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Thinking Spatially with Maps DeMers: Chapter 3

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Thinking Spatially with Maps DeMers: Chapter 3 The map is the fundamental device by which we abstract our environment s space, and within which the GIS will operate ... – PowerPoint PPT presentation

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Title: Thinking Spatially with Maps DeMers: Chapter 3


1
Thinking Spatially with Maps DeMers Chapter 3
  • The map is the fundamental device by which we
    abstract our environments space, and within
    which the GIS will operate to analyze it.

2
Overview
  • Maps
  • Shift in Cartography
  • Scales
  • Projections
  • Grid systems for mapping
  • The cartographic process
  • Symbols
  • Some problems related to specific thematic maps

3
Important considerations
  • The cartographic method
  • How do we depict spatial features and their
    relationships?
  • How do we portray a 3D world in 2D?

4
Maps
  • Maps are a graphic form of spatial data
  • Map as Model The Abstraction of Reality a map
    is an abstraction of reality not meant to show
    every detail implies selective inclusion/exclusion
    of objects and phenomena (as well as their
    attributes)
  • Types
  • Reference
  • Thematic

5
3. Shift in Cartography
  • Communication paradigm
  • Assumed that the map itself was a final product
    designed to communicate spatial pattern through
    the use of symbols, class limit selection, and so
    on. E.g. Tourism maps
  • Map is end result and the user is incapable of
    regrouping the data into forms more useful
  • Analytical (holistic) paradigm
  • Maintains the raw attribute data inside a
    computer storage and displays data based on user
    needs and classification
  • The map allow for both communication and analysis

6
Fig. 3.1 - State Park
7
4. Illustrating scale
  • Scale The ratio of distance on the map to the
    same distance as it appears on the earth

8
Effect of scale on accuracy
The rule of thumb It is always better to reduce
a map after analysis than to enlarge it for
analysis
9
5. Map Projections
  • 3D Earth -gt -gt 2D surface?
  • Families of projections
  • Distortions (shape, distance, direction, area)

10
Definition
  • Map projections are attempts to portray the
    surface of the earth or a portion of the earth on
    a flat surface. Some distortions of conformality,
    distance, direction, scale, and area always
    result from this process.
  • Some projections minimize distortions in some of
    these properties at the expense of maximizing
    errors in others. Some projection are attempts to
    only moderately distort all of these properties

11
Classes of map projections
  • CylindricalResult from projecting a spherical
    surface onto a cylinder.When the cylinder is
    tangent to the sphere contact is along a great
    circle (by a plane passing through the center of
    the Earth).
  • Conic Result from projecting a spherical surface
    onto a cone. When the cone is tangent to the
    sphere contact is along a small circle.
  • Azimuthal Result from projecting a spherical
    surface onto a plane.When the plane is tangent to
    the sphere contact is at a single point on the
    surface of the Earth.

12
Classes of map projections-continue
  • Miscellaneous projections Include unprojected
    ones such as rectangular latitude and longitude
    grids and other examples of that do not fall into
    the cylindrical, conic, or azimuthal categories

13
Historically - light source projected features on
a transparent globe
Three families of map projections (a) Flat
surfaces (b) Cylinders (c) Cones
14
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15
Distortions
  • When projecting from 3D sphere to 2D globe, there
    will be some distortions in shape, distance,
    direction, area
  • Conformal or orthomorphic map projection When
    the scale of a map at any point on the map is the
    same in any direction, the projection is
    conformal. Meridians (lines of longitude) and
    parallels (lines of latitude) intersect at right
    angles. Shape is preserved locally on conformal
    maps.
  • It retains the property of angular conformity,
    but results in distortion of areas

16
Distortions-continue
  • Equal area or equivalent projections Preserves
    areas, but distorted angles, i.e. areas and
    angles cannot be preserved at the same time
  • Equidistant projections Preserves distance along
    standard parallels or from one or two points
  • Azimuthal projection A map preserves direction
    when azimuths (angles from a point on a line to
    another point) are portrayed correctly in all
    directions (navigation)

17
No flat map can be both equivalent and
conformal.
18
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19
Selection of a projection
  • The first step in choosing a projection is to
    determine Location, Size, and Shape
  • These three things determine where the area to be
    mapped falls in relation to the distortion
    pattern of any projection. One "traditional" rule
    says
  • A country in the tropics asks for a cylindrical
    projection.
  • A country in the temperate zone asks for a
    conical projection.
  • A polar area asks for an azimuthal projection.

20
Selection of a projection-continue
  • Implicit in these rules of thumb is the fact that
    these global zones map into the areas in each
    projection where distortion is lowest
  • Cylindricals are true at the equator and
    distortion increases toward the poles.
  • Conics are true along some parallel somewhere
    between the equator and a pole and distortion
    increases away from this standard.
  • Azimuthals are true only at their center point,
    but generally distortion is worst at the edge of
    the map.

