Modeling feature classes

Modeling feature classes
Release 9.3

Task 1. Design simple feature classes.

• Pick a geometry type (also known as the feature class type) such as point, line, polygon, or annotation. You'll need to use one common geometry type for all the features in each feature class. See Feature class basics.
• Determine the attribute fields and column types. See Geodatabase field data types.
• Determine geometry properties. Will you have z-coordinates? M-coordinates? What kind of coordinate resolution? What kinds of line segments for lines and polygon feature classes? Most often, users only need the default, which uses simple, straight-line segments. However, sometimes you might need curved segments, for example, to represent cul-de-sacs and roads. See Feature class basics.
• Define the coordinate system for each feature class. See An overview of map projections.
• Do you need to use this dataset at multiple scales? How will the representations change at each map scale? You may find that you'll need alternative feature class representations for use at other scale ranges. In these cases, you can consider additional feature classes for representing the same data theme for each scale range.

• If you need to validate a dataset before you import and use it in your system (for example, to ensure the dataset adheres to a series of spatial integrity rules)
• If you will need to edit the data and maintain its spatial integrity
• If you want to use the feature class for advanced GIS work such as modeling and analysis

Task 2. Organize related feature classes into feature datasets.

• Add a topology.
• Add a cadastral fabric dataset (must have Survey Analyst to use).
• Add a network dataset (must have Network Analyst to use).
• Add a geometric network.
• Add a terrain dataset (must have 3D Analyst to use).

• Topology
• Network dataset
• Terrain
• Geometric network
• Cadastral fabric

Task 3. Add geodatabase elements to facilitate data editing and to manage data integrity.

• Do you want to manage the integrity of attribute values? You can use domains, which are rules for assigning valid values in an attribute field.

• Do you want to use subtypes to help manage subsets of features in a feature class? Subtypes allow you to set up special behaviors for each subclass. They can be used to set default rules for managing feature subsets. For example, you can use subtypes to automatically assign default attribute values as new features are added during editing, to set spatial integrity rules for how new features connect to others, and to add other feature behaviors.

• Determine if there are related tables and if you need relationship classes. Relationship classes allow you to work with features in one table by selecting features in related tables, a very common relational database capability.

• Determine if there are spatial relationships between features in this feature class or with other feature classes that need to be modeled. For example, do you have parcels that share common boundaries? Do they share geometry with a feature class of parcel boundaries and another of parcel corners? Do you want to ensure that your road segments connect to one another or that electrical lines meet at junctions and switches? Do you have county boundaries that nest within states and do not overlap? Do you have vegetation classes that share boundaries with other environmental layers such as slope, aspect, and soil type polygons? In these kinds of cases, topology is very useful; in fact, it is essential.
The feature classes that participate in any topology must be organized into the same feature dataset. See topologies to read more about how you can use them within feature datasets to organize and manage the integrity of topological relationships during editing and update operations.

Task 4. Add capabilities for advanced data uses, analytic models (such as network analysis and geocoding), and advanced cartography.

• Do you want to model and use topological relationships to navigate through the nodes, edges, and faces of a topology? Will shared feature geometry help you more realistically model your features? For example, the polygon and line boundaries of numerous terrain data layers, such as feature classes for vegetation, slope, aspect, soil types, geology, water bodies, watersheds, ecological zones, and other environmental layers, nest within one another. Integrating their common boundaries using topology enables you to build much more robust and consistent attribute combinations. These greatly impact suitability/capability models and the ability to gain real insight into a problem. Topologies can also help you integrate features such as parcel systems, census units, administrative boundaries, and many other information sets. GIS users sometimes refer to this as vertical integration of GIS data layers.
• Do you want to model a transportation network? The geodatabase uses a network dataset to model these situations. A network dataset is a collection of edges, turns, and junctions through which you can model navigation and the flow of goods and resources. Each network has a set of navigation properties. These include the "cost" to travel along each edge and transfer onto another edge as well as the ability to model one-way, left-turn, and other travel restrictions and multimodal networks (combining trips using an automobile, a bus, walking).
A network dataset uses feature classes as data sources for edges, junctions, and turns. You specify the role each feature class will play in the network along with its navigation properties. The feature classes that participate in a network must be organized into the same feature dataset.

• Do you want to model utility networks? Electrical utilities and water, storm water, and sewer systems are modeled using a geometric network in the geodatabase. A geometric network is a set of connected edge and junction features used to model the flow of things such as electricity, water, gas, and storm water runoff. Each feature class is assigned a role in the geometric network as a collection of edges or junctions. The connectivity of the network is defined by feature properties and geometric coincidence. For example, valves (which are held as a point feature class) are connected to the endpoints of pipe segments (stored as line features). If the valve is open, water can flow through it in a specified direction.

• Do you want to use geocoding? For address geocoding, you add an address locator to your geodatabase. A locator is a combination of one of more feature classes containing addressable features (such as address range information for street centerlines) and a set of address styles and matching rules. Each locator dataset is used as the source for matching a single address or a large file of addresses to find address locations.

You can create locators and save copies of them independent of the geodatabase. This allows you to share your locators with many kinds of users for use in their own geocoding work.
• Do you want to use linear referencing to locate events or facilities along transportation lines? 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, stream lines, and pipelines. You can assign an m-value to each vertex in a feature. One very common 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 include 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:

• Do you want to model elevation using triangulated irregular networks? Or do you need to manage lidar or bathymetric point collections? The geodatabase has a terrain dataset to model surfaces using triangulated networks and to manage large multipoint collections such as lidar and bathymetry data. Terrains are used to manage massive 3D point collections (such as billion-point lidar collections) as well as other 3D features and to generate multiresolution TINs from these collections.

• Do you want to manage parcels or a cadastral database? A cadastral fabric is a dataset of connected parcels. In a cadastral fabric, parcels are represented by parcel line features, parcel point features, and parcel polygon features. Cadastral fabrics are created and managed using the ArcGIS Survey Analyst extension.

• Do you want to include cartographic representations and rules in your feature classes? A cartographic representation can be added to a feature class to hold drawing rules or alternative graphic representation for the map display of features. In GIS, most users automate mapping by defining a set of map layers. A map layer is a set of rules for how to symbolize and label features on each map. Sometimes layers are not enough to convey the information properly. For example, you might have street centerlines that connect at intersections. But, if you want to show bridges, overpasses, tunnels, and so forth, you cannot easily show these on your map.
A cartographic representation enables users to apply special overrides, rules, and graphics to ensure that the map representation is clear. For example, in a map display, road symbols exaggerate the size of roads and can cause conflicts with other features such as streams and buildings. With cartographic representations, you can offset some feature symbols to remove the conflicts—without having to change the underlying geographic location of the features. You can move the road representations off of the rivers and offset buildings from road symbols.