Transcript Map Basics - University of Colorado Boulder

Map Basics
Lecture #3, Intro to GIS
spring 2006
Topics
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Map definitions
Types of maps
Map projections
Geodetic Datums
Coordinate Systems
What are maps?
• Maps are models of reality
• Attempts to represent the twodimensional curved surface of the Earth
or other body on a plane (flat surface)
• main data source for GIS
The beginnings of Cartography
• Ancient Greece:
Ptolemy (90 - 168 BC)’s Geographia
– bases of coordinate systems
– maps
– list of places
Ptolemy’s world map
Sri Lanka
oversized
Muhammed
Al’Idrisi’s
view of the
world
(1154)
What’s strange about
this map?
Kepler’s map of the world
Types of maps
1) Topographic map:
"Topography”: pertaining to the shape of the
surface, represented by contours and/or
shading, but topographic maps also show roads
and other prominent features
Topographic maps
Thematic map (choropleth):
show geographical concepts such as the
distribution of
 population
 climate
 land use etc.
Thematic maps
Map elements
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Map features: points, lines, areas, …
Scale
Accuracy
Extent
Scale
the amount of reduction in the
representation of a real world
geographic phenomenon on a map.
or, the ratio of map distance to earth
distance
verbal scale
1:24,000
1 inch = 2000 ft representative fraction
1
2000 ft
bar scale
Accuracy:
what is it?
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Quality of the source data
Map scale
mapping skills
distortions from flattening Earth’s surface to
fit on a sheet of paper
Construction of a map:
summary
Steps:
1. Select projection
2. Select a model for the shape of the earth
(sphere or ellipsoid)
3. Transform geographic coordinates
(longitude and latitude) to plane coordinates
(eastings and northings or x,y)
4. Reduction of scale
Map Projections
•Used to display locations on the curved surface of
the earth on a flat sheet or surface according to some
set of rules
•Involve distortions in:
shape (of a region)
distance (between two points)
direction (bearing from one point to another)
area (of a region)
Properties of map projections
1. Equal-area: correctly represents areas
sizes of the sphere on the map
e.g. Lambert cylindrical equal-area projection
2. Equidistant: correctly represents distances
e.g. Plate Carrée projection
3. Conformal: represents angles and shapes
correctly at infinitely small locations.
e.g. Mercator projection
The classification of
Map Projections
A. Planar (azimuthal)
• flat sheet is placed in contact with a globe,
and points are projected from the globe to
the sheet
Ex: Lambert's azimuthal equalarea projection
• Preserves
area
• Distorts shapes
& distances
B. Conical projections
Ex: Alber’s equal area conical
projection
• distorts scale and
distance except
along standard
parallels
C. Cylindrical projections
• project the sphere onto a cylinder tangent to
a central meridian
• meridians and parallels intersect at right
angles
E.g.Mercator projection (conformal)
•Preserves angles
•Distorts scale, distance,
direction and area
•distortions increase
away from the central
meridian
Note an obvious distortion?
•used in sailing
(direction more important
than distance)
Plate carrée (geographic
projection)
• latitude and longitude coordinates
• many distortions; mostly used in thematic mapping
•Compare the size of Greenland
D. “Compromise” projections
• Distortions
are balanced
• Make things "look right”
• shape distorted more in the polar regions
than at the equator
E.g. Robinson projection
Mainly used for geographic maps, e.g National Geographic
Earth’s gravitational field
• Gravity = force of
attraction between two
bodies with a mass
• Earth is not a perfect
sphere
• density not uniform
• local topography (e.g
presence of mountains) and
geology (the density of
rocks) also influence the
gravitational field.
Geoid and Ellipsoid
Geoid = Earth’s shape (minus topographic relief)
Ellipsoid = approximation to the shape of the geoid,
defined mathematically
Ellipsoid parameters
Geoid-ellipsoid relationships
Mean Sea Level (MSL) = an approximation to the geoid,
used as reference surfaces for height measurements
(orthometric heights).
Common datums:
•World Geodetic System (WGS84): GLOBAL
•NORTH AMERICAN DATUM OF 1927
•NORTH AMERICAN DATUM of 1983
Why the need for a common Reference System?
For historical reasons each country has its own
geodetic network and national geodetic reference
frame.
Projections demo:
ArcIMS
Projections Demo: ArcGIS
• Latitude, Longitude, Map Projections
and Great Circles