Data Structure

Objectives:

• Reminder of Map Booby Prize preliminary heats in lab sections (Map Library Suzallo basement Wednesay/Thursday lab sections)
• Reminder of check out procedures for GPS units for Project 6 (Geography Main Office)
• Cartographic data structures
• Event 20: diagram two data structures

A data structure encodes a schema of the objects to be represented, their attributes and their relationships to each other.

Some simple data structures only allow one class of objects and their attributes, creating a matrix or "flat file" (see Brian Berry, 1964 Geographical Matrix). An example of this would be statistical packages, like SPSS.

An object has one "special" attribute - its IDENTIFIER
Linkage from one object to another by reference to ID,
or through pointers to address in memory (pointers)

Big Distinction in Cartographic Data Structures

As described earlier under methods of measurement (borrowing from Sinton)
Space, Theme and Time: one can be measured if another is controlled and the last is fixed.
RASTER : control space, measure attribute (theme) (TIFFs, GIFs, JPEGs and image processing)
the basic unit is a PIXEL (a rectangular unit of space, coded with some attribute value)

VECTOR : create OBJECTS that represent FEATURES, (based on attribute)
measure their location. (PostScript ... and more)

This distinction used to separate the software solutions. Now ArcMap does both. You have worked with the vector side (shape files), but there is Spatial Analyst that works with GRIDs (raster).

Data Structure alternatives (mostly within vector approach)

Vector data structure follow progression of increasing complexity, internalizing the logic
of map interpretation into the data structure -- increase in EXPLICIT relationships.

Spaghetti Files

Some storage of cartographic images is "no more structured than spaghetti on a plate"
The image consists of lines and points which the reader must organize.

Shape Files (simple loops for polygons)

One advance is to store the polygon as object (see def. of polygon)

This permits shading of the polygon (ArcView shape files and ESRI's "spatial database" SDE - the high end product - computes the topological relationships on demand from a shape file)
Problems: slivers between adjacent polygons because boundaries not nec. the same.

Topological Data Structure

Organizes Points, lines and areas as nodes, chains and polygons.

Boundary and coboundary:

• nodes bound chains, chains bound polygons....
• chains cobound nodes, polygons cobound chains....
  

Provides for contiguity, better quality control, and other features...
Examples: DIME (US Census), ODYSSEY and modern GIS (Arc/INFO "coverages")

What is this TOPOLOGY stuff anyway?

Understanding Topology:

What it is NOT: Topography: measurement/representation of earth elevation
and related features (a form of general/reference map)

Topology: study of basic spatial relationships based on intuitive notions of space
(those not requiring measurements - just dimensions)

[the higher levels of mathematics of space (collectively called Geometry):
Graph theory adds direction (even distances) to a network (still based on connectivity)
Analytical Geometry: coordinate measurements; metrics of distance, orientation
Differential geometry: projections

The basic spatial relationships: connectivity, contiguity

• points (nodes) bound lines
• lines (chains) bound areas
• areas (polygons / faces) bound volumes

Why topology in Cartography?

Some readings:

• ESRI shape file description
• ESRI on topology in ArcMap