Co-ordinate systems Flashcards

Lecture 3

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1
Q

y and x

A

latitude (y) and longitude (x) co-ordinates

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2
Q

what are latitude and longitudes measurements of?

A

geographic co-ordinate system

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3
Q

what is longitude?

A

measured angle subtended from the centre of the earth to the surface of the earth, east and west of the greenwich meridian - prime meridian (0)

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4
Q

longitude values range from?

A

180degrees E to 180 degrees W

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5
Q

what is latitude?

A

anfle subtended from the centre of the earth North and south of the equator (0)
from North to south.

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6
Q

latitude values range from?

A

90N to 90S

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7
Q

Define the graticule.

A

A planar grid system of intersecting latitude and longitude lines.

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8
Q

how to the positives and negative of longitude and latitude change with the position around the equator and greenwich meridian?

A

NW- +latitude -longitude
SW- -latitude -longitude
NE- +latitude +longitude
SE- -latitude +longitude

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9
Q

how do we convert DMS to DD?

A

(degree + min/60 + sec/3600)

NB 1degree =60min
60 min = 3600sec

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10
Q

what is problem with the shape of the earth?

A

oblate spheroid - need to make a model of earth to be able to meausure it.

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11
Q

what is a model?

A

a simplified idealised representation of reality

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12
Q

What are the 3 surfaces to consider, deciding on the best model for earth?

A

True earth surface
Geoid (mean sea level)
ellipsoid

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13
Q

What is the ellipsoid model?

A

Mathematical represenation of the earth
reference for depicting geographical co-ordinates
a - equitorial axis
b- from poles the centre.

different ellipsoids work better to depict different parts of the earth.
Not a universal model.

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14
Q

What is a datum?

A

mathematical model of the Earth
erves as a reference for calculating the geographic
coordinates of a location.

A datum consists of two components:
1. A reference ellipsoid
2. A reference point or location
The further the distance of a location from the reference point of
a datum on an ellipsoid, the greater the geometric distortion of
those geographic coordinates

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15
Q

What was the old datum for South Africa?

A

Cape Datum
Modified Clarke 1880 reference ellipsoid
Initial point - Buffelsfontein, PE

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16
Q

What is the current datum for South Africa?

A

Hartebeeshoek94 Datum
WGS84 reference ellipsoid
initial point - Hartebeeshoek, Pretoria

17
Q

Summary - conclusion

A

The further the distance of a location from the reference point of
a datum on an ellipsoid, the greater the geometric distortion of
those geographic coordinates
GIS and maps use planar projections which allows for distance and area calculations.