Transcript Getting Oriented with Maps

Getting Oriented with Maps
Maps are useful for finding our way
around; to an Earth Systems Scientist,
they are Models of our Planet.
Compass Orientation
North is a direction parallel to
Earth’s Axis and in the direction of
Star Polaris
The Compass
• North is often (but not always) oriented as
up on a map. Moving clockwise at ninety
degree angles are East, South, and West.
Intermediate angles are North-East, or
North-North-East, etc.
• For greater position, directions are
described using angles, with North as Zero
Degrees. We will practice this with a
worksheet.
Magnetic Declination
• A magnetic compass’s needle orients
toward the Earth’s north magnetic pole.
• The Magnetic Pole is not at the same
place as the True North pole.
• At different places on Earth, a magnetic
compass will deviate from true north. This
is shown on topographic maps. Here, it is
about 4.5 degrees.
Coordinate Systems
• For small land areas a grid system is
simple and easy to understand.
• A football field is a partial grid, in fact it is
called a “gridiron”
• The city of Washington DC was designed
as a grid by Pierre L’Enfant. It is very
much like a Cartesian graph in algebra
class, with the addition of avenues.
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Problems with Grid Maps
• If your city isn’t built on a swamp, there
may be hills, or streams to get around.
• At some scale, the world gets too big for a
flat grid to fit it.
• At sea, what do you use for reference to
mark the corners?
Getting Around “On the Ball”
A Spherical Planet has no corners, but if it is
spinning we can define some key places;
The Poles – the Rotational Axis passes
through them. In our solar system, North
is defined by the “right hand rule” – wrap
your fingers in the direction of spin, your
thumb points north
The Equator – Extend a plane perpendicular
to the axis halfway between the poles.
The Equator
• Imagine someone standing at the center of
a hollow Earth. If they looked straight out
from the axis at the center, and turned
around, their eyes would be following the
equator.
• Using a neat model, with a turning “Earth”,
your instructor will show how an Equatorial
line can be constructed.
Parallels of Latitude
• Lines of latitude are generated the same way as
the Equator, but are north or south of it.
• The angular measurement from the center of the
earth to the new line is its latitude, measured in
degrees (and minutes and seconds if we are
getting precise).
• Latitude lines are parallel, but only the Equator
is a Great Circle; the others all have less
circumference.
DMS
• Just as we used the DMS system to locate
stars and planets in astronomy, angles are
broken down.
• A full circle is 360 degrees (in latitude, we
only go north or south by 90 degrees –
past that you are going around the other
side)
• 1 degree = 60 minutes (60’)
• 1 minute = 60 seconds (60”)
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RASC Calgary Centre - Latitude and Longitude
By: Larry McNish
Page last updated August 28, 2005
The Shape of the Earth
The Earth is not a Sphere - it is an "Oblate Spheroid" - it is 134.397 Km further around
the Equator than it is around the Poles.
The following diagram is exaggerated to show what we mean. (The blue line is a circle.
1/3
6378.14 Km
(3963.19 miles)
6356.75 Km
(3949.90 miles)
6371.00 Km
(3958.76 miles)
(99.66 % of the Equatorial radius)
facweb.bhc.edu/academics/science
/harwoodr/GEOG101/Study/LongL
at.htm
• Here is a nice link if we can follow it
Determination of Latitude
• A sextant in an instrument for determining
latitude. In the northern hemisphere you
take a sighting of Star Polaris (the North
Star) and measure the angle between in
and a vertical (a string tied to a weight).
• Your instructor has a simple instrument to
show you how this is done (demonstrate).
The Problem of Longitude
• The other coordinate for location is
longitude. Longitudes, or Meridians, are
all great circles which pass through both
poles.
• Longitudes are also measured in degrees,
from 0-180, east or west of…What?
• There is no obvious reference as there is
with the equator.
The Longitude Problem,
continued.
• There is a certain prestige to be the starting
point for all maps.
• Observatories were the starting points because
they had the equipment to make the
measurements.
• The US Naval Observatory in Foggy Bottom,
The Royal Greenwich Observatory outside of
London England, and The Paris Observatory in
France, all wanted the honor.
The Problem of Longitude, III
• Longitude is also tied in to time of day.
Once it was realized that ships would
change days crossing the 180 degree
meridian, the world finally settled on the
Greenwich Observatory as the Prime
Meridian, although French maps did not
accept this until 1923. The “line states” of
the US western territories were based on
the Naval Observatory’s meridian.
Another Longitude Problem
• Accurate determination of longitude requires
knowing what time an observation is made. If
you measure the time the sun is most directly
overhead, your local noon can be compared to
correct time to determine longitude.
• At sea, a pendulum clock is unreliable because
the ship rocks.
• Tables of moons transiting planets will work, if
you can wait to make the measurement exactly
as they are eclipsed and the sky is clear.
The Longitude Problem Solved
William Harrison, a clever English man, was an
extraordinary clockmaker. Since he was not
formally educated, he faced great hurdles and
ruined his health but ultimately was awarded a
huge prize by the British Royal Society for
developing a chronometer which worked at sea.
One of the main jobs of the US Naval Observatory
was making sure chronometers ran correctly.
Time and Date on Earth
• Local noon used to be good enough. The US
railroad industry had to syncronize railroad
timepieces to avoid collisions on shared rail
lines.
• Today the world is divided into 24 time zones.
Each keeps the same minutes. The hours
change. Official time (“Zulu”, or GMT, or UCT, is
based on Greenwich.
• Each time zone is about 15 degrees apart
(360/24 = 15).
• The International Date Line is at 180 degrees
longitude.
The System of Latitude and
Longitude
• Latitude is specified first, then longitude.
• Latitude must be expressed as North or
South, and Longitude as East or West;
otherwise a pair of numbers could
describe four different places.
• You live at around 38 deg 28 min North, 78
deg 53 minutes West.
• Let’s practice finding latitude and longitude
on a flat paper map!