Intro to GIS | Fall 2010 Georeferencing & Map Projections · Earth's Shape • Geoid:...

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