GeoReferencing

GeoReferencing
- What is it?
  - = geographic reference
  - provides a unique description for locations on earth's surface
- Non-metric
  - without measure
  - examples
    - Placename (London, Ontario)
    - Landmark (Eifel Tower)
    - Street Address (900 Fifth Street, Nanaimo, BC)
    - Postal Code (V9R 5K5)
    - Phone Exchange (250, 604)
    - Census Tract
    - Political Riding
    - Cadastre
  - nominal or ordinal in nature (placename vs. street address)
  - "point" or area based (street address vs. postal code)
  - Issues
    - may be 'local in nature" (meet you at Chadwick's field)
    - at 'global scale' may have repeats (Paris - France, Cdn: ON, YU, USA: AK, ID, IL, IN, IA, KY, MN, MI, MS, MO, NH, NY, OH, OR, TN, TX, VA)
    - often adequate for cultural features, not so much for 'the natural world'
    - anomalies - street addresses in some cities in Japan are chronological not 'linear'
- Metric
  - measured
  - examples
    - "Linear Offset" (e.g. 124 m north of Oak & Main)
    - Latitude / Longitude
    - UTM eastings & northings
    - Others
      - Public Land Survey System (w US only)
      - State Plane Coordinate System (US only)
  - point-based (producing coordinates)
  - advantages (wrt lat/long & UTM)
    - infinite resolution
    - good for all parts of globe (cultural or natural)
    - no ambiguity
      - only 1 location possible
      - but ...
        
        lat/ long
        
        E vs W and N vs. S
        
        digital ... + vs. -
        
        UTM
        
        need zone
        
        N vs. S
- Closer look at Lat. / Long.
  - parallels & meridians
  - how are (were) latitudes & longitudes measured?
    - latitude ... N star (surface measure)
    - longitude ... local noon & Greenwich time
  - DMS -> dd.ddd (computers need decimal degrees)
    - 30 min = 0.5 degree (30/60)
    - 30 sec = 0.008 degrees (30/3600)
    - convert 36 (d) 14 (m) 44 (s) to dd.dddd
    - convert 59.7025 to dd.mm.ss
  - shape of the earth
    - sphere not flat
    - ellipsoid
      - centrifugal force
      - polar flattening
      - 1 / 298 (~ 0.3%)
      - not imp. for small scale (large area) maps
      - very imp. for large scale (small area) maps
      - latitude on sphere
      - geographic latitude on ellipsoid
    - geoid
      - = MSL ... extended thru continents
      - where gravity has = force
      - variation in gravity (density) ...
      - ... geoid undulates
  - datums
    - datum = a reference surface used to determine location (coordinates)
    - ellipsoid and how it is "attached" to earth
    - also ...
      - a survey control network (Meades Ranch, Kansas)
      - MSL for elevation (i.e. vertical datum)
    - local vs. global
      - excellent "local fit", v. accurate, but ...
      - intercontinental ballistic missiles
      - navigation (long distances)
    - time ... NAD 27 vs. NAD 83
    - can be significant difference btwn NAD 27 & NAD 83
      - Nanaimo: 49.1642N. 123.9364W
      - Sydney NS: 46.1378N, 60.1829W
- How to determine (interpolate) a map coordinate
  - method 1: two ratios ... X-multiple & divide
  - method 2: prorate ... calc. proportion, then multiply
  - UTM
    - cartesian ("square") grid
    - often light blue grid lines on map
    - NOT true N-S / E-W
    - can interpolate "within the box"
  - Lat/Long
    - usually not projected "square"
    - UTM grid often "tilted" ... beware!
    - may need to "transfer" location to neatline ...
    - ... or look for tics (and "make a box")