Transcript No Slide Title

EARTH SCIENCE: LOCATION, GRID SYSTEMS, MAPS,
GIS.
GRID SYSTEMS:
Since geography deals with the position of features on the
surface of the Earth, it is important to have some way to
accurately specify precise locations. The obvious way of doing
this is to use some form of grid system.
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The trouble is that the Earth is a sphere*, not a flat piece of
paper - however, there are natural points of reference on the
Earth that can be used to construct a grid system (next page).
*Note that the
earth is not
completely
spherical - it
bulges slightly
around the equator
- but for our
purposes this is
negligible.
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The Earth rotates about the axis of rotation, which
connects the north and south poles; the plane of the
equator is midway between the poles and at right angles
to the axis.
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Lines connecting the poles run north-south and are called
MERIDIANS or lines of LONGITUDE; lines running parallel
to the equator run east-west and are called PARALLELS or
lines of LATITUDE.
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Latitude is measured by the angle from the Earth's center
north or south from plane of the equator (00 = equator; 900
North = North Pole; 900 South = South Pole).
Longitude is measured by the angle from the Earth's
center east or west from the Prime Meridian running through
Greenwich, England (Prime Meridian = 00).
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On the globe, latitude and longitude lines form a
geographic grid that can be used to specify the location of any
point. To increase accuracy, degrees of latitude and longitude
are divided into minutes - a 60th of a degree, and seconds - a
60th of a minute.
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It should be noted that because longitude lines converge,
the distance of 1 degree of longitude changes with latitude; i.e. at
the Equator, 1 degree of longitude = 40 000 km/3600 = 111 km; 1
minute = 1.85 km; 1 second = 30 m.
At 600 N, 1 degree of longitude = 20 088 km/3600 = 55.8 km; 1
minute = 0.93 km; 1 second = 15.5 m.
Unlike longitude lines, latitude lines do not converge,
therefore the distance of 1 degree of latitude is constant at 111
km. One minute of latitude at the equator defines the
NAUTICAL MILE (1.85 km) and the KNOT (1 nautical mile
per hour).
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Topographic Maps, GIS (pages 25-34):
Maps are "scaled-down" versions of the real world. The map scale
is the ratio of map distance to true distance. There are two main
types:
1. Graphic Scale - a line is drawn which is divided into distances.
2. Representative Fraction (RF) - for example, 1:63,360; this
means that 1 unit of distance on the map represents 63,360 of the
same units of distance in the real world i.e. 1 inch represents
63,360 inches; 1 mm represents 63,360 mm and so on.
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In the U.S., the most common map is the United States
Geological Survey 7.5 minute quadrangle. These have an RF of
1:24,000 (approximately 2.64 inches to one mile) and cover an
area 7.5 minutes of longitude wide and 7.5 minutes of latitude
long.
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Example: On a map with RF = 1:50000,
if the map distance is 5.36 inches, then
the ground distance is 5.36x50000 inches = 268000 inches
There are 63360 inches in one mile, therefore
= 268000/63360 miles = 4.23 miles
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Contour lines, spot heights and bench marks are used to
show elevation on the map (topographic = “variation in
height over surface”).
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Contour line = line of constant elevation above mean sea
level e.g. 700 feet (contour interval is stated on map =
vertical separation of contour lines).
Spot height = accurately surveyed height shown as a small
cross on map (not marked on the ground) e.g. x 730
Bench mark = same as spot height, shown by BM x 680
AND marked on ground by a monument.
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Contours: imaginary lines of constant elevation. Every 5th
contour is bold to facilitate tracing – index contours.
Index contours are numbered at a break in the line, with the
number “upright” if possible.
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The difference in elevation between adjacent contours is the
Contour Interval.
The contour interval varies depending on the relief of the map – it
is usually a multiple of 10 feet.
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Bench marks are bronze disks, usually
set in concrete in sidewalks, bridges,
roadways and other structures. They are
accurately surveyed geodetic control
points. “Geodetic” refers to the size and
shape of the earth. Bench marks are
used to provide an elevation reference
for engineering projects such as road
building.
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Topographic Profiles show the
shape of the surface between
two points on a topographic
map. Contour elevations are
transferred from the map
(usually using a piece of paper
as shown) to graph paper. The
elevations are plotted on the
graph paper with reference to a
Y-axis showing elevation.
Because distances on the map
are transferred directly to the
graph paper, the horizontal
scale of the profile is the same
as the map (i.e. if the map is
1:24,000, the profile horizontal
scale is 1:24,000).
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However, unlike the map, the profile also has a vertical scale
determined by the Y-axis. For example if the Y-axis is 1 inch = 200
feet, this is a vertical scale of 1:2,400.
Because of this, profiles usually have vertical exaggeration
(vertical scale is larger than horizontal scale). In the example
above, the vertical exaggeration is 24,000/2,400 = 10x.
How is the vertical scale chosen? It is arbitrary i.e. 1” to 100’ =
1:1,200; 1” = 50’ = 1:600 and so on. So the creator of the profile
chooses what the vertical exaggeration will be.
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Example:
A profile has a horizontal scale of 1:24,000 and a vertical scale of
one inch = 100 feet. What is the vertical exaggeration?
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Slopes or gradients: slope expresses the relationship between the
change in height of the surface ('rise') with respect to a horizontal
shift in position ('run'). On a topographic map, slope is proportional to
the spacing of contour lines: closely-spaced contours indicate a steep
slope; widely-spaced contours indicate a gentle slope. To quickly
determine which way the ground slopes, look for a stream – when
contours cross a stream, they form a “V” shape pointing in the
upstream direction.
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There are a variety of ways in which slope can be expressed. For
example, the slope AB, rises 200 feet over a distance of 2 miles.
A common way to express slope is in terms of feet per mile; so
this is a gradient of 100 feet per mile.
A
200 feet
2 miles
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Gradients, expressed in feet per mile, can also be easily obtained
from the map; e.g. Pecan Creek drops from the 700' contour to
the 650' contour over a distance of 1.14 miles (map distance = 3
inches; RF = 1:24000; real world distance = 3x24000 = 72000
inches = 72000/63360 miles = 1.14 miles); this represents a drop
of 50 feet in 1.14 miles or 44 feet per mile.
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GIS.
Geographic
Information Systems
are essentially
computer-based maps
and analysis systems.
Usually map features
are stored in layers in
the computer and can
be superimposed on
screen.
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The utility of GIS is the ability to manipulate and analyze very large
amounts of spatial (map) data very quickly. The following GIS has
potential sites for a new apartment building in Memphis.
Geographic information such business locations, competition and
population can be used to select the best site for a new development.
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GIS showing 14minute drive-time
polygon for
customers of a
department store
in Atlanta. This
gives you an idea of
the market area
required by your
store (size and
population). This
can help answer
questions such as
“Is there room for
another store in
this city?”
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A big area of GIS growth is web-based GIS such as Google
Maps…
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