Understanding Maps

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Understanding Maps

• Traditional method for storing, analyzing and
  presenting spatial data1
• 2-dimensional representation of the real world
• Many different forms and scales
• Small Scale e.g., 11,000,000 or 1250,000,
• Large Scale e.g., 110,000 or 125,000
• Maps are integral to GIS process

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Spatial Entities

• Map symbols representing real world features
• Points, lines and areas (polygons)
• Used by cartographers to depict, on paper,
  3-dimensional features in 2-dimensions
• Type of spatial representation depends on scale
  used

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Spatial Entities
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Map Generalization

• All spatial data are simplifications of real
  world features
• Level of generalization is determined by map
  scale OR
• Dictated by limitations of data production OR
• Introduced by humans purposely to highlight
  particular features

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Scale Related Generalization
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Mapping Involves

• Determining geographic locations of
  earth features (impacts datum, scale, etc.)
• Considering method for referencing locations
  e.g., coordinate system
• Transforming locations to a position on a
  flat map through use of projection
• Graphically symbolizing these features

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Earth Size Shape Generalization

• Earths surface AREA is equal to that of a sphere
  having a radius of 6,370 km
• r 637,000,000 cm r 3,960 miles
• (generalized radius commonly used in map
  creation)
• Not really a sphere - generalized for mapping
• radius at Equator 6,378 km
• radius between Poles 6,356 km

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Earth Size Shape Generalization

• Datum
• Reliable starting point from which accurate
  measurements of the earths size can be made
  (e.g., NAD27, NAD83)
• Ellipsoid
• Hypothetical, nonspherical shape of earth that
  resembles a flattening of the earth at the poles
  due to rotation. Basis for datums.
• Example
• Most 124,000 topographic maps from the US
  Geological Society use polyconic projection, the
  Clarke 1866 ellipsoid, NAD27 datum. NAD83
  datum uses GRS80 ellipsoid.

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World Datum Examples
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Coordinate Systems Spatial Referencing

• Spherical Coordinate Systems
• Geographical coordinate system (Longitude
  Latitude)
• Rectangular Coordinate Systems
• UTM (Universal Transverse Mercator)
• UPS
• State Plane
• Non coordinate systems

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Geographical Coordinate System

• Common globe referencing system
• based on perfect sphere not true earth shaped
  ellipse
• Lines of Latitude (a.k.a. parallels)
• Lines of Longitude (a.k.a. meridians)

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Latitude and Longitude
North Pole
900 longitude
Lines of Latitude (East/West - parallels)
Lines of Longitude (North/South - meridians)
South Pole
Central Parallel
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Reading Latitude Longitude

• reference system uses degrees minutes
• 550 45 N 360 0 E -- reads as
• 55 degrees 45 minutes north 360 degrees 0 minutes
  east
• first set of numbers (550 45 N) latitude
• N tells us this locational reference is north of
  the equator
• second set (360 0 E) longitude
• E indicates the remaining part of this locational
  reference is east of the prime meridian
• N and E tells us which quarter of the globe

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Example USGS Topographic Map

• http//plymouth.ces.state.nc.us/staff/ameijer/for5
  54/usgs.html
• http//plymouth.ces.state.nc.us/staff/ameijer/for5
  54/graphics/map-information-2.jpg

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Rectangular Coordinate Systems

• Also referred to as Planar or Cartesian
  Coordinate Systems (X,Y)
• used for locating positions on a flat map of the
  earths curved surfaces
• principal axes
• X horizontal (easting)
• Y vertical (northing)

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Grid Coordinate System
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Universal Transverse MercatorSystem (UTM)

• Most widely used grid system in GIS
• Derived from Transverse Mercator projection
• Divides earth from latitudes 840 N 800 S into
  60 numbered vertical zones 60 of longitude wide
• Each UTM zone has its own central meridian which
  covers 30 west 30 east of CM
• Most USGS topographic maps use
• Areas outside of UTM grid (Polar areas) use the
  UPS grid system
• Measurement metric system

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UTM Grid
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UTM Grid
Source Clarke, Keith. (2001). Getting Started
with Geographic Information Systems. p. 57.
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UTM Cell
Source Clarke, Keith. (2001). Getting Started
with Geographic Information Systems. p. 58.
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U.S. UTM Zones
Source Clarke, Keith. (2001). Getting Started
with Geographic Information Systems. p. 54.
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State Plane Coordinate System

• SPCS is a grid system developed by USGS in 1938
• Widely used in public works land surveys
• Divides US, Puerto Rico US Virgin Islands into
  over 120 zones
• Each zone has an assigned number which defines
  its projection parameters

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State Plane Coordinate System

• Three projections used for SPCS determined by
  geometric direction of state
• Lambert Conformal Conic for states longer in E/W
  direction
• Transverse Mercator for states longer in the N/S
  direction
• Oblique Mercator for panhandle of Alaska

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SPCS U.S. Map
Source Demers, Michael. (2000). Fundamentals of
Geographic Information Systems. p. 65.
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SPCS New Mexico
Source Demers, Michael. (2000). Fundamentals of
Geographic Information Systems. p. 65.
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Projections

• Transfer spherical Earth into 2-dimensions
• The method where the world is laid flat
• Approximates true shape introduces errors
• distance preserved gt direction distorted
• shape preserved gt area calculations inaccurate
• Wide range of map projections in use
• Universal Transverse Mercator (UTM), Lambert
  Conformal Conic, etc.

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Map Projection Process
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Azimuthal Projection
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Cylindrical Projection
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Conical Projection
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Projection type features

• Conformal preserves shape
• Equivalent or equal area preserves area
• Combination preserves neither

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Source Clarke, Keith. (2001). Getting Started
with Geographic Information Systems. p. 49.
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