Learning outcomes

• Explain the difference between data and information
• Describe the main characteristics of spatial data
• Give examples of map projections and why they are important
• Provide details of different methods of spatial referencing
• Define topology
• Explain the thematic characteristics of spatial data
• List the main sources of spatial data
• Explain why data standards are an important issue in GIS

Introduction

• Simplified / filtered view of the World
• Data and Information
  • Data – raw facts
  • Information – data with meaning and context added
• Modes
  • Spatial, Temporal, Thematic
  • A piece of data is 'spatial' only if it is referenced to location

Data sources

• Primary – captured specifically for use in your project
• Secondary – created for some other purpose but useful in your project
• Representations
  • Are needed to convey information
  • Fit information into a standard form or model
  • Almost always simplify the truth that is being represented

• Representations can rarely be perfect
• Details can be irrelevant, or too expensive and voluminous to record
• It’s important to know what is missing in a representation
• Representations can leave us uncertain about the real world

The Fundamental Problem

• Geographic information links a place, and often a time, with some property of that place (and time)
  • “The temperature at 34 N, 120 W at noon local time on 12/2/99 was 18 Celsius”
• The potential number of properties is vast
  • In GIS we term them attributes
  • Attributes can be physical, social, economic, demographic, environmental, etc.

• The number of places and times is also vast - potentially infinite
• The more closely we look at the world, the more detail it reveals - potentially ad infinitum
• The geographic world is infinitely complex
• Humans have found ingenious ways of dealing with this problem
• Many methods are used in GIS to create representations or data models

Discrete Objects and Continuous Fields

• Two ways of conceptualizing geographic variation

• The most fundamental distinction in geographic representation

• Discrete objects
  • The world as a table-top
  • Objects with well-defined boundaries
• Continuous Fields
  • Properties that vary continuously over space

Discrete Objects

• Points, lines, and areas
• Countable
• Persistent through time, perhaps mobile
• Biological organisms
• Animals, trees
• Human-made objects
• Vehicles, houses, fire hydrants

Continuous Fields

• Properties that vary continuously over space
• Value is a function of location
• Property can be of any attribute type, including direction
• Elevation as the archetype
• A single value at every point on the Earth’s surface
• The source of metaphor and language
• Any field can have slope, gradient, peaks, pits

Spatial entities - Vector

• Points
  • Single point (0 dimension), (x,y)
• Lines
  • Drawn between two point locations (1 dimension)
• Polyline – multiple line segments – list of point locations – (x1y1, x2y2 ... xnyn)
• Polygons (Areas)
  • As polyline but first & last points are equal thus ensuring closure
  • Represents area (2 dimensions)
• Note chosen representation varies with scale

Raster

Raster representation. Each colour represents a different value of a nominal-scale field denoting land cover class.

Maps and their influence
on the character of spatial data

• Traditional method of storing & analysing spatial data
• Shapes the way we think about space
• Purpose
  • Thematic
  • General purpose / topographic
  • Prerequisite for judging quality / accuracy / completeness of data

Scale

Ratio of distance on the map to distance on the ground
Large scale => small area & vice versa

Scale-related generalization

Generalization issues

• To avoid clutter some compromises must be made
• Selection
• Simplification
• Displacement
• Smoothing & enhancement

Projections

• Problem: how to flatten an (approximate) sphere (the earth) onto a 2d surface
• There are many reasons for wanting to project the Earth’s surface onto a plane, rather than deal with the curved surface
• The paper used to output GIS maps is flat
• Flat maps are scanned and digitized to create GIS databases
• Rasters are flat, it’s impossible to create a raster on a curved surface
• The Earth has to be projected to see all of it at once
• It’s much easier to measure distance on a plane

Distortions

• Any projection must distort the Earth in some way
• Two types of projections are important in GIS
• Conformal property: Shapes of small features are preserved: anywhere on the projection the distortion is the same in all directions
• Equal area property: Shapes are distorted, but features have the correct area
• Both types of projections will generally distort distances

