Transcript No Slide Title

Map Scale.
The Elements of a Map: All maps should have 5 essential elements:
1. Title
2. Grid – covered last week
3. Data and Legend
4. Direction indicator
5. Scale
Title: should include WHAT, WHERE & WHEN.
Example: Energy Consumption In France, 1978-1982.
Data and Legend: Data can simply be general tangible surface
features such as streams or intangible features such as county
boundaries. The legend is required to explain features shown on the
map, which are usually simplified, classified and symbolized. For
example, roads have small curves removed, they are classified as
types of road and they are symbolized by line width, color and
pattern.
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Example USGS map symbols for roads:
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Direction Indicator:
The familiar north arrow may be added to maps of small areas to
provide a directional reference for the reader.
N
On maps with a grid system, the grid lines
provide a directional reference – although it
should be remembered that meridians are
not parallel on many projections (such as
the conic). On maps of familiar areas (for
example, the U.S., a direction indicator
may not be necessary - it’s assumed
everyone knows the orientation of the U.S.
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On USGS topographic maps, a declination diagram containing
three norths is shown in the bottom margin of the map sheet:
The three norths are:
1. True north – points to the
north pole; aligned to
central meridian on the map
sheet.
2. Grid north – shows the
difference between true
north and the UTM grid
north.
3. Magnetic north – points
to the magnetic north pole;
same as compass north.
(note: the angles are NOT
drawn accurately, but the
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numbers are correct).
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Scale:
Map scale is the ratio of the map distance to the true distance.
Three types of scale:
1. A statement in words: “six inches to one mile”, “ten centimeters
represents one kilometer”. By convention, one of the
measurements is always “one”. This method gives an immediate
impression of distance on the map without having to use math.
Limited accuracy.
2. Linear, graphic or bar scale:
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Found in bottom margin of map. Allows direct measurement with
ruler or dividers (without math). Automatically adjusts to
photographic enlargement or reduction. Limited accuracy.
3. Representative Fraction (RF): e.g. 1:24,000 or 1/24000. This
means 1 unit of distance on the map represents 24,000 units of
distance in the “real world”. The left-hand side is always “1”. Can
be any units – cm, inches, feet etc.
Example: At an RF of 1:50,000, how many miles are represented by
a map distance of 3 inches?
3 inches on map represents 3x50,000 inches = 150,000 inches
=150,000/63360 miles = 2.37 miles. Advantages: greater accuracy;
works with any unit of measurement.
Example 2 (for class): At an RF of 1:24,000, how many km are
represented by 10 cm?
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Area Measurement on Maps:
1. Based on geometry: OK for regular shapes e.g. squares,
rectangles, circles, triangles. Very important to understand that linear
and areal conversions are NOT the same e.g. 1000 m in 1 km;
1,000,000 m2 in 1 km2.
Example: RF = 1:24,000. Find the area in km2
3 cm
6 cm
3 cm represents 3x24,000 cm = 72,000 cm = 0.72 km.
6 cm represents 6x24,000 cm = 144,000 cm = 1.44 km.
0.72 km x 1.44 km = 1.04 km2.
Always covert to required units first, then multiply.
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2. Grid Square Method: the
irregular shape is traced onto
a regular grid (e.g. 1 cm
squares). The number of
whole and “half” squares
within the shape are counted
e.g. 64 whole squares; 62
“half” squares. The 62 “half”
squares equal 31 whole
squares, so there are 95
whole squares total. At an RF
of 1:24,000, each square
represents 24,000 cm x
24,000 cm = 0.24 km x 0.24
km = 0.0576 km2. Therefore,
total area = 95 x 0.0576 =
5.472 km2.
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Whole square
“half” square
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Polar Planimeter: this
device is used to trace
the perimeter of an
irregular shape.
Readings from the
dials are then used to
calculate the area. It
will be demonstrated
in the lab.
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