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Cartography: the science of map making

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Benefits of Maps (including electronic maps) Maps: are the geographers most important tool. Benefits: ... property lines, vegetation, topography, ... – PowerPoint PPT presentation

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Title: Cartography: the science of map making


1
Cartography the science of map making
A Round World in Plane Terms
2
Geographic Coordinate System
  • The earth is modeled as a sphere or spheriod.
  • The GCS consists of latitude and longitude lines
    to describe location on a spherical surface.
  • When latitude and longitude are projected on to a
    flat piece of paper distortions occur.

3

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.
  • Globes can not show detail and are big and clumsy.

4
Benefits of Maps (including electronic maps)
  • Maps are the geographers most important tool.
  • Benefits
  • reproduced easily and inexpensive
  • can create different scales
  • can put an enormous amount of information on a
    map
  • roads, buildings, property lines, vegetation,
    topography, etc.

5
Important Map Features
  • Areas
  • Lines
  • width exaggeration
  • Points
  • size exaggeration

6
On a globe four properties are true
  • 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

7
Map Projection
  • A map projection is a mathematical formula for
    representing the curved surface of the earth on a
    flat map.
  • wide variety of projections possible
  • each projection will create a different type of
    distortion

8
Think of a light bulb
9
Distortions
Distortions are inherent in maps The Earth
is round, a map is flat
  • distance
  • area
  • shape
  • direction

10
Map Projections Types
14
11
Variations of Azimuthal Projections (Planar)
12
Azimuthal Projection example most 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.

13
Variations on conic projections
14
Conic exampleLambert Conformal Conic
Projection
  • One or more cones tangent (or secant) 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

15
Variations on Cylindrical Projection
16
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.

17
There are many map projections and each one is
good at representing one or more spatial
properties, but no projection preserves all four
properties.
18
GCS - unprojected
Geographic coordinates displayed as if the
Latitude-longitude values are linear units
19
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.

20
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.)

21
Transformations
  • The conversion between projections involving
    mathematical formulas.
  • Good GIS packages can do this.
  • Overlaying different projections is not possible
    unless the GIS program has on-the-fly
    reprojection capabilities.

22
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23
Sphere vs. Spheroid
A sphere is okay for small scale maps
(lt15,000,000). For larger scale maps a spheroid
is necessary, the spheroid used will depend upon
the purpose, location, and accuracy of the data.
24
Datums
  • A reference frame for locating points on Earths
    surface
  • It consist of
  • A spheroid (ellipsoid) with a spherical
    coordinate system and an origin.
  • A network of points that have been meticulously
    surveyed.

25
Position of the Capital of Texas
26
N. American Datums
  • NAD27
  • Clarke 1866 spheroid
  • Meades Ranch, KS
  • Local datum
  • NAD83
  • GRS80 spheroid
  • Earth-centered datum
  • GPS-compatible
  • NAD27 NAD83 up to 500 shift
  • WGS84 (very similar to NAD83)

27
Datums and Elevation
  • Horizontal and Vertical Datums
  • Is the Sea level?
  • Panama Canal
  • Height Above Ellipsoid (HAE)
  • Height Above Geoid (HAG)
  • Common Vertical Datums
  • National Geodetic Vertical Datum of 1929
    ((NGVD29)
  • North American Vertical Datum of 1988 (NAVD88)
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