Transcript Maps as Numbers

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.
No projection can preserve all four simultaneously, although some combinations
can be preserved, such as Area and Direction
2.
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
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There are many reasons for wanting to
project the Earth’s surface onto a plane,
rather than deal with the curved surface
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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
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Spatial data are referenced to locations on the earth’s
surface using coordinate systems
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Ensure all map layers share a common coordinate system
Recognized global coordinate systems consist of:
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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
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Projection properties
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Projection uses
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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)
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Projection properties
All Transverse properties
 Standard line is a meridian
 60 zone divided
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Projection uses
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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
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Is essential in GIS, since all information must
be linked to the Earth’s surface
The method of georeferencing must be:
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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
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Some georeferences are metric
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They define location using measures of distance
from fixed places
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Others are based on ordering
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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
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Placenames do not involve ordering or measuring
Placenames
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The earliest form of georeferencing
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Many names of geographic features are
universally recognized
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Others may be understood only by locals
Names work at many different scales
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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
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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
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Transportation authorities
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To keep track of pavement quality, signs,
traffic conditions on roads
Police
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To record the locations of accidents
Converting Georeferences
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GIS applications often require conversion of
projections and ellipsoids
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Street addresses must be converted to
coordinates for mapping and analysis
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These are standard functions in popular GIS
packages
Using geocoding functions
Placenames can be converted to coordinates
using gazetteers