|Using Spatial Data with SAS/GIS Software|
SAS/GIS software uses two basic types of data:
|Spatial Data Features|
SAS/GIS software uses spatial data to represent the following three types of map features:
To represent point, line, and area features in the map, SAS/GIS software defines the following topological features in the spatial data:
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.
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:
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.
All chains must be
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.
These relationships must be complete, so the following two rules apply:
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.
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:
Note: Graphically overlaid data may have overlapping polygons,
chains, and nodes and have no topological interconnectivity
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.
Topological errors in the spatial data cause the following types of problems:
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:
Attribute data can be used as follows:
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