Geography 176B Technical Issues in GIS

Lecture 3: Geocoding Spatial Data

Models of the World

World model used for GIS work includes the sphere and the geoid.

Model allows selction of the map geodetic datum.

• Most often this reflects the local coordinate system in use.
• Common geoids for North America are: NAD27, NAD83 and WGS84.
• Errors of datum can be hundreds of meters vertically and horizontally.
• Translations are possible using several alternative special purpose progams or a GIS.

Map Projections

• Map projections can use sphere, ellipsoid or the geoid.
• Simple basis os the "developable form" (plane, cylinder, cone)
• Scale is twofold: (1) Scale multiplier and (2) local scale.
• No flat map can be both conformal and equivalent.
• Map projections always sacrifice something, such as continuity, one-to-one etc.
• Can be interrupted in many places.
• Can be equatorial, transverse or oblique.
• Can be tangential or secant. Secant "cuts" the earth along a line.
• 
• Very common US map is lambert conformal conic, secant at 45 and 30 degrees North.
• GIS packages can convert between map projections. e.g. Tralaine and Atlas*GIS
• In Arc/Info use the PROJECT command
• Arc: project
• Usage: PROJECT <COVER | GRID | FILE> <input> <output> {projection_file}
• {NEAREST | BILINEAR | CUBIC} {out_cellsize}
• 

Coordinate Systems

• Most basic coordinate system is the geographic.
• 
• Most GIS systems store map reference data in geographic coordinates in decimal degrees.
• Data are projected as needed.
• Also common to store GIS files in single coordinate system.
• Other systems in use;
• UTM
  • World Divided into 60 zones, six degrees wide
  • Civilian version, 84N to 80S.
  • Strip map of earth drawn on Transverse Mercator, centered on central meridian.
  • False origin for easting, central medridian placed at 500,000mE
  • Origins for northing at S. Pole and Equator.
  • Zones are adjusted (made secant) by scaling along central meridian
  • System printed on many maps, including USGS
• UPS
• Military Grid
• 
• 
• State Plane
• World Geographic Reference System
• Landsat WRS (Path and Row)
• National Grid Systems e.g. UK.

Land Partitioning Systems

Cadastral Systems

• In East: Metes and Bounds
• Majority of Country covered by USPLS.
• Series of state systems with Base and "geographers" lines.
• Township and Range System

Projection Distortion

• Has a spatial distribution.
• Can be scale, direction or area distortion.
• Often visually shown by Tissot's indicatrix.

Address Matching

• Conversion of a street address to a georeference in a standard coordinate system.
• Try your own house at the ETAK MapsOnUs site
• System works by using TIGER features and topology.
• 

K. Clarke Last Update 1/13/97