Week 8 Flashcards
(27 cards)
What are the different ways to describe a position?
Coordinate system, relative position, and absolute position.
What is relative position used for?
Setting out and monitoring displacements of a structure, usually for small areas that can be assumed as a plane.
What must be considered for large project areas when describing position?
The shape of the Earth.
What is the Geoid?
An equipotential surface that best fits Mean Sea Level (MSL) and is defined by gravity.
How does gravity affect the shape of the Earth?
Gravity varies, defining the shape of the Earth and creating multiple closed level surfaces of different potential energy.
What is the reference datum for terrestrial measurements?
Mean Sea Level (MSL).
How does the Geoid differ between countries?
Local geoids can be parallel to the Geoid but offset, such as in mainland UK where it is 80cm below the Geoid.
What is EGM96?
The Earth Gravitational Model 1996, which is the most precise global geoid.
What is the shape of the Earth described by the ellipsoid?
A mathematical shape, an ellipse of rotation around its semi-minor axis.
What are the best-fitting global ellipsoids?
GRS80 (Geodetic Reference System 1980) and WGS84 (World Geodetic System 1984), which is based on GRS80.
How does the Geoid relate to the Reference Ellipsoid?
The Geoid does not coincide with the Reference Ellipsoid and can differ by up to 100m in some places.
What is the local ellipsoid used in Great Britain?
Airy (1830).
What is the geographic coordinate system used for?
To define a position on an ellipsoid, requiring knowledge of the specific ellipsoid.
What are the angles measured in the geographic coordinate system?
Longitude (East-West) and Latitude (North-South).
What is ellipsoidal height?
The distance of the Earth’s surface above or below the ellipsoid.
What is orthometric height?
Geoid height used in most engineering surveys.
What is the significance of the Geoid model in height conversion?
Different Geoid models can result in different orthometric heights for the same ellipsoidal height.
What is the datum in the context of positioning?
A set of parameters that define the coordinate system and its position relative to the Earth’s surface.
What is a datum realization?
Known points that make the coordinate system accessible to users, such as OSGB36.
What is the purpose of map projections?
To compute map coordinates from latitude and longitude of an ellipsoid, minimizing distortion.
What are the three main types of surfaces used for map projections?
Cylindrical, conical, and azimuthal (planar).
What is projection distortion?
The unavoidable distortion that occurs when a curved surface is flattened.
What is a conformal projection?
A projection where the ratio between two lengths is preserved, making it favorable for engineering applications.
What is the Transverse Mercator projection?
A conformal projection ideal for countries with a north-south extent, where the central meridian is at 90 degrees to the equator.
