Textbook: Slocum,
T. A. 1999. Thematic
Cartography and
Visualization. Prentice Hall:
New Jesey.
Instructor: Dr. M.
M. Yagoub URL : https://www.angelfire.com/mo/yagoub
Chapter 7: Proportional Symbol
Mapping (See Page: 118-134)
List
and define the basic types of
point data that can be displayed with proportional symbols?
1. True point
data
Data can be measured at a point location, for example, the production
of an oil well.
2. Conceptual point data
Data collected over an area or
volume, but the data are conceived as being located at a point for the purpose
of symbolization, for example, the number of people in each emirates.
What is meant by data
standardization, illustrate by figures?
Data standardization is a process
whereby raw data are standardized by considering raw counts (or totals) for one
variable relative to values of another variable. For example, we might divide
the gallons of water pumped at a well by the area covered by the associated
center-pivot irrigation system (compare Figure 7.1. A and B, page 119).
List
and define the main kinds of proportional symbols, illustrate by figures?
1. Geometric
For example circles, squares. Circles
have been the most frequently used because they are visually stable, users
prefer them and they conserve map space (Compare Figure 7.11.D and Figure 7.12
on page 129 and 130 respectively).
2. Pictographic
Pictographic symbol mirror the
phenomena being mapped (e.g. photo of camel in a camel distribution map, see
also Figure 7.3, page 121). Pictographic symbols are becoming common because of the ease with which they can
be created with software. One of the problem of pictographic symbols is that
when overlapped, they may be more difficult to interpret (Compare the
northeastern proportions of Figure 7.1 A and 7.3 on pages 119 and 121).
List
and define the methods of scaling (sizing) proportional symbols, illustrate by
figures?
1. Mathematical
scaling
In mathematical scaling symbols are
sized in direct proportion to the data, for example, a data value 20 times
another is represented by an area 20 times as large (Figure 7.6, page 122).
2. Perceptual
scaling
In perceptual scaling, a correction
is introduced to account for visual underestimation of larger symbols; thus
larger symbols are made bigger than would normally specified by mathematical
scaling (Figure 7.6 , page 122).
3. Range grading
In range grading data are grouped
into classes, and a single symbol size is used to represent all data falling in
a class. An advantage of range grading is readers can easily discriminate
symbol sizes and thus can readily
match map and legend symbol. However, one of the disadvantages of range grading is that readers may misinterpret specific
information if they do not pay careful attention to the legend (Figure 7.11. A,
page 129).
List and define the
basic legend arrangements that are used on proportional symbol maps, illustrate
by figures?
1. Nested-legend arrangement
Smaller symbols are drawn within larger symbols (Figure
7.13, page 130).
2. Linear-legend arrangement
Symbols are placed adjacent to each
other in either a horizontal or vertical orientation depending on the space in
the map (Figure 7.13, page 130).
Which symbols to
include in a proportional symbol maps, illustrate by figures?
With range grading , the symbols
shown in the legend are determined by the classes displayed on the map (e.g.
four classes). For mathematical and perceptual scaling, there are two methods
for selecting the symbols. One is to include the smallest and largest symbols
actually shown on the map and then interpolate several intermediate-sized
symbols (Figure 7.14.A, page 131). A second is to select a set of symbols that
are most representative of those appearing on the map, which should minimize
estimation error.
How to handle symbols overlap in a
map?
There are two basic solution to
handle symbols overlap in a map. These are transparent and opaque symbols.
Transparent symbols enable readers to see through overlapping symbols while opaque symbols display smaller
symbols stacked on top of larger symbols (Figure 7.16, page 133). Transparent
symbols ease the problem of estimating symbol size, while opaque symbols enhance figure-ground contrast. Other
solutions for handling overlap
symbols include use of Zoom function and moving symbols slightly away from the
center of congested area (compare Figure 7.9.C with 7.9.B, page 127).
