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Lecture 5

GGR272H1 Lecture Notes - Lecture 5: International Date Line, Sexagesimal, Spheroid


Department
Geography and Planning
Course Code
GGR272H1
Professor
Don Boyes
Lecture
5

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Lec 5 Coordinate Systems
-Longitude and latitude
based on angular unit of measure
two axes to measure angles from equator and prime meridian
o East or West PM, North or South E
How to read angles (they are not coordinates)?
o 48 degrees, 51 minutes, 300 seconds north of the equator and 2 degrees, 17
minutes, 40 seconds east of the prime meridian
o we read latitude (N/S of Equator) first and then longitude (E/W of Prime
Meridian) (opposite in GIS)
Axes for frame of reference
o earth’s ais – the earth spins on it using it as the prime meridian vertical
0 degrees going down from the north pole, 90 degrees on each of the
sides (left side is wester and right side is east) and it goes all the way to
the top to 180 degrees called the International Date Line
o equator it is horizontal 0 degrees goes up by 10 degrees all the way until 90
degrees up means North and down means South
o where they cross call it origin
o once you make all the lines it turns into a grid defined from lat and long called
the graticule
lat and long are good at describing location on the surface of the earth, but not good at
measuring distances
-What are geographic coordinate systems?
-Defining the prime meridian
runs from the north pole through Greenwich England to the south pole
o why Greenwich
chosen in 1884
was already basis for U.S. time zones, most sea charts
o different countries had their own prime meridian and were competing to
standardize
it goes through the royal observatory goes through the airy transit
circle within it goes through the eye piece of the telescope spider web
cross hair
airy transit was developed by sir George airy in 1850
-Recording lat/long coordinates
long/lat coordinates
o sexagesimal (base 60) system
o Degrees, Minutes, Seconds (DMS)
E.g. 142 degrees, 32 minutes and 23 seconds
1 degree = 60 minutes
1 degree = 3600 seconds
o two ways of describing lat and long on gis DMS and Decimal degrees or DD
rather work with a base 10 system (DD) than a base 60 system (DMS)
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Decimal Degrees
o Signs
No N/S/W/E use positive/negative lat or positive/negative long
-Converting DMS-DD coordinates
DMS TO DD
o Keep the degree + minute divided by 60 (m/60) + (s/3600)
Turning minutes into degrees or fraction of degrees
And then add all the numbers up EX. 121 + 8/60 +6/3600 = 121.135
DD TO DMS
o 121.135
121 stays the same
.135 times 60 = 8.1 keep the 8 for minutes
0.1 times 60 = 6 6 is now the seconds (CAN HAVE DECIMAL SECONDS
BUT NOT THE SAME AS DECIMAL DEGREES)
-Converting degrees-minutes-seconds values to decimal degree values
-The Earth as an Ellipsoid
the earth is flat a little due to the fact that it is spinning
Earth can be modeled as an ellipsoid
o As a semi-minor axis (b along the y axis) and semi-major axis (a along the x axis)
Semi major is bigger
o Amount being flattened f= a-b/a f=1/300 (approx.)
o The distances are not the same from one place or the other
Geocentric latitude
o If you do treat the earth as sphere and measure a latitude from that model, then
you are using geocentric latitude
o In this case, if we drew an equatorial plane and 45 degrees from it a tangent line,
then that line will go straight through the middle or center of the earth then it
becomes geocentric latitude
Geodetic latitude
o If ou do the eat sae thig, ut ith a ellipsoid the it o’t go through the
center of the earth it will pass through the equatorial plane but not the center
Distance measurements
o If you treat the earth as a sphere to measure distance, it will be off by 1 km for
every 110 km (>1%)
o If you are making a map 1:5,000,000 or smaller: not noticeable, use geocentric
o But, 1: 1,000,000 or larger: noticeable, use geodetic
Not all data comes in geodetic
-Spheroids and spheres
-Horizontal datum
Important when trying to describe data more accurately and taking consideration
flattening of earth and ellipsoid
Ellipsoids
o Eah ellipsoid is desiged to appro. the earth’s shape for oe part of the plaet
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