Geodesy Projections Flashcards
What type of measurements are longitude and latitude?
Horizontal
What type of measurements are geoids?
Vertical
Perpendicular to gravity fields
Why are 2D maps never fully accurate?
Placing a 3D object onto a 2D surface
Distortion occurs
What is a Cartesian grid?
Eastings- longitude lines
Northings - latitude lines
What type of map projections are there?
Cylindrical
Conical
Planar / azimuthal
What do conformal maps do?
Preserve local shape and angles
What do equal area maps do?
Preserve areas of displayed features
What do equidistant maps do?
Preserve distances between certain points
Conformal projections?
Graticule lines intersect at 90° angles
At any point on the map the scale is the same in all directions
Commonly used for navigation or meteorology
Equal area projections?
All mapped areas have the same proportional relationship to the areas on earth they represent
Used for reference and educational graphs
Equal distance projections?
Cake constant in all directions
Used for radio, seismic mapping and navigation
What projections do the UK use?
Transverse Mercator projections
Ordnance survey national grid
Universal transverse Mercator
What is the Transverse Mercator Projection?
Cylindrical map projection - conformal so distance is distorted
The further away from the meridian the more distortion
Scale factor = 1
How do you calculate scale factor?
F = 1 + e^2/2R^2
e - distance as a difference of eastings
R - radius of ellipsoid at the point
What is the scale factor between sub parallels?
0.9996
What is the universal transverse Mercator?
Low distortion projection 60 zones Constant longitude Each zone = 6° of longitude Zone one is at 177° W
Curvature of Geodesics?
A line on the projection must contain the same intermediate points as that on the ellipsoid
The angle between the tangent to any two lines on the ellipsoid should have a corresponding angle on the projection
What is OSGB36?
The triangulation of GB
1783-1853
Retriangulation
Concrete pillars placed on hilltops
1935-1962
100 to now 6000 points
Used to create the ordnance survey national grid
Uses the Airy 1830 ellipsoid
Central meridian 2°W
What is the Ordnance datum Newlyn?
Geoid model - mean sea level as reference
Corresponds to the average sea level measured by the tide gauge in Newlyn 1915-1921
What is the WGS84?
Used for GNSS positioning
Designed for positioning anywhere on earth with the origin being the centre of earth
Established from 1600 points
Designed to best fit the global Geoid
Most widely used
Correct and valid New Year’s Eve 1993
What is ETRS89?
Earth centred earth fixed
Eurasian plate is static
Uk national coordinate system for 3D GNSS positioning
Monitored by 250 GNSS tracking systems
What are the ellipsoids relative to the UK?
Airy 1830
International 1924
GRS80
What are the projections relative to the Uk?
National grid
Irish national grid
UTM zones 29,30 and 31
