Concept of Map ProjectionConcept of Map Projection

Presented byPresented by

Reza WahadjReza WahadjUniversity of California ,San Diego (UCSD)

Map ProjectionMap ProjectionA map projection is a A map projection is a set of rulesset of rules for for transforming featurestransforming features from the from the

threethree-dimensional earth onto a -dimensional earth onto a twotwo-dimensional display. No flat -dimensional display. No flat representation of the earth can be completely accurate, so many representation of the earth can be completely accurate, so many different projections have been developed, each suited to a different projections have been developed, each suited to a particular purpose. Map projections differ in the way they handle particular purpose. Map projections differ in the way they handle four propertiesfour properties::

AreaArea, , AnglesAngles, , DistanceDistance and and DirectionDirection. .

Rules:Rules:

1.1. No projection can preserve all four simultaneously, although some combinations No projection can preserve all four simultaneously, although some combinations can be preserved, such ascan be preserved, such as AreaArea and and DirectionDirection

2.2. No projection can preserve both No projection can preserve both AreaArea and and AnglesAngles, however. The map-maker, however. The map-maker must decide which property is must decide which property is most importantmost important and choose a projection and choose a projection based on based on

that.that.

http://mac.usgs.gov/mac/isb/pubs/MapProjections/projections.htmlLearn more:

Projections and CoordinatesProjections and Coordinates

There are many reasons for wanting to There are many reasons for wanting to project the Earth’s surface onto a plane, project the Earth’s surface onto a plane, rather than deal with the curved surfacerather than deal with the curved surface The paper used to output GIS maps is flatThe paper used to output GIS maps is flat Flat maps are scanned and digitized to create GIS Flat maps are scanned and digitized to create GIS

databasesdatabases Rasters are flat, it’s impossible to create a raster Rasters are flat, it’s impossible to create a raster

on a curved surfaceon a curved surface The Earth has to be projected to see all of it at The Earth has to be projected to see all of it at

onceonce It’s much easier to measure distance on a planeIt’s much easier to measure distance on a plane

Coordinate SystemsCoordinate Systems

Spatial data are referenced to locations on the earth’s Spatial data are referenced to locations on the earth’s surface using coordinate systemssurface using coordinate systems Ensure all map layers share a common coordinate systemEnsure all map layers share a common coordinate system

Recognized global coordinate systems consist of:Recognized global coordinate systems consist of: A Spheroid: a mathematical description of the earth’s shapeA Spheroid: a mathematical description of the earth’s shape A Map Projection: a mathematical conversion from spherical A Map Projection: a mathematical conversion from spherical

to planar coordinatesto planar coordinates

Map ProjectionMap Projection

Normal or Polar Oblique Transverse orEquatorial

Different View

Normal or Polar ObliqueTransverse or

Equatorial

Different View

Different familyDifferent family

coniccylindricalazimuthal

Projection different familyProjection different family

coniccylindricalazimuthal

Distortion patterns

Direction of Distortion Direction of Distortion

Transverse MercatorTransverse Mercator

Projection propertiesProjection properties Conformal cylindricalConformal cylindrical Rhumb lines Rhumb lines notnot projected as straight lines projected as straight lines Transverse -- standard line is a meridianTransverse -- standard line is a meridian

Projection usesProjection uses Topographic maps (USGS)Topographic maps (USGS) As a base for the UTM coordinate systemAs a base for the UTM coordinate system As a base for state plane coordinatesAs a base for state plane coordinates

conformal distancedirectionequal area

Universal Transverse Universal Transverse Mercator (UTM)Mercator (UTM)

Projection propertiesProjection properties All Transverse propertiesAll Transverse properties Standard line is a meridianStandard line is a meridian 60 zone divided 60 zone divided

Projection usesProjection uses World MapWorld Map

conformal distancedirectionequal area

conformal distancedirectionequal area

Azimuthal Equidistant

Projection properties• Distances are correct outward from center• Possible to show the entire sphere

Projection uses• Sea and air navigation planning• Distance from a specified location to all

others

conformal distancedirectionequal area

Robinson

Projection properties• Compromise

Projection uses• World atlas maps

Uses tabular coordinates rather than mathematical formulasto make the world "look right."

Directions true along all parallels and along central meridian

Distances constant along Equator and other parallels

GeoreferencingGeoreferencing

Is essential in GIS, since all information must Is essential in GIS, since all information must be linked to the Earth’s surfacebe linked to the Earth’s surface

The method of georeferencing must be:The method of georeferencing must be: Unique, linking information to exactly one locationUnique, linking information to exactly one location Shared, so different users understand the Shared, so different users understand the

meaning of a georeferencemeaning of a georeference Persistent through time, so today’s georeferences Persistent through time, so today’s georeferences

are still meaningful tomorroware still meaningful tomorrow

Georeferences as MeasurementsGeoreferences as Measurements

Some georeferences are metricSome georeferences are metric They define location using measures of distance They define location using measures of distance

from fixed placesfrom fixed places E.g., distance from the Equator or from the Greenwich E.g., distance from the Equator or from the Greenwich

MeridianMeridian

Others are based on orderingOthers are based on ordering E.g. street addresses in most parts of the world E.g. street addresses in most parts of the world

order houses along streetsorder houses along streets Others are only nominalOthers are only nominal

Placenames do not involve ordering or measuringPlacenames do not involve ordering or measuring

PlacenamesPlacenames

The earliest form of georeferencingThe earliest form of georeferencing And the most commonly used in everyday activitiesAnd the most commonly used in everyday activities

Many names of geographic features are Many names of geographic features are universally recognizeduniversally recognized Others may be understood only by localsOthers may be understood only by locals

Names work at many different scalesNames work at many different scales From continents to small villages and neighborhoodsFrom continents to small villages and neighborhoods

Names may pass out of use in timeNames may pass out of use in time

ZIP code boundaries are a convenient way to summarize data in the US. The dots on the left have been summarized as a density per square

mile on the right

Linear ReferencingLinear Referencing

A system for A system for georeferencing positions georeferencing positions on a road, street, rail, or on a road, street, rail, or river networkriver network

Combines the name of Combines the name of the link with an offset the link with an offset distance along the link distance along the link from a fixed point, most from a fixed point, most often an intersectionoften an intersection

Users of Linear ReferencingUsers of Linear Referencing

Transportation authoritiesTransportation authorities To keep track of pavement quality, signs, To keep track of pavement quality, signs,

traffic conditions on roadstraffic conditions on roads PolicePolice

To record the locations of accidentsTo record the locations of accidents

Converting GeoreferencesConverting Georeferences

GIS applications often require conversion of GIS applications often require conversion of projections and ellipsoidsprojections and ellipsoids These are standard functions in popular GIS These are standard functions in popular GIS

packagespackages

Street addresses must be converted to Street addresses must be converted to coordinates for mapping and analysiscoordinates for mapping and analysis Using Using geocodinggeocoding functions functions

Placenames can be converted to coordinates Placenames can be converted to coordinates using using gazetteersgazetteers