Georeferencing Projections

Concept of Map Projection Presented by

Reza Wahadj University of California ,San Diego (UCSD)

Map Projection A map projection is a set of rules for transforming features from the three-dimensional earth onto a two-dimensional display. No flat representation of the earth can be completely accurate, so many different projections have been developed, each suited to a particular purpose. Map projections differ in the way they handle four properties: Area, Angles, Distance and Direction. Rules: 1.

2.

No projection can preserve all four simultaneously, although some combinations can be preserved, such as Area and Direction No projection can preserve both Area and Angles, however. The map-maker must decide which property is most important and choose a projection based on that.

Learn more:

http://mac.usgs.gov/mac/isb/pubs/MapProjections/projections.html

Projections and Coordinates ■

There are many reasons for wanting to project the Earth’s surface onto a plane, rather than deal with the curved surface ■ ■ ■ ■ ■

The paper used to output GIS maps is flat Flat maps are scanned and digitized to create GIS databases Rasters are flat, it’s impossible to create a raster on a curved surface The Earth has to be projected to see all of it at once It’s much easier to measure distance on a plane

Coordinate Systems ■

Spatial data are referenced to locations on the earth’s surface using coordinate systems ■

■

Ensure all map layers share a common coordinate system

Recognized global coordinate systems consist of: ■ ■

A Spheroid: a mathematical description of the earth’s shape A Map Projection: a mathematical conversion from spherical to planar coordinates

Map Projection

Different View

Normal or Polar

Oblique

Transverse or Equatorial

Different View

Normal or Polar

Oblique

Transverse or Equatorial

Different family

azimuthal

cylindrical

conic

Projection different family

azimuthal

cylindrical

conic

Distortion patterns

Direction of Distortion

Transverse Mercator ■

Projection properties ■ ■ ■

■

Projection uses ■ ■ ■

conformal

Conformal cylindrical Rhumb lines not projected as straight lines Transverse -- standard line is a meridian Topographic maps (USGS) As a base for the UTM coordinate system As a base for state plane coordinates

equal area

direction

distance

Universal Transverse Mercator (UTM) ■

Projection properties All Transverse properties ■ Standard line is a meridian ■ 60 zone divided ■

■

Projection uses ■

conformal

World Map

equal area

direction

distance

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

equal area

direction

distance

Robinson

Uses tabular coordinates rather than mathematical formulas to make the world "look right." Directions true along all parallels and along central meridian Distances constant along Equator and other parallels

Projection properties •

Compromise

Projection uses • conformal

World atlas maps equal area

direction

distance

Georeferencing ■

■

Is essential in GIS, since all information must be linked to the Earth’s surface The method of georeferencing must be: ■ ■

■

Unique, linking information to exactly one location Shared, so different users understand the meaning of a georeference Persistent through time, so today’s georeferences are still meaningful tomorrow

Georeferences as Measurements ■

Some georeferences are metric ■

They define location using measures of distance from fixed places ■

■

Others are based on ordering ■

■

E.g., distance from the Equator or from the Greenwich Meridian

E.g. street addresses in most parts of the world order houses along streets

Others are only nominal ■

Placenames do not involve ordering or measuring

Placenames ■

The earliest form of georeferencing ■

■

Many names of geographic features are universally recognized ■

■

Others may be understood only by locals

Names work at many different scales ■

■

And the most commonly used in everyday activities

From continents to small villages and neighborhoods

Names 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 Referencing ■

■

A system for georeferencing positions on a road, street, rail, or river network Combines the name of the link with an offset distance along the link from a fixed point, most often an intersection

Users of Linear Referencing ■

Transportation authorities ■

■

To keep track of pavement quality, signs, traffic conditions on roads

Police ■

To record the locations of accidents

Converting Georeferences ■

GIS applications often require conversion of projections and ellipsoids ■

■

Street addresses must be converted to coordinates for mapping and analysis ■

■

These are standard functions in popular GIS packages

Using geocoding functions

Placenames can be converted to coordinates using gazetteers