Intro to GIS Fall 2010 Georeferencing & Map Projections

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 45.759167 Degrees, Minutes, Second or Decimal Degrees

Longitude 122 43' 59'' or 122.733056 Degrees, 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

Albers Equal-Area Conic Lambert Conformal Conic

Lambert Conformal Conic Standard Parallels 33 N & 45 N Albers Equal-Area Conic Standard Parallels 30 N & 46 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