2 Representing the world spatially

Model connecting signs, thoughts and reality

Semantic triangle

What does the semantic triangle explain?

We always abstract from the real world and abstractions are mediated by thought. These representations may be influenced by culture, experience or language

What do the historical examples show us?

There are many ways of representing geographic information

Ancient Chinese map carved into stone

Grid

Yu Ji Tu

Printed Swiss map

South at the top

Tabula Nova Heremi Helvetorium

Written text describing Norse exploration of North America using place names, routes and descriptions of places

Icelandic saga

Document describing ownership of a land parcel

Property deeds on sheepskin

Marshallese charts tracking island locations, currents and swells of the Pacific, used to memorise the locations beforehand

Rebbelib

Newspaper print describing the previous day’s weather with isobars

Early weather map

What do these representations have in common?

• communicate geographic information

• abstract real world with signs or symbols

• influenced by culture and mental models

• decisions were made to decide how to represent what

How do these representations differ?

intents and purposes

What two key choices have to be made when describing objects?

Multidimensional building blocks of geographical information

Atoms of geographic data a = < l, t, d >

What are the two conceptual models of space?

Discrete objects

Continuous fields

What do conceptual models help us with?

understand ways of describing space

What is the limit of conceptual models?

no direct solution for digital representation

Features with clear,

definable boundaries

Discrete objects

Geographic phenomena measurable everywhere

Continuous fields

Conceptual model for counting distinct entities

Discrete objects

Conceptual model for elevation or temperature

Continuous fields

Core triplet elements of geographic data

Location

Dimensionality of point data features

0D

Dimensionality of line data features

1D

Dimensionality of area or polygon data

2D

Attribute type used for IDs or names

Nominal

Attribute type with true or false choices

Binary

Attribute type with naturally ranked values

Ordinal

Attribute type where differences can be calculated

Interval

Attribute type that can be ranked and used to calcualate percentiles

Ratio

Data structure dividing space into regular cells with a given resolution, each cell has values e.g. height or RGB values

Raster

Data structure storing discrete objects as points, liner or areas, each object has at least one attribute

Vector

Which data structure is used for continuous fields?

raster

How to model choosing routes?

• data structure

• attributes

vector: discrete objects (lines)

attributes: nominal (road names or IDs), ratio (speed limits)

How to model managing and dispatching resources?

• data structure

• attributes

vector: discrete objects (points representing accident, helicopter and hospital locations)

attributes: vary, nominal (names of those involved), ordinal (severity of injuries)

How to model managing and dispatching resources?

• data structure

• attributes

raster: continuous field

attribute: ordinal (priority of sites)

How to model managing and dispatching resources?

• data structure

• attributes

raster: continuous field

attributes: ratio (measure spectral signals)

How to model monitoring deforestation?

• data structure

• attributes

continuous field or discrete object

continuous field: total deforestation

discrete object: identifying patches

How can you add the third dimension?

represent elevation with raster

add attributes to represent information about volume

(simplified)

Human-readable aspatial data file format

CSV

Human-readable vector data file format

GeoJSON

Human-readable raster format with a text header

ESRI ASCII raster

Proprietary vector format (from ESRI) requiring three files

Shapefile

Non-human-readable raster format for remote sensing, used for sharing raster data

GeoTIFF

Database format storing both vector and raster, default in QGIS

GeoPackage

Which files are exclusively for raster data?

ESRI ASCII

GeoTIFF

Which files are exclusively for vector data?

Shape files

GeoJSON