Lecture Notes for Clarke, K. C. Analytical and Computer Cartography

Lecture 7: Spatial Data Structures for Computer Cartography

"The Spatial Data Transfer Standard has provided cartographers with a consistent set of terminology and concepts, known as a data model, around which data structures can be developed." (Clarke, p. 115)

Data Structures and Programming

• Data Model maps onto a data structure
• Data structure eventually implies programming structure
• Unstructured computer programming languages did not support data structures well
• Structured languages (e.g. C, Pascal) allow definition of structures directly
• Link between the physical storage of data and the data's use in mapping systems

Spatial Data Transfer Standard

• Draft standard published in The American Cartographer (1988)
• FIPS 173 approved 1992
• Standard consists of several sections
• Introduction
• Spatial Data Concepts
  • Formalization of spatial data and models
  • Entity Instance: A spatial phenomenon of a defined type
  • Entity Object: A digital representation of an entity instance.
• Spatial Data Quality
  • Lineage
  • Positional Accuracy
  • Attribute Accuracy
  • Logical Consistency
  • Completeness
• General Specification
  • Uses Modules
  • Transfer takes place in ISO 8211 formats
  • Large number of feature types and language descriptions at 2 levels

Zero Dimensional Objects

• Most primitive object is the POINT
• Can be (x,y) or (x,y,z)
• Consists of geocodes for location in a standard system
• Should be in world not image geometry
• If significant topologically, is a node.
• Can identify a feature (entity) or a label (label)
• Can be INSIDE an area and carry its identification information

One Dimensional Objects

• Divide up by lines with and without topological significance
• Primitive object is the segment
• Segments connect to make a string (line or polyline)
• If defined mathematically, use arc
• If line segment connects nodes, called a link (for a network)
• Topological versions carry end node and or left and right polygon data
• Complete, area and network chain versions
• Area-like objects are G-ring and GT-ring

Two Dimensional Objects

• Interior area is the space contained by the polygon, i.e. the object not the boundary
• G-polygon contains graphical objects that form a polygon, e.g. a ring
• GT-polygon contains conplete topology
• Toplogical encoding requires universe and void polygons.
• Special objects
  • pixel (the smallest non-divisible element of a digital image)
  • Grid cell (same as pixel but for a grid)

Aggregate Objects

DIGITAL IMAGE

two dimensional array of regular pixels

GRID

Set of grid cells forming a regular or near regular tesselation

LAYER

• Areally distributed set of spatial data representing entity instances within on theme, or with a common attribute.
• Usually registered with other layers.

RASTER

One or more overlapping layers from the same grid or digital image.

RED

GREEN

BLUE

GRAPH

Planar Graph: Node and link/chain set as applied to a plane surface

Two-dimensional Manifold: Planar graph with all included objects

Network: A graph without two-dimensional objects (links do not have to intersect)

Limitations

• Three dimensional objects
• time-sensitive objects
• Links to other standards e.g. PHIGS, GKS
• Implementation slow via profiles

Keith C. Clarke Last Update 4/21/97 Copyright Prentice Hall (1995)