features such as roads, streams, and pipelines. You can assign an m value to each vertex in a feature. A common application example is a landmark measurement system used by transportation departments to record road conditions, speed limits, accident locations, and other incidents along highways. Two commonly used units of measurement are the distance of a mile pole from a specific location, for example. B. a circle boundary and the distance from a reference mark.
Vertices for measurements can be x,y,m or x,y,z,m.
Support for these data types is often referred to aslinear reference. The geolocation process of events that occur along these measurement systems is calleddynamic segmentation.
The measured coordinates form the basic components of these systems. In the linear reference implementation in ArcGIS, the termrotarefers to any linear feature, e.g. a city street, highway, river, or pipeline 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 incorporated into a line feature class as follows:
verAn overview of linear referencingfor more information.
Resource Tolerances
Location accuracy and support for a highly accurate data management framework are critical in GIS data management. A key requirement is the ability to store coordinate information accurately enough. The precision of a coordinate describes the number of digits used to record the location. This defines the resolution at which spatial data is collected and managed.
Because geodatabases and databases can record high-precision coordinates, users can create high-precision, high-resolution datasets as data collection tools and sensors improve over time (surveying and data entry civil engineering, cadastral and COGO data collection, higher image resolution, Lidar, CAD construction drawings, etc.).
ArcGIS records coordinates using integer numbers and can handle locations with very high precision. In many ArcGIS operations, feature coordinates are processed and managed using some important 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 geometric and spatial processing operations:
- X,y Resolution: The precision with which coordinates are recorded within a feature class
- x,y tolerance—A cluster tolerance used to group features that have matching geometry; Used in topology, feature overlay and related operations
- Z Tolerance and Z Resolution: The tolerance and resolution properties for the vertical coordinate dimension in 3D datasets (for example, a height dimension)
- M Tolerance and m Resolution: The tolerance and resolution properties for measurements along line features used in linear reference datasets (for example, the distance along a road in meters).
resolution x,y
The x,y resolution of a feature class or feature dataset is the numerical precision used to store x,y coordinate values. Accuracy is important for accurate representation, analysis, and mapping of features.
The x,y resolution defines the number of decimal places, or significant digits, used to store the feature coordinates (in x and y). You can think of resolution as defining a very fine grid to which all coordinates fit. Coordinate values are stored and used in ArcGIS as integers. Therefore, this grid is sometimes called the entire grid or the coordinate grid.
Resolution defines the distance between meshes in a coordinate grid into which all coordinates fit. The XY resolution is expressed in the units of the data (depending on the coordinate system), eg. B. Declare flat feet, UTM gauges or Albers gauges.
The default XY resolution for feature classes is 0.0001 meters or the equivalent in units of the dataset's coordinate system. For example, if a feature class is stored in State Plane Feet, the default precision is 0.0003281 feet (0.003937 inches). If the coordinates are in latitude and longitude, the default x,y resolution is 0.000000001 degrees.
The graphic below provides a conceptual view of a coordinate grid with all coordinate values snapped to the grid's mesh. The grid covers the scope of each dataset. The fineness of this mesh (the distance between the grid lines) is defined by the very small x,y resolution.
If necessary, you can override the default XY resolution value and set a different value for each feature class or feature dataset. Specifying a lower XY resolution value can potentially increase data storage and processing time for datasets compared to datasets that use higher XY resolution values.
X,y-Tolerance
When creating a feature class, you will be asked to define the XY tolerance. XY Tolerance is used to define the minimum distance between coordinates in grouping operations such as topology validation, buffer generation and polygon overlapping, and some editing operations.
Feature processing operations are affected by the XY Tolerance, which determines the minimum distance that separates all feature coordinates (nodes and vertices) during these operations. By definition, it also defines the distance a coordinate can be moved in x or y (or both) during grouping operations.
The XY tolerance is an extremely small distance (default is 0.001 meters in ground units). It is used to resolve imprecise coordinate intersections during grouping operations. When processing feature classes with geometric operations, coordinates whose x and y distances are within the x,y tolerance are considered to be coincident (in other words, they have the same x,y location). Therefore, the grouped coordinates are moved to a common location.
Typically, the less precise coordinate is moved to the more precise coordinate location, or a new location is calculated as a weighted average distance between the coordinates in the group. In these cases, the weighted average distance is based on the accuracy ranges of the grouped coordinates.
For more information on defining accuracy ranges for each feature class, seeTopologies in ArcGIS.
The grouping process works by moving around the map and identifying groups of coordinates that are within XY tolerance of each other. ArcGIS uses this algorithm to discover, clean up, and manage shared geometry between features. This means that the coordinates are considered to be coincident (and snapped to the same common coordinate position). This is fundamental to many GIS concepts and operations. see for exampleAn overview of topology in ArcGIS.
