Lecture 3: Spatial Reference Systems

Longitude

• Measurement of location east to west of Prime Meridian, which passes through geographic poles and Greenwich, England

• X-coordinate value

• A.k.a. Meridians

Latitude

• Measurement of location north or south of the Equator

• Y-coordinate value

• A.k.a. Parallels

Degree, Minutes, Seconds (DMS)

• Sexagasimal (base 60) system

• Assumes 1 degree = 60 minutes (1 minute = 60 seconds)

• Decimal Degrees (DD) = Degrees + (Minutes/60) + (Seconds/3600)

Geographic Coordinate System

• Based on 3D ellipsoid model of the Earth

  • Ellipsoid → elongated sphere used to approximate the shape of the Earth (geometrical model of the Earth)

  • Geoid → conceptual model used to approximate the shape of the Earth (represents Mean Sea Level under gravity and rotation)

  • Horizontal Datum → comprises an ellipsoid and reference points to anchor the model to known points on the Earth’s surface (local + global)

• Uses angular measurements (i.e., longitude and latitude)

• Prime Meridian

Projected Coordinate System

• Transforms 3D surface of the Earth to a 2D plane

• Uses linear measurements (e.g., metres)

• Benefits:

  • Easier interpretation of 2D representation of data

  • Detailed local views of data

  • Easier to measure distance, area, and directions

• Datum

• Projection method

• Projection properties

• Central Meridian

• Standard parallels (latitude of origin)

• Coordinate origin

• Scale factor

Projection Surfaces

1. Planar

• Touches Earth at a single point

• Suitable for polar regions

2. Conical

• Suitable for mid-latitude regions with a greater east-west extent

3. Cylindrical

• Suitable for equatorial regions and global maps

Tangent Projections

Surface touches the Earth at a single line

Secant Projections

Surface intersects the Earth at two lines

Projection Types

1. Equal area

• Preserves area

• Distorts shape + distance

• Suitable for thematic maps and measuring areas

2. Conformal

• Preserves shape

• Distorts area + distance

• Suitable for local maps and navigation

3. Equidistant

• Preserves distance

• Distorts area + shape

• Suitable for measuring distance

Mercator Projection

• Cylindrical conformal

• Preserves shape and distorts area

• Appropriate for navigation but not world maps

• Variations in Mercator include versions designed for web mapping (based on sphere model of the Earth)

• Tissot’s indicatrix circles illustrate scale of distortion on a map

Universal Transverse Mercator (UTM) Projection

• Secant cylindrical conformal

• Preserves shape but minimally distorts area and distance within each zone

• Appropriate for large-scale maps such as city or construction plans