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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
• 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
• 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

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
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
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

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
• 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,