The maximum distance a coordinate can move to its new position during such operations is the square root of twice the x,y tolerance. The clustering algorithm is iterative, so in some cases it is possible for coordinate positions to change by more than this distance.
The default XY tolerance is defined as 0.001 meters or the equivalent in the dataset's real-world coordinate system units (in other words, 0.001 meters on the ground). For example, if your coordinate system is plotted in state plane feet, the default XY tolerance is 0.003281 feet (0.03937 inches).
The default XY tolerance is 10 times the default XY resolution and is recommended for most cases. You have the option of setting a higher tolerance value for data with lower coordinate precision or a lower value for an extremely high precision data set.
It is important to note that the x,y tolerance is not intended to generalize about geometric shapes. Rather, it is intended to integrate linework and boundaries during topology operations. This means integrating very close coordinates. Since coordinates can change by the x,y tolerance in x and y, many potential problems can be solved by processing data sets with commands that use the x,y tolerance. This includes handling extremely small overshoots or undershoots, automatic splitting of double segments, and coordinated roughing along boundary lines.
Here are some useful tips:
- In general, you can use an XY tolerance that is 10 times the XY resolution and expect good results.
- Keep the XY tolerance small to keep the coordinate movement small. However, a very small XY tolerance (for example, 3 times the XY resolution or less) may not correctly integrate the linework with corresponding coordinates and boundaries.
- On the other hand, if your XY tolerance is too large, the coordinates of the features might get mixed up. This can affect the accuracy of representations of feature boundaries.
- Your XY tolerance should never come close to your data acquisition resolution. For example, on a map scale of 1:12,000, 1 inch equals 1,000 feet and 1/50th of an inch equals 20 feet. You must keep the coordinate movement well below these numbers with the x,y tolerance. Remember that the standard x,y tolerance in this case would be 0.0003281 feet, which is a very reasonable standard x,y tolerance; In fact, it's best to use the default x,y tolerance values in all exceptional cases.
- In topologies, you can specify the coordinate space of each feature class. You should set the coordinate range of your more precise features (for example, search features) to 1 and your less precise features to 2, 3, etc. in descending order of accuracy. This causes other feature coordinates with higher precision rating (and therefore lower coordinate precision) to snap to lower rated more accurate features.
Resource class memory
Each resource class is managed in a single table. A shape column in each row is used to contain the geometry or shape of each feature.
In the entity class table:
- Each resource class is a table.
- Individual features are retained as rows.
- Resource attributes are recorded in columns.
- The shape column contains the geometry of each feature (point, line, polygon, etc.).
- The ObjectID column contains the unique identifier for each function.
When you create a line feature class in a geodatabase, an additional field is automatically added to the feature class to record the length of the line. When you create a polygon feature class, two additional fields are automatically added to record the length (perimeter) and area of each polygon feature. The units of measure for these values depend on the spatial reference defined for the feature class. The names of these fields vary depending on the database and spatial type used. These are mandatory fields and cannot be changed.
Extend entity classes
Each feature class is a collection of geographic features with the same geometry type (point, line, or polygon), the same attributes, and the same spatial reference. Resource classes stored in geodatabases can be expanded as needed to achieve a variety of goals. Here are some ways and reasons to extend resource classes using the geodatabase.
Working with resource classes in the geodatabase
| To use | if you need |
|---|---|
resource record | Maintain a collection of spatially related feature classes or create topologies, networks and terrains. |
subtypes | Manage multiple entity subclasses within a single entity class. This is commonly used in entity class tables to manage different behavior for subsets of the same entity type. |
The domain of the attribute | Provide a list of valid values or a range of valid values for the attribute columns. Use domains to ensure the integrity of attribute values. Domains are often used to enforce classifications of data (such as road classes, zone codes, and land use classifications). |
relationship classes | Create relationships between entity classes and other tables using a shared key. For example, find related rows in a second table based on selected rows in the feature class. |
Topology | Model how features share geometry. For example, neighboring counties share a common border. Also, county polygons nest inside states and cover them completely. |
network record | Connectivity model and transport flow. you must have itArcGIS Network Analyst ExtensionAArcGIS for the desktopFurnished. |
geometric networks | Model networks and utilities layout. |
land registration | Model triangulated irregular networks (TINs) and manage large sets of lidar and sonar points. you must have itArcGIS 3D Analyst ExtensionAArcGIS for the desktopFurnished. |
address locator | Geocode addresses. |
linear reference | Locate events along linear features with dimensions. |
cartographic representations | Manage multiple map views and advanced map drawing rules. |
version control | Manage a variety of important GIS data management workflows; For example, it supports long-update transactions, archives, and multi-user editing. |