Cylindrical Projections

• Conceptualized as the result of wrapping a cylinder of paper around the Earth
• The Mercator projection is the best-known cylindrical projection
• The cylinder is wrapped around the Equator
• The projection is conformal
• At any point scale is the same in both directions
• Shape of small features is preserved
• Features in high latitudes are significantly enlarged

Conic Projections

• Conceptualized as the result of wrapping a cone of paper around the Earth
• Standard Parallels occur where the cone intersects the Earth
• The Lambert Conformal Conic projection is commonly used to map North America
• On this projection lines of latitude appear as arcs of circles, and lines of longitude are straight lines radiating from the North Pole

The “Unprojected” Projection

• Assign latitude to the y axis and longitude to the x axis
• A type of cylindrical projection
• Is neither conformal nor equal area
• As latitude increases, lines of longitude are much closer together on the Earth, but are the same distance apart on the projection
• Also known as the Plate Carrée or Cylindrical Equidistant Projection

Projections:
(a) cylindrical, (b) azimuthal, (c) conic

Commonly used projections

Transverse Mercator

Irish Grid

Irish Grid

Irish Grid

Spatial referencing

• Geographic co-ordinate systems
  • Latitude/Longitude
• Rectangular co-ordinate systems
  • Projections
  • Grid
• Non co-ordinate systems
  • Placenames
  • Postal addresses
  • Postcodes

Latitude and Longitude

• The most comprehensive and powerful method of georeferencing
• Metric, standard, stable, unique
• Uses a well-defined and fixed reference frame
• Based on the Earth’s rotation and centre of mass, and the Greenwich Meridian

Definition of longitude

The Earth is seen here from above the North Pole, looking along the Axis, with the Equator forming the outer circle. The location of Greenwich defines the Prime Meridian. The longitude of the point at the centre of the red cross is determined by drawing a plane through it and the axis, and measuring the angle between this plane and the Prime Meridian.

Definition of Latitude

• Requires a model of the Earth’s shape
• The Earth is somewhat elliptical
• The N-S diameter is roughly 1/300 less than the E-W diameter
• More accurately modelled as an ellipsoid than a sphere
• An ellipsoid is formed by rotating an ellipse about its shorter axis (the Earth’s axis in this case)

Latitude of a point is the angle between a perpendicular to the surface and the plane of the Equator

• WGS 84
  • Radius of the Earth at the Equator 6378.137 km
  • Flattening 1 part in 298.257

The History of Ellipsoids

The History of Ellipsoids

• Because the Earth is not shaped precisely as an ellipsoid, initially each country felt free to adopt its own as the most accurate approximation to its own part of the Earth
• Today an international standard has been adopted known as WGS 84
• Its US implementation is the North American Datum of 1983 (NAD 83)
• Many US maps and data sets still use the North American Datum of 1927 (NAD 27)
• Differences can be as much as 200 m

Projection File example in WKT

PROJCS["TM65 / Irish Grid",
 GEOGCS["TM65",
  DATUM["TM65",
   SPHEROID["Airy Modified 1849",6377340.189,299.3249646, AUTHORITY["EPSG","7002"]],
   AUTHORITY["EPSG","6299"]],
  PRIMEM["Greenwich",0, AUTHORITY["EPSG","8901"]],
  UNIT["degree",0.01745329251994328, AUTHORITY["EPSG","9122"]],
  AUTHORITY["EPSG","4299"]],
 UNIT["metre",1,
     AUTHORITY["EPSG","9001"]],
 PROJECTION["Transverse_Mercator"],
 PARAMETER["latitude_of_origin",53.5],
 PARAMETER["central_meridian",-8],
 PARAMETER["scale_factor",1.000035],
 PARAMETER["false_easting",200000],
 PARAMETER["false_northing",250000],
 AUTHORITY["EPSG","29902"],
 AXIS["Easting",EAST],
 AXIS["Northing",NORTH]]

Georeferencing

• Is essential in GIS, since all information must be linked to the Earth’s surface
• The method of georeferencing must be:
  • Unique, linking information to exactly one location
  • Shared, so different users understand the meaning of a georeference
  • Persistent through time, so today’s georeferences are still meaningful tomorrow

