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Lecture 2 Flashcards

1
Q

What is Spatial Data?

A

Data that defines something about a location

Location- where?

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2
Q

What are example of vectors?

A

Points
Lines
Polygons
Discrete objects

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3
Q

What is raster data?

A

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.

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4
Q

What are Vector Points?

A

Represent discrete specific locations on the ground

A pair of X and Y coordinates

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5
Q

What are Vector Lines?

A

Represent linear features, such as rivers, roads and transmission cables.
A linked sequence of X and Y coordinates (joining the dots)

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6
Q

What are Vector Polygons?

A

Form bounded areas

A closed link sequence of X and Y coordinates

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7
Q

Raster don’t have…

A

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

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8
Q

What can raster data information allow for the understanding of?

A

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.

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9
Q

What are raster attributes?

A

Typically single numerical attributes stored in a pixel. Can have bands e.g aerial imagery

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10
Q

Raster precision/ resolution

A 
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
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11
Q

Layers

A

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

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12
Q

What is Topology?

A

Branch of maths and science

Looks at geometric characteristics and relationships

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13
Q

What are Map Projections?

A

•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

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14
Q

What are distortions?

A

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

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15
Q

Projection types

Conformal Map

A

–Preserves local angles and therefore shapes

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16
Q

Projection types

Equal-area (or Equivalence)

A

–Preserves areas

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17
Q

Projection types

Equidistant

A

–Preserves distance from a reference point or line

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18
Q

Projection types

Direction

A

–Azimuthal

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19
Q

Projection types

Compromise

A

–Concentrates on making the map ‘look right’ instead of preserving measurable properties

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20
Q

Projection surfaces (developable)

Tangent/Secant

A
  • 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
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21
Q

Projection surfaces (developable)

Mercator

A

•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

22
Q

Projection surfaces (developable)

Transverse Mercator

A
  • 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
23
Q

Projection surfaces (developable)

Azimuthal Equidistant

A
  • Azimuthal or planar projections
  • Distances are preserved
  • Distortion of area and shape increases from centre point
  • Typically used for polar or hemisphere maps
24
Q

Projection surfaces (developable)

Albers Equal Area

A
  • 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
25
Projection surfaces (developable) Gall-Peters Projection
* Equal-area * Cylindrical (Normal) * Controversy surrounding political and marketing issues
26
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
27
Projection surfaces (developable) Tissot’s indicatrix
Demonstrates spatial change and distortion caused by map projections No distortion in shape but is distortion in size
28
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 ```
29
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) ```
30
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)
31
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
32
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
33
Geoid
* Model used to represent the shape of the Earth * Earth not a sphere- (looks squashed and fatter round the middle) * Ellipsoid - Oblate Spheroid
34
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)
35
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
36
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
37
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.
38
OS GB National Grid – Coordinates in GIS
•For GIS need coordinates in XY –Cartesian Coordinate System –Use False Origin (0, 0) –Units in metres
39
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
40
Examples of vector points
Trees Buildings Sampling points
41
Examples of vector lines
Roads Rivers Contour lines
42
Examples of vector polygons
Boundaries Buildings River catchments
43
Examples of raster data
Scanned maps Aerial photography Surfaces (elevation, house prices)
44
Examples of topology
Connectivity (roads and bridges) Adjacency Containment Distance
45
What type of map projection is OS GB National Grid?
Transverse Mercator
46
What type of map projection is UTM – Universal Transverse Mercator?
Cylindrical Transverse Mercator Projection
47
What type of map projection is Robinson Projection?
Compromise projection
48
What type of map projection is Gall-Peters Projection?
Equal-area Cylindrical (Normal)
49
What type of map projection is Albers Equal Area?
Conic Projection
50
What type of map projection is Azimuthal Equidistant?
Azimuthal or planar projections
51
What type of map projection is Transverse Mercator?
Transverse cylindrical projection
52
What type of map projection is Mercator?
Normal cylindrical projection

GIS Lectures (10 decks)

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