Cartography:
the science of map making
Locating yourself on a Globe
• You need a frame of reference
• That is the purpose of Latitude and Longitude
• Defining these parameters:
– Earth rotates on an imaginary axis ~ North and South
Poles
• Equator: is a great circle that lies equidistant
between them.
Great Circles:
• ..are imaginary circles of the surface of the earth
who's plane passes through the center of the earth.
• The circumference of the earth is 25,000 miles of
40,000 km
• "Great" because it is the largest possible circle
Great Circles:
• 1) cut the earth in half and each half is known as a
hemisphere
• 2) are the circumference of the earth
• 3) provide the shortest routes of travel on the
earth's surface.
– ** Planes travel in great circles.
– ** We were always taught a line is the shortest distance
between two points - Not True.
• Small circles: circles whose planes do not pass
through the center of the earth.
Latitude
• Latitude: is the angular distance north or south of
the equator.
•
1° of latitude = 112 km 360°/40,000 km
•
1 degree = 60 minutes
•
1 minute = 60 seconds
36°49'52" N
• ArcView uses: Decimal Degrees
• Sextant measures the angular distance between 2
points (sun & horizon)
• **So it easy to determine latitude.
Longitude:
• Longitude: no natural reference point
• In 1884 by International Agreement
Greenwich England was the chosen starting
point.
• This is called the prime meridian or zero
degrees and everything is east or west of
that.
– (angular distance from Greenwich, England)
The global grid:
• Parallels: lines of latitude, only the equator is a
great circle all other parallels are small circles
(they never meet)
• Meridians: these are line of longitude and when
joined with its mate half way around the globe
form great circles
• * the distance between meridians will vary with
latitude
Global Coordinate System
• Longitude and Latitude
– Degrees, minutes, seconds
– 1o latitude = 110.5 km (equator)
– 1o longitude = 111.3 cos(latitude)
• Meridian
• Parallel
• Great and Small Circles
How the Earth is Divided
• Hemispheres: Northern, Southern, Eastern,
Western
Time Zones
• Solar noon: most towns used this, defined
as when a vertical stake cast the shortest
shadow.
• By the 19th century transportation and
communications (namely railroads and
telegraph) connected towns and cities, the
adopt of a standard time was necessary.
Time Zones (continued)
• 1884 at the International Meridian Conference 24
time zones were established.
• Greenwich Mean Time (GMT) = Universal time
= Zulu time
• 360°/24 = 15° for each time zone, however for
convenience many time zones follow state and
country lines.
• International Date Line: where each new day
begins 180th meridian
• Chronometer
Time Zones
Globes
• The Globe is a nearly perfect representation
of the earth, it shows the shape and spatial
relationships of land and water.
• Problem: Can only look at 1/2 at a time.
• However globes can not show detail and are
big and clumsy.
Benefits of Maps
• Maps: are the geographers most important tool.
• Benefits:
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reproduced easily and inexpensive
different scales
can put an enormous amount of information on a map
roads, buildings, property lines, vegetation, topography
distribution of land forms
Map Features important in GIS
• Areas
• Lines
– width exaggeration
• Points
– size exaggeration
On a globe four properties are true:
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•
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1) parallels of latitude are always parallel
2) parallels are evenly spaced
3) meridians of longitude converge at the poles
4) meridians and parallels cross everywhere at
right angles
Map Projection:
• A map projection is a
mathematical formula for
representing the curved
surface of the earth on a
flat map.
Think of a light bulb
Distortions
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distance
area
shape
direction
You must make a choice between:
• Equivalence: equal area relationship
throughout the map, however you get distorted
shapes.
• Conformal: shapes are true and meridians and
parallels are at right angles, however land masses
are greatly enlarged at high latitudes.
• Except for very small areas Conformality and
Equivalence are mutually exclusive.
• There are over 1000 different projections.
Other types of considerations
• Equidistant projections – However scale is not
maintained correctly by any projection throughout
an entire map
• True-direction projections or azimuthal
projections, maintain some of the great circle
arcs. (The shortest distance between 2 points on a
globe is the great circle route.)
Map Projection
• Distortions are inherent in maps
– Earth is round, map is flat
• Projection is the term used to describe the
process of mapping a round surface to flat paper
– wide variety of projections possible
– each projection causes different distortions to
information
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Map Projections Types
Planar Projection
Conical Projection
Cylindrical Projection
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Cylindrical Projection:
example: Mercator
• Tangent to the globe at the equator. No distortions
at the equator but it increases moving North or
South. Nice rectangular grid.
• Why are they used in Navigation?
*A straight line drawn anywhere on a Mercator
projection is a true compass heading: this is called
a rhumb line.
• However, the distance along this line may vary.
Variations on Cylindrical Projection
Azimuthal Projection
example: Many Polar projections
• Plane is tangent to the globe at some point N or S
of the equator or one point on the equator. No
distortion at the point of tangency but it increases
moving away. All directions from the center are
accurate. It is like a view from space. Can only
see half the world at once.
• All great circles passing through the point of
tangency appear as straight lines.
• Good for knowing the great circle path (I.e.
shortest distances, important to navigators.
Variations of Azimuthal Projections
Conic:
example:Lambert Conformal Conic Projection
• One or more cones tangent to one or more
parallels. Best for mid-latitudes in an E-W
direction (U.S.)
• A straight line is almost a perfect great
circle route (planes use this)
• Can be conformal or equivalent
Variations on conic projections
Transformations
• The conversion between projections
involving mathematical formulas.
• Good GIS packages can do this.
• Overlaying different projections is not
possible.