intro to gis | fall 2010 georeferencing & map projections · earth's shape • geoid:...
TRANSCRIPT
Intro to GIS | Fall 2010
Georeferencing & Map
Projections
SHAPE OF THE EARTH
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
How do we measure points on the Earth's Ellipsoid?
Geographic Coordinate System
GEOGRAPHIC COORDINATE SYSTEM (GCS)
GCS• Latitude/ Longitude coordinate system based on a
geodetic datum, which is based on a reference
ellipsoid
Latitude 45 45' 33'' or 45.759167Degrees, Minutes, Second or Decimal Degrees
Longitude122 43' 59'' or 122.733056Degrees, Minutes, Second or Decimal Degrees
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
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)
GEODETIC DATUMS
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
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)
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)
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
How do we represent the Earth's ellipsoid on a flat surface?
Projected Coordinate Systems
PROJECTED COORDINATE SYSTEMS (PCS)
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
Projection Types
Cylindrical Projections
Planar (Azimuthal)
Conical Projections
Projection Tangency
What type of projection is this?
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
Projection Resources
COMMON PCS
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
UTM coordinate: 10 357800E 4276750N
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
Lambert Conformal Conic
Albers Equal-Area Conic
Lambert Conformal Conic
Albers Equal-Area Conic
Standard Parallels 30 N & 46 N
Standard Parallels 33 N & 45 N
Oregon Statewide PCS
PROJECTIONS IN ARCGIS
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
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)
Define Projection v. 'Project' Tools
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
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)
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
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
Projections Demo in ArcGIS