The Earth in Space and
finding where we are.
The Earth in Space
Where are we, and what does that
mean?
June 22
Another way of looking at June 22…
www.time.gov
Daylight pattern on 22 September… near the equinox
What would December 22 look like?
What significance do the
tropics of Cancer and
Capricorn have?
Kepler’s Laws of
Planetary Motion
Weather and
Ocean currents
are caused by the
imbalance of heat
from vertical solar
radiation between
the tropics and
very little direct
solar radiation at
the poles (or none)
Describing Where: spatial
referencing systems
• Many common applications use only a
simple reference grid
Geographic coordinates (Latitude
and Longitude).
Simple conversion of angles at the
earth’s center gives a basic
equirectangular projection
Common and simple*
• the Babylonian system is based on the number
60. Latitude and Longitude are based on a a
"sexagesimal" system. A circle has 360 degrees,
a degree has 60 min. and each minute is divided
into 60 seconds. This nomenclature is known as
DMS (degrees, minutes, seconds)
• Perhaps a better system is to convert the
minutes/seconds to a decimal part of a degree
this is known as DD (decimal degrees).
e.g. 120 30’ 00” = 120.5
‘Great Circle’ routes
• The shortest distance between any two
points on the surface of a sphere can be
defined by a plane that connects the two
points and intersects with the center of the
sphere….
The Great Circle Route from HSV (Huntsville AL) to FCO (Rome):
The curved flight path north and east crossing Labrador is the shortest route
between the two points.
The Great Circle route from Chicago to Hong Kong…. http://gc.kls2.com/
Defining ‘0’
• In terms of latitude, 0 is easy to define, it it the
line between the north and south poles where
the earth's rotational speed is at its maximum
• 0 degrees longitude is much more difficult, as all
lines of longitude are equally valid as the base
line or ‘0’
• The French (of course) wanted 0 to run through
the center of the civilized world… Paris
• However, England was the true global maritime
power when ‘0’ was established… therefore…
as the ‘big dog’ the ‘Prime’ meridian, the
baseline runs through the Royal Observatory at
Greenwich England.
The importance of ‘time’
• SO how does one determine exactly
where you are…. We have established a
grid system, Latitude and Longitude… we
have ‘0’s fixed in place… now… where are
you?
• Latitude can be determined from a fixed
stellar reference, usually Polaris, the ‘north
star’.
The Sextant
A device for measuring the
angle of a star above the
visible horizon
Longitude… no stars are
available to show longitude… it
requires TIME.
The sun requires 24 hours to cover 360
degrees of the earth…. 1 degree every
4 minutes. 15 degrees every hour….
IF
I can compare accurate times between
2 locations, I can find the difference in
longitude!
A sundial is dandy, but it only keeps
solar time… there is no way to know
how far you are from another location
A pendulum clock
cannot be used at
sea….
In 1714 a prize was offered: 20,000 pounds
In 1727 John Harrison started building clocks to attack the problem of longitude….
Isaac Newton (who was on the search committee was convinced that no clock would
ever be invented that could do the job…)
John Harrison developed a highly accurate clock which proved to lose only 1.25
minutes in a 2 month long voyage to Jamaica…even though is final time piece was
proved in 1762…..it was not until 1772 that the prize was awarded (note that he
worked on this for 35 years before success and it was another decade before he got
paid!)
GPS: Global Positioning System
• The Geographer’s best friend!
• You can say with confidence… “I’m
not lost!, I’m never lost!”*
*Of course, where everybody else is or how to find your destination….
That’s a different question!
For all the ‘high tech’ involved
the concept for the system is
actually quite simple….
• A network of 24 Satellites in geosynchronous
orbit
• Each Satellite has an extremely accurate
atomic clock and a radio transmitter with a
unique frequency
• The GPS receiver also has a highly accurate
clock and the ability to receive radio signals
from multiple satellites at the same time.
Each Satellite constantly broadcasts the time and
every 30 min or so it also broadcasts an
ephemeris which is the projected location of all
satellites in the constellation.
The satellite broadcasts the time, the receiver
compares the time from the satellite to its
internal clock… even at the speed of EMR
propagation there is a slight delay… this delay
provides a distance to each satellite.
It is not triangulation but rather trilateration
Standard Time
• For the average person, feeding the pigs,
hoeing the cotton… accurate time was not
necessary
• Everybody can see when the sun comes
up, when its directly overhead its noon and
when it goes down its dark….
• The spread of the railroads and more rapid
transportation made it necessary to
establish standard time zones.
Each time zone is 15 degrees of longitude (the distance the sun travels in one
hour). Noon is true solar noon only at the center of the time zone… to the east
its past solar noon and to the west it is not yet solar noon….
The negative numbers in each time zone are the offset from GMT or ‘zulu’
Why are the time zone boundaries not straight… nor even along
state boundaries?
The international date line… by convention where Saturday becomes Sunday!
Cross this line going east and lose a full day, cross going west and arrive before
you leave
Making Maps
• The earth is round (ish): maps and
computer screens are flat
• How do I make a spherical object appear
to be flat?
While the Earth is ‘roundish’, maps/display screens are FLAT
Map Projections are different ways that a curved surface can be
displayed FLAT.
A projection can intersect the surface in many different places.
There is no distortion at the points of intersection… distortion
increases as the distance from the intersecting points
increases.
The direction of projection can be changed according
to the needs of the person using the data
A complicating factor: the Earth is not
really ‘round’. It is in fact an ‘oblate
spheroid’*.
Distortion:
It is impossible to project a curved
surface to a flat display without
causing distortion of the features.
An almost unlimited number of
projections have been developed
for the purposes of individual
users.
The UTM Grid (Universal Transverse Mercator) Projection
Each Cell is 6 degrees of longitude and 8 degrees of
latitude. For civilian applications, the critical issue is the
zone.
UTM zones for the Conterminous US:
Note that most states are split between 2 or 3 zones… it is
critical to know which zone your data are in.
Nevada is one of the few states where virtually all state data are
found in UTM coordinates.
The “State Plane” Coordinate
System
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•
•
•
•
Each state has a unique set of coordinates
‘wide’ states use a Lambert projection
‘tall’ states use UTM projection
Larger states have multiple zones
The goal is to create minimal distortion
between the curved surface and flat
display and to have simple, positive
coordinates
Wisconsin has 3 zones and uses a
Lambert conic projection.
Oregon has 2 zones and also uses the
Lambert projection.
Illinois uses a UTM projection
Public Land Survey System
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•
•
•
The idea of Thomas Jefferson
Set up in 1785 for the ‘western’ US
Creates ‘square’ landuse patterns.
Commonly used in parcel descriptions
Land use patterns in
Ohio from
‘uncontrolled’ survey.
Land use patterns in
Ohio from Public Land
Survey System (PLSS)
survey.
USPLSS baselines for the US… note the patterns of history and
politics evident in the locations of this system.
TN
AL
N-S divisions are
‘TOWNSHIPs’
E-W divisions are
‘RANGE’
Each section is 1 mile
square (640 Acres)
The various ‘homestead’
acts gave rights of claim
to a ¼ section…160 acres.
1 mile = .6km
1hectare = 2.2 acres