21
6. Grid systems for mapping
  • Need a grid (coordinate system) for distance and
    direction on the earth.
  • Also need grid system that take into account the
    distortions introduced by projecting world onto
    2D map.
  • Rectangular coordinates (plane coordinates)
  • Basic Cartesian coordinate system (x,y)
  • Plane coordinate system are used to represent
    large areas and not small scale maps. For small
    scale maps, adjustment must be made to compensate
    for the distortions.
  • The Universal Transverse Mercator (UTM) is the
    most prevalent plane grid system used in GIS
    operations

22
A cartesian coordinate system (X,Y) (N,E)
Digitizers are based on cartesian coordinate
system
23
The Universal Transverse Mercator (UTM)
  • UTM system is used to define horizontal,
    positions world-wide by dividing the surface of
    the Earth into 6 degree zones, each mapped by the
    Transverse Mercator projection with a central
    meridian in the center of the zone. UTM zone
    numbers designate 6 degree longitudinal strips
    extending (60 zones) from 80 degrees South
    latitude to 84 degrees North latitude. The zones
    numbering starts at 180th meridian in east ward
    direction
  • Eastings are measured from the central meridian
    (with a 500km false easting to insure positive
    coordinates). Northings are measured from the
    equator (with a 10,000km false northing for
    positions south of the equator).

24
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25
UTM principles
26
The cartographic process
  • The main four general steps in cartographic
    process are
  • Data collection Field survey
  • Data compilation Development of base map
  • Map production Output of a map with all features
  • Map reproduction Quantitative production at
    different scales (Magnification, reduction)
  • Although the analytical or holistic paradigm may
    not follow the same steps, the process is almost
    similar

27
Map symbolism
  • Geographic objects (point, line, area, surface)
    are represented by symbols on the map
  • Symbol geometry and dimensionality are sometimes
    not a true representation of an object, but are
    often manipulated to achieve a particular visual
    response (e.g. area symbol represent a point
    feature)
  • A major difference between communication and
    holistic paradigms is the classification-oriented
    manipulation of data prior to map production
    (Class interval selection-see Chap9)

28
Class interval selection methods
  • Constant interval Same number of areas/data in
    each category/class (contour interval)
  • Variable intervals Isolating certain high or
    low values, for highlighting variations in value
    (Creating a discrete set of point symbols to show
    variation in attribute variable)
  • Considerations must be paid, during the input to
    GIS, to symbols, method of classification, and
    graphic simplification (if road, river, and
    railway are very near, they can be displaced from
    their original location to improve visualization)
    (feature elimination (filtering) and
    smoothingrivers-roads)

29
Map abstraction and cartographic database
  • Cartographic database are collected from existing
    cartographic documents, which may include some
    filtering and smoothing of spatial feature,
    therefore the GIS input will not be accurate
  • Geographic database are collected from field
    (surveying, GPS) or remotely sensed data, which
    are more accurate and sometimes the GIS input
    device (scanner, digitizer) may not give the same
    accuracy
  • Incompatibility between maps generated from
    different sources or scale may arise in GIS
  • The scale of input for a cartographic database
    should be as nearly identical as possible

30
Some problems related to specific thematic maps
  • Soil maps Provide information for agricultural
    activities. Problems associated with soil maps
    are method of sampling using aerial photographs
    (distortion, relief, projection- Orthophotomaps)
  • Zoological maps Provide information about animal
    locations (point or area). Problems associated
    with these maps is the movement of animal,
    therefore time domain must be encounter
  • Remote sensing imageryGeometry and manipulation
    (resolution,enhancement,classification)
  • Vegetation maps Sampling and classifications
  • Historical maps Use for spatiotemporal analysis,
    different tools for data collection and
    classification,

31
Questions
  • 1. What is the difference between communication
    and holistic paradigms in cartography
  • 2. How scale is illustrated on a map and the
    potential problems in analyses when scale is
    changed.
  • 3. Briefly discuss the classes of map
    projections.
  • 4. What basic properties of the spherical earth
    are affected by using map projection?
  • 5. What are the factors that considered in
    selection of a projection

32
References
  • Anson, R. W., 1996. Basic Cartography for
    Students and Technician. Butterwork.
  • Clarke, K. C., 1990. Analytical and Computer
    Cartography, Prentice Hall, New York.
  • Maling, D.H. 1992. Co-ordinate Systems and Map
    Projections, 2nd Ed. Pergamon Press. Oxford.
  • Muehrcke, Phillip C. 1986. Map use Reading,
    Analysis, Interpretation. Madison, WI JP
    Publications.
  • Robinson, A. H., J. L. Morrison, P. C. Muehrcke,
    A. Jon Kimerling, and S. C. Guptil, 1995.
    Elements of Cartography, 6th ed., John Wiley
    Sons, Inc., New York. (Very Important Reference).
  • Snyder, John P. 1987. Map Projections a working
    Manual. USGS Professional Paper 1395. Washington,
    DC United States Government Printing Office.
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