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GGR272H1 (20)

Don Boyes (20)

Lecture 5

Department

Geography and PlanningCourse Code

GGR272H1Professor

Don BoyesLecture

5This

**preview**shows page 1. to view the full**5 pages of the document.**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 ais – 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 eat sae thig, 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 Eah ellipsoid is desiged to appro. the earth’s shape for oe part of the plaet

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