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Intro to GIS | Fall 2010
Georeferencing & Map
Projections
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SHAPE OF THE EARTH
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Earth's Shape
• Geoid: shape of earth minus topographic features
(irregular due to local variations in gravity)
• Ellipsoid: elongated sphere used to represent geiod for
mapping purposes (+/- 110m variation from geoid)
• Geodesy: study of the Earth's shape & representation
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How do we measure points on the Earth's Ellipsoid?
Geographic Coordinate System
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GEOGRAPHIC COORDINATE SYSTEM (GCS)
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GCS• Latitude/ Longitude coordinate system based on a
geodetic datum, which is based on a reference
ellipsoid
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Latitude 45 45' 33'' or 45.759167Degrees, Minutes, Second or Decimal Degrees
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Longitude122 43' 59'' or 122.733056Degrees, Minutes, Second or Decimal Degrees
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Geographic Coordinates
• meridian: line of constant longitude
• parallel: line of constant latitude
• prime meridian: 0 longitude; passes through the
Royal Observatory in Greenwich, England
• international date line: 180 (approx) longitude; date
changes when the line is crossed
• equator: 0 latitude; separates northern & southern
hemispheres
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Geographic Coordinate System (GCS)
• Lat/long is not a projection and therefore cannot be
shown on a flat surface (map, screen)
• Shown in ArcGIS as the “Platte Carre” cylindrical
projection (the “unprojected” projection)
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GEODETIC DATUMS
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Geodetic Datums
• Define the size and shape of the Earth for a particular
location based on a set of precisely surveyed points
and an ellipsoid
• Nations and agencies use different datums, as a basis
for coordinate systems
• Referencing geodetic coordinates to the wrong datum
can result in position errors of hundreds of meters
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North American Datum 1927 (NAD27)
• North American datum based on the Clarke 1866
ellipsoid
• Uses a single survey point (in Kansas) as reference
• Not as accurate as newer datums (which use more
precise ellipsoids)
• Still used on most USGS topographic quads (DRGs,
DLGs)
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North American Datum 1983 (NAD83)
• North American datum based on the Geodetic
Reference System of 1980 (GRS80) ellipsoid
• Compatible with modern survey techniques
• Official datum of the Federal government and most
states
• Current version is NAD83 HARN (High Accuracy
Reference Network)
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World Geodetic Survey 1984 (WGS84)
• Based on the WGS84 ellipsoid
• Compatible with modern survey techniques
• World reference system first developed by Department
of Defense in the 1950’s
• Official datum of bombs (and Google Earth)
• Used by the GPS system
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How do we represent the Earth's ellipsoid on a flat surface?
Projected Coordinate Systems
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PROJECTED COORDINATE SYSTEMS (PCS)
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PCS
• Translates geographic coordinate system to a flat plane
• Transforms lat/long geographic coordinates to x/y
(Cartesian) coordinates using mathematical formulas
• Varying distortions of shape, area, distance, and
direction always result from this process
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Projection Types
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Cylindrical Projections
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Planar (Azimuthal)
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Conical Projections
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Projection Tangency
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What type of projection is this?
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Projection Properties
• Conformal: preserves shape at the expense of area &
distance; meridians & parallels intersect at right
angles;
• Equal Area: preserves area at the expense of shape;
mostly used for analysis
• Equidistant: preserves distance at the expense of
shape & size; equal area projections can also be
equidistant
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Projection Resources
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COMMON PCS
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Universal Transverse Mercator (UTM)
• UTM zone numbers designate 6 longitudinal strips
from 80 south latitude to 84 North latitude
• UTM coordinates are in “northings” and “eastings”
(meters)
• “northings” are relative to the equator; “eastings” are
relative to false origin 500,000 meters west of the
zone’s central meridian
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UTM coordinate: 10 357800E 4276750N
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State Plane
• Developed in the 1930’s to provide local reference systems
that were tied to a national datum (NAD 83 or NAD 83
HARN)
• Projections are chosen to minimize distortion based on the
state’s shape (good for surveying)
• Feet or meters
• Smaller states use a single state plane zone; larger states are
divided into several zones
• NOT a projection; system is based on different projections
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Lambert Conformal Conic
Albers Equal-Area Conic
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Lambert Conformal Conic
Albers Equal-Area Conic
Standard Parallels 30 N & 46 N
Standard Parallels 33 N & 45 N
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Oregon Statewide PCS
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PROJECTIONS IN ARCGIS
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Geographic Coordinate Systems
• Latitude/longitude system based on a particular datum
(i.e., NAD 83)
• In ArcGIS, named “GCS_{datum name}”
• Lat/long is not a projection and therefore cannot be
shown on a flat surface (map, screen)
• Shown in ArcGIS as the “Platte Carree” projection
• Units = degrees, minutes, seconds or decimal degrees
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Projected Coordinate Systems
• Method used to translate the GCS on to a flat plane
• In ArcGIS, named “{projection name}_{projection
type}_{datum name}”
• Uses real-world units (meters, feet)
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Define Projection v. 'Project' Tools
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Define Projection tool
• When the data does not have a defined projection or
coordinate system (no .prj file)
• Term “projection” is misleading (why?)
• Projection or coordinate system can be created,
selected from a predefined list, or imported from
another dataset
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Project tool
• When you want to change the defined projection or
coordinate system
• If the geographic coordinate system changes, requires
a geographic transformation
• Projection or coordinate system can be created,
selected or imported
• Creates a new dataset (user specifies name & storage
location on computer)
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ArcMap Data Frame
• Takes on the projection or GCS of the first dataset
added
• All other datasets are then projected “on-the-fly” to
match it (assuming they have a defined projection)
• The data frame’s projection/GCS can be set manually in
the data frame “Properties”
• When you change the projection or GCS of the data
frame, it changes to appearance of the map data
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ArcMap & Projections
• GIS data files & data frames (in an .mxd) need to have
defined projections
– GIS data files have a .prj file that stores projection info
• GIS data MUST be defined in the correct projection,
which is the projection used to create the data (GPS,
Satellite image, digitizing from a paper map, etc.)
– If downloading data, it will either have a .prj file that
saves the projection information OR the projection
info is available in the documentation on website
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Projections Demo in ArcGIS