Lecture 2 Flashcards
What is Spatial Data?
Data that defines something about a location
Location- where?
What are example of vectors?
Points
Lines
Polygons
Discrete objects
What is raster data?
Consists of a matrix of cells (or pixels) organized into rows and columns (or a grid) where each cell contains a value representing information, such as temperature.
What are Vector Points?
Represent discrete specific locations on the ground
A pair of X and Y coordinates
What are Vector Lines?
Represent linear features, such as rivers, roads and transmission cables.
A linked sequence of X and Y coordinates (joining the dots)
What are Vector Polygons?
Form bounded areas
A closed link sequence of X and Y coordinates
Raster don’t have…
Discrete objects but models of continuous data over an area
A grid of pixels or cells where each cell has a value and an area
What can raster data information allow for the understanding of?
Something that is going to vary over a surface
Arranged in a way that is complementary to computer use
Can be used to overlap data on maps
You can take a spectral fingerprint of a surface to find out about it.
What are raster attributes?
Typically single numerical attributes stored in a pixel. Can have bands e.g aerial imagery
Raster precision/ resolution
Higher precision = smaller cell size More cells required for same coverage More detail More processing power required This will take the software a lot longer to work through
Layers
Location and attribute
Layers can be combined visually
Can be moved up and down (reorder)
Information can be extracted from attributes based on location
Can understand relationships between different types of data
What is Topology?
Branch of maths and science
Looks at geometric characteristics and relationships
What are Map Projections?
•A method to represent the surface of the Earth (or other 3D body) on a plane
•Taking a globe and flattening it onto a map
•Why?
–Create Maps
–Enables easier measurement
What are distortions?
Map projections will cause distortions to the surface of the Earth, e.g
Area
Shape
Direction
Distance
Scale
Some projections can preserve different properties of the Earth
No projection can preserve all
Projection must be chosen to suit the purpose of the map
Projection types
Conformal Map
–Preserves local angles and therefore shapes
Projection types
Equal-area (or Equivalence)
–Preserves areas
Projection types
Equidistant
–Preserves distance from a reference point or line
Projection types
Direction
–Azimuthal
Projection types
Compromise
–Concentrates on making the map ‘look right’ instead of preserving measurable properties
Projection surfaces (developable)
Tangent/Secant
- Tangent – touches the Earth’s surface (i.e. the equator if projection is normal)
- Secant – intersects the globe creating parallel secant lines
- Scale at tangent and secant lines is constant – no distortion
Projection surfaces (developable)
Mercator
•Normal cylindrical projection
•Straight rhumb lines (line of constant direction) good for navigation
•Distortion increases away from equator
–E.g. Greenland appears to equal Africa in size, in reality Africa 14x larger
Projection surfaces (developable)
Transverse Mercator
- Transverse cylindrical projection
- Distances are only true along the central meridian (if cylinder is tangent) selected by mapmaker
- Accuracy reasonable within 15° of central meridian
- Used for national to large scale mapping
- Projection used by OS for their maps
Projection surfaces (developable)
Azimuthal Equidistant
- Azimuthal or planar projections
- Distances are preserved
- Distortion of area and shape increases from centre point
- Typically used for polar or hemisphere maps
Projection surfaces (developable)
Albers Equal Area
- Conic Projection
- Secant
- Equal area but the shape and scale of countries is changed as you move around the map
- Shape and scale are accurate at secant intersections
- Distorted elsewhere
Projection surfaces (developable)
Gall-Peters Projection
- Equal-area
- Cylindrical (Normal)
- Controversy surrounding political and marketing issues
Projection surfaces (developable)
Robinson Projection
- Compromise projection
- Used to show whole world
- Aims to make the world “look right”
- Uses tabulated coordinates to project map
Projection surfaces (developable)
Tissot’s indicatrix
Demonstrates spatial change and distortion caused by map projections
No distortion in shape but is distortion in size
Spatial Referencing
•Geographic co-ordinate systems Latitude and Longitude •Cartesian co-ordinate systems OS National Grid Axes and direct measurements are available •Geocodes Postcodes Descriptive term to represent an area
Latitude
•Used to indicate North-South position on the Earth’s surface •Lines run East-West •Lines are parallel •Equator at 0° latitude •Range: –0° to 90° North (e.g. 35°N) –0° to -90° South (e.g. 65°S)
Longitude
•Used to indicate East-West position on the Earth’s surface
•Lines run North-South
•Converge at Poles
•Lines of Longitude also called meridians
•Prime meridian runs through the Royal Observatory in Greenwich, London - 0° longitude
•Range:
–0° to 180° East (e.g. 10°E)
–0° to -180° West (e.g. 142°W)
Given as an angle
λ – Longitude
Angle between line connecting a point on Earth’s surface with the centre of the Earth and a line between the prime Meridian and the centre of the Earth
Given as an angle
φ – Latitude
Angle between line connecting a point on Earth’s surface with the centre of the Earth and a line between the equator and the centre of the Earth
Geoid
- Model used to represent the shape of the Earth
- Earth not a sphere- (looks squashed and fatter round the middle)
- Ellipsoid - Oblate Spheroid
Cartesian Coordinates
- Locate a point uniquely by measuring the perpendicular distances from a pair of axes
- Where the axes meet is called the origin.
- Coordinates typically reported X, Y
- Can have a third dimension – height (Z)
UTM – Universal Transverse Mercator
•Cylindrical Transverse Mercator Projection
•Divides globe into 60 vertical projection zones to increase accuracy
•Units are in metres
•Grid squares are consistent in size and shape (lat lon grid squares are not)
–aids measurement
•Regions that cross multiple projection zones can lead to problems
OS GB National Grid
•Transverse Mercator
–Conformal at large scale. Shape, area and distance are correct
–Transverse as country runs North-South
•Secant
–Two lines of zero distortion. Minimal distortion on land
OS GB National Grid - Coordinates
•Great Britain covered by grid squares measuring 100 km x 100 km
–Each has a two letter code as identification
•Each square can then be divided into 100, 10 km x 10 km squares
–The squares are numbered from 0 to 9 from the South-West corner in Easterly (left to right) and Northerly (upwards) directions allowing them to be further identified
•That square can be subdivided again, and again until the required level of precision is reached.
OS GB National Grid – Coordinates in GIS
•For GIS need coordinates in XY
–Cartesian Coordinate System
–Use False Origin (0, 0)
–Units in metres
Geocodes
•Geographic data which gives location of a feature
•E.g. Addresses, Postcode
•Geocoding will get direct coordinates for this data
–This is required for plotting in GIS
Examples of vector points
Trees
Buildings
Sampling points
Examples of vector lines
Roads
Rivers
Contour lines
Examples of vector polygons
Boundaries
Buildings
River catchments
Examples of raster data
Scanned maps
Aerial photography
Surfaces (elevation, house prices)
Examples of topology
Connectivity (roads and bridges)
Adjacency
Containment
Distance
What type of map projection is OS GB National Grid?
Transverse Mercator
What type of map projection is UTM – Universal Transverse Mercator?
Cylindrical Transverse Mercator Projection
What type of map projection is Robinson Projection?
Compromise projection
What type of map projection is Gall-Peters Projection?
Equal-area Cylindrical (Normal)
What type of map projection is Albers Equal Area?
Conic Projection
What type of map projection is Azimuthal Equidistant?
Azimuthal or planar projections
What type of map projection is Transverse Mercator?
Transverse cylindrical projection
What type of map projection is Mercator?
Normal cylindrical projection
