Harry Williams, Earth Science1 EARTH SCIENCE: LOCATION, GRID SYSTEMS, MAPS, GIS. GRID SYSTEMS: Since geography deals with the position of features on the

Harry Williams, Earth Science 1

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.



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.



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.



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.




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).



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.


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).


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.


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.




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


Contour lines, spot heights and bench marks are used to show elevation on the map (topographic = “variation in height over surface”).


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.


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.


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.


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.


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).


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.


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?


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.


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

B

200 feet

2 miles


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.



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.


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.


GIS showing 14-minute 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?”


A big area of GIS growth is web-based GIS such as Google Maps…

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Harry Williams, Earth Science1 EARTH SCIENCE: LOCATION, GRID SYSTEMS, MAPS, GIS. GRID SYSTEMS: Since geography deals with the position of features on the