Using Spatial Data with SAS/GIS Software

The SAS/GIS Data Model

SAS/GIS software uses two basic types of data:

Spatial Data: Describe the location, shape, and interrelationships of map features
Attribute Data: Provide information that relates to the map features.

Spatial Data Features

SAS/GIS software uses spatial data to represent the following three types of map features:

Point Features: Consist of individual locations that are shown as symbols, representing real-world locations of special points of interest.
Line Features: Consist of sequences of two or more coordinates that form zero-width shapes, either closed or unclosed. Line features represent entities that either have no width, such as political boundaries, or those that can be represented as having no width, such as streets or water pipes.
Area Features: Consist of sequences of three or more coordinates that form polygons (with single or multiple boundaries and with or without holes.) Area features represent two-dimensional entities such as geographic areas (countries, states, and so forth) or floorplans for buildings.

SAS/GIS Topology

To represent point, line, and area features in the map, SAS/GIS software defines the following topological features in the spatial data:

Chains

Are sequences of two or more points in the coordinate space. The end points (that is, the first and last points of the chain) are nodes. Each chain has a direction, from the first point toward the last point. The first point in the chain is the from-node and the last point is the to-node. Relative to its direction, each chain has a left side and a right side.

Points between the from-node and the to-node are detail points, which serve to trace the curvature of the feature that is represented by the chain. Detail points are not nodes.

Nodes

Are points in the spatial data coordinate space that have connections to one or more chains.

Areas

Are two-dimensional finite regions of the coordinate space. One or more chains, called boundary chains, separate two different areas. Chains that lie completely inside an area are called internal chains and are bounded on the left and right sides by the same area.

The spatial data coordinate space can be represented in any numeric units even those that include arbitrary values. Coordinates that are stored as longitude and latitude values have a maximum usable precision of about one centimeter.

Representations of map features are implemented with one or more chains, as follows:

Point features: Are implemented with one chain, one node (that is, the from-node and to-node for a point feature are the same node), and no detail points.
Line and Area Features: Are implemented with one or more chains and one or more nodes.

Rules for Topological Correctness

SAS/GIS spatial data must obey the following rules in order for the topology to be correct. These rules are similar to the rules for TIGER/Line files from the U.S. Bureau of the Census. For more information on these rules, see Gerard Boudriault's 1987 article, "Topology in the TIGER File" in AUTO-CARTA 8, Proceedings, pages 258-263, published by the American Society for Photogrammetry and Remote Sensing and the American Congress on Surveying and Mapping.

Rule: Topological Completeness

All chains must be

Bounded by two nodes, the from-node and the to-node
Note: In chains for point features and for single-chain closed-loop line features or area boundaries, the from-node and the to-node are the same node, but both are still included in the chain definition.
Bounded by two areas, one on the left and one on the right.

These relationships must be complete, so the following two rules apply:

The sides of all the chains incident to any given node must form a cycle. A cycle consists of one or more chains that start or end at the same node.
The end points of chains that bound an area must form one or more disjoint cycles.

For each unique area ID or unique set of area IDs, all the boundary chains that have the ID value (either on the right or left, but not both) form one or more closed loops or cycles.

Rule: Topological-geometric Consistency

The collection of chains, nodes, and areas must have coordinates that make the collection a disjoint partitioning of the coordinate space. The following four conditions must be true to avoid problems with displaying the spatial data:

No two points in the combined set of nodes and detail points share the same coordinate.
No two line segment interiors share a common coordinate.
No two areas share a common coordinate.
Note: Graphically overlaid data may have overlapping polygons, chains, and nodes and have no topological interconnectivity
Polygons that form the boundaries of holes inside areas must fall completely within the enclosing areas.

Note: Edge-matched data share coordinates along the common boundaries, but each chain should have the proper polygonal ID values on the side that represents the outside edge of their respective physical coverages as well as the inside edges. [cautionend]

Problems Resulting from Topological Errors

Topological errors in the spatial data cause the following types of problems:

A polygonal index cannot be built for all the polygons for a particular area set.
A successfully indexed polygon does not close because
- the chains for a node do not form a cycle, which is sometimes the result of left- and right-side values being swapped for one or more of the connected chains
- a chain crosses another chain's interior coordinated space.
Multiple features are selected when only one selection is desired because of overlapping features in a coordinate space.
Select Like Connected processing fails to select apparently connected chains.

Attribute Data

Attribute data are all other data that are related to map features in some way, including the data that you want to analyze in the context of the map. Attribute data can be stored in the spatial database by the following methods:

Directly with the spatial data as variables in the chains data set
Indirectly in SAS data sets that are joined to the chains data set by a link that is composed of one or more variables.

Attribute data can be used as follows:

As themes for map layers.
By actions that display or manipulate the attribute data when features are selected in the map. Actions can be defined to display the attribute data, create new SAS data sets that contain subsets of the attribute data, or submit SAS programs to process the attribute data.

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