Coverages

(Getting Started with ArcGIS starting on p. 50)

Coverages contain primary, composite and secondary feature types.

Primary features in coverages are

• label points,
• arcs, and
• polygons.

Composite features are built from primary features.

Secondary features are

• tics,
• links,
• annotations which are used to provide text about the geographic features on the maps.

Primary Features

Label points can represent individual point features (e.g. wells, intersections etc). The points label or link the attributes to polygons. Each polygon in a coverage has a single label point with its feature ID number, usually at the center.

Arcs are connected sets of line segments, with nodes at the endpoints. Arcs can be organized into polygons that represent areas.

Nodes are the endpoints or and connections between arcs. Nodes can have attributes so that they can represent point features in a network such as values in a network of water mains. Nodes are important because they connect "features" to each other. (form the topology???) Although Nodes are not technically listed as features they act like a primary feature Essentially a node is a label point. Not every conjunction between two lines is a node. An arc may be made up of several line segments, but only the ends of the arc are nodes.

Polygons represent areas bounded by arcs, including island polygons. We DEFINE polygons such that they do NOT OVERLAP. a point can fall into at most one polygon.

Secondary Features

Annotations are just that, annotations. They are stored "in geographic coordinates" so that they maintain position and scale relative to other coverage features. I'm guessing you can think of these as vector elements that scale with the map.

Tics are geographic control points. They represent known locations on the ground and are used to register and transform the coordinates of a coverage. For our archaeology project, we should use our actual predetermined points as control points.

Links are displacement vectors that are used to adjust the shape of coverages, for example, to match the edges of adjacent coverages. Links consist of a "from-point" and a "to-point" from a location on one coverage to a identical location on a second coverage.

Coverage Topology

Topology is the procedure for explicitly defining and using the spatial relationships inherent in feature geometry. The three major topological relationships that coverages maintain are:

1. Connectivity
2. Area
3. contiguity

Coverages implement topology and explicitly record these spatial relationships in special files.

Connectivity is stored by coverages by recording the nodes that mark the endpoints of arcs. Arcs that share a node are connected. They call this arc-node topology.

Areas are defined by coverages by keeping a list of connected arcs that form the boundary for the polygon - called polygon-arc topology.

Contiguity is defined by coverages by storing a list of the polygons on the left and right side of each arc - called left-right topology.