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CS 128/ES 228 - Lecture 4b 1

Spatial Data Formats


Stages of development:

1. Conceptual model: select the features of reality to be modeled and decide what entities will represent them

2. Spatial data model: select a format that will represent the model entities

3. Spatial data structure: decide how to code the entities in the model’s data files


2. Spatial data models

1. Raster

2. Vector

3. Object-oriented

Spatial data formats:


Raster format

Features represented by cell contents

Spatial precision limited by cell size

Surfaces modeled as continuous values


Vector format Spatial precision

limited by number format

Discrete features explicitly represented

Surfaces shown by contours rather than continuous values


Object-oriented formats

Leave details for Dr. Levine!


Thematic data (a.k.a. “attribute data”)

Quantitative or descriptive

May represent 1 or many themes

Tied to a spatial reference

Represented differently in raster vs. vector formats


Scales of measurement

Data Unit Scale

Resort name text Nominal

Resort ranking value Ordinal

Winter temp. oC Interval

Size of ski area m2 Ratio

Heywood et. al. 1998 – Table 2.1


Spatial modeling in raster format

Basic entity is the cell

Region represented by a tiling of cells

Cell size = resolution

Attribute data linked to individual cells


Tesselation

A closed shape or polygon that repeats on all sides without any gaps or overlaps

Three regular polygons tesselate the plane:

Square Equilateral triangle Hexagon


Tilings

In 1922 Escher visited the Alhambra palace and saw the wall tilings of the Moors. He was excited to find other artists who had been captivated by tilings, but also made this revealing comment: "What a pity their religion forbade them to make graven images."


Escher’s “tesselations”


Raster model – close up


Advantages of raster format

many data sets available

different file formats readily inter-converted

fast computer lookupand display

easy to overlay multiple themes

able to represent multiple continuous surfaces


Limitations of raster format

poor representation of discrete objects

constant resolution throughout region modeled

exact boundary location difficult

network analysis nearly impossible

difficult to change projection or coordinate system