Georeferencing - Uniqueness

• A georeference may be unique only within a defined domain, not globally
• There are many instances of Springfield in the U.S., but only one in any state
• The meaning of a reference to London may depend on context, since there are smaller Londons in several parts of the world

Georeferences as Measurements

• Some georeferences are metric
• They define location using measures of distance from fixed places
  • E.g., distance from the Equator or from the Greenwich Meridian
• Others are based on ordering
  • E.g. street addresses in most parts of the world order houses along streets
• Others are only nominal
  • Placenames do not involve ordering or measuring

Placenames

• The earliest form of georeferencing
• And the most commonly used in everyday activities
• Many names of geographic features are universally recognized
• Others may be understood only by locals
• Names work at many different scales
• From continents to small villages and neighbourhoods
• Names may pass out of use in time
• Where was Camelot?

Postal Addresses and Postcodes

• Every dwelling and office is a potential destination for mail
• Dwellings and offices are arrayed along streets, and numbered accordingly
• Streets have names that are unique within local areas
• Local areas have names that are unique within larger regions
• If these assumptions are true, then a postal address is a useful georeference

Where Do Postal Addresses
Fail as Georeferences?

• In rural areas
• Urban-style addresses have been extended recently to many rural areas
• For natural features
• Lakes, mountains, and rivers cannot be located using postal addresses
• When numbering on streets is not sequential
  • E.g. in Japan
• See http://www.openpostcode.org/ for the reasons why the Eircode proposal for Ireland is flawed

Postcodes as Georeferences

• Defined in many countries
  • E.g. ZIP codes in the US
• Hierarchically structured
  • The first few characters define large areas
  • Subsequent characters designate smaller areas
  • Coarser spatial resolution than postal address
  • Useful for mapping

UK Postcode System

Converting Georeferences

• GIS applications often require conversion of projections and ellipsoids
• These are standard functions in popular GIS packages
• Street addresses must be converted to coordinates for mapping and analysis
• Using geocoding functions
  • Placenames can be converted to coordinates using gazetteers

Topology

• Geometric characteristics of objects which do not change under transformation
• Three elements
  • Adjacency
  • Containment
  • Connectivity
• Useful in spatial analysis, routing etc.

Thematic characteristics of spatial data

• Entities and attributes
  • Entity -> something in which we have an interest
  • Attribute -> characteristics of an entity
• How does a line know it's a road as opposed to a river?
• Scales of measurement

Types of Attributes

• Nominal, e.g. land cover class
• Ordinal, e.g. a ranking
• Interval, e.g. Celsius temperature
  • Differences make sense
• Ratio, e.g. Kelvin temperature
  • Ratios make sense
• Cyclic, e.g. wind direction

Cyclic Attributes

• Do not behave as other attributes
• What is the average of two compass bearings, e.g. 350 and 10?
• Occur commonly in GIS
• Wind direction
• Slope aspect
• Flow direction
• Special methods are needed to handle and analyse

Sources of spatial data

• Apart from maps ...
  • Census and survey data
  • Aerial photographs
  • Satellite imagery
  • Field survey and GPS
• Anything that can record location

The Global Positioning System

• Allows direct, accurate measurement of latitude and longitude
• Accuracy of 10m from a simple, cheap unit
• Differential GPS capable of sub-meter accuracy
• Sub-centimetre accuracy if observations are averaged over long periods

GPS satellite constellation

Aerial photographs

LIDAR

Lidar is a remote sensing technology that measures distance by illuminating a target with a laser and analysing the reflected light

Data issues

• Data quality
  • GIS nearly always relies on externally sourced data – unlike other IT systems
  • Need to be confident in data source
  • Need to be aware of characteristics of data – provenance, age, method of collection, resolution, known error
  • Quality => fitness for purpose – comes at a cost
• Data standards
  • Open Geospatial Consortium (OGC)
  • Technical standards – file formats, database representations etc
  • Interchange standards - XML-based or, more recently based on formats such as JSON