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Geodesy
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- 1What is geoinformatics?• How information science is used as the framework to capture, process, and display spatial data
• Utilises geographical data sets, geographical information systems (GIS), location techniques and remote sensing methods to study the state of the environment
• Geoinformatics uses geo-computation and geo-visualisation for analysing geoinformation - 2What components are included in geoinformatics?• Cartography
• Geodesy
• GNSS
• Photogrammetry
• Remote sensing
• Spatial analysis
• Web mapping - 3What is geodesy?• Science of measuring/monitoring the size and shape of the Earth
- 4Why monitor the shape and size of the Earth?- Earths surface displaces by 30cm due to the influence of the Moon and Sun – the tides
- Plate tectonics - to understand how they move etc - 5What is the history of geodesy?• (500 BC) Most people thought the Earth was a sphere
• (384-322 BC) Aristotle estimated the Earth’s circumference as 400000 stades (approx. 45,500 miles)
• (250 BC) Eratosthenes, used the equation c = (360° ÷ θ) x (s)
θ = angle between two reference points on the surface of the sphere/circle
S = distance between those two reference points on the surface - 6How did Eratostenes calculate the circumference of the Earth?• Valid assumption that the Sun's rays are parallel
• He had two reference points a deep well in Syene (which had no shadow at the bottom on the longest day) and a column in Alexandria
• He measured the shadow of the column when there was no shadow in the well
• Important to do it at the same time
• He calculated the circumference as
(360° ÷7.12°)=50
50 x 4,400=220,000
= 25,000 miles
Our current estimate is 24,855 miles - 7What were the two schools of thought on the shape of the Earth?o Prolate
- Axis between the poles is longer than around the equator
o Oblate
- Wider around the equator and squashed at the poles
- Based on Newton - 8How do you decide on what the shape of the Earth is?o You need to measure the curvature of the Earth at different points
- As the curvature is joined to the radius
o This was done by measuring the time it takes for the sun to set - 9How do you measure the curvature?• Find a place where you can see the sunset over the ocean
• Make your first measurement lying down on the ground and your second measurement while standing
o If viewing a sunrise, you do the opposite
• Before the sunset, measure the height of your eyes while you are lying down and still able to see the horizon where you expect the sunset
• Prepare a stopwatch to begin counting and water for the sunset
• When the last bit of sun has disappeared, start the stop watch and quickly get up and stand in position that is directly above where your eyes were when you were laying down. You should be able to see the sunset again
• Then you stop the stopwatch when you see the last bit of sun disappear again
• Measure the height to your eyes in the standing position
• The time is essentially the radius of the Earth at that point
o Turns out the Earth is oblate - 10What is a coordinate system?o A reference system used to represent the locations of geographic features, imagery and observations within a common geographic framework (ESRI)
o A common geographic framework is important, as there are lots of different systems - 11What are coordinate systems defined by?o Measurement framework
- Geographic - Spherical coordinates are measured from the Earth’s centre and are measured with angles, such as latitude and longitude
- Planimetric - The Earth’s coordinates are projected onto a 2D planar surface such as ordnance Survey GB grid
o Unit of measurement
Metres, decimal degrees
o The definition of the map projection for projected coordinate systems
o Other parameters
A spheroid of reference, a datum, projection parameters, a central median, and possible shift in the x/y directions - 12What are the two common types of coordinate systems for geospatial data?o A global or spherical coordinate system (latitude – longitude) also called a geographic coordinate system
o A projected coordinate system based on map projection (map projections) : E.g. Transverse Mercator, Albers equal area, or Robinson (among others) which provide a mechanism to project maps of the Earth’s spherical surface onto a two-dimensional Cartesian coordinate plane - 13What is the geoid?o Surface of the Earth’s gravity field. Approximately the same as mean sea level and perpendicular to the direction of gravity pull. Since the mass of the Earth is not uniform at all points the shape of the geoid is irregular
Imagine trying to work out what mean sea level is even over continents by removing all the topography but keeping the same irregular gravity - 14What is the spheroid?o A three-dimensional shape created from a two-dimensional ellipse
Simplified shape compared to a geoid
Might also be referred to as an ellipsoid, but being technically this is when all 3 axes are different lengths but in the literature they’re the same - 15What is the datum?o Built on top of a selected spheroid and can incorporate local variations in elevation
Essentially you need a reference point on a spheroid, such as the Greenwich median for a latitude longitude system, where both 0°s cross is the origin
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- Fichas sobre Geodesy, creado por Stephen Booth el 24/11/2018.
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- Stephen Booth
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