Lecture 3: Georeferencing Flashcards
What is georeferencing?
The principles and the process of transforming spatial data from an arbitrary system into a geographic or projected system. It is also known as registeration, coregisteration, geometric transformation
What is a geoid?
Geoid represents the true shape of the Earth. It accurately represents the shape and the size. It is based on the equipotential gravity surfaces at mean sea level. Rock densities cause the geoid to deviate from ellipsoid
What is a datum?
A datum is a reference point or surface on the earth’s surface against which position measurements are made, and an associated model of the shape of the earth for computing positions
Horizontal datums are used to describe points on the earth surface, in latitude and longtitude, or another coordinate system
Vertical datums are used to measure elevations or underwater depths
What is a geodatic datum used for?
Define the references for our characteristics of the size and shape of the earth and the origin and orientation of the coordinate systems used to map the earth
Datums have used to frame position descriptions since the first estimate of the earth’s size were made by Aristole
Transformation
When you move from one projection to another projection
Projection
a mathematical transformation that take spherical coordinates (latitude and longitude) and transforms them to an XY (planar) coordinate system. This enables you to create a map that accurately shows distances, areas, or directions.
Georeferencing
When you transform a 3D image to a projection
Types of map projection and what do they preserve?
Conformal/orthomorphic projection: the accurate transformation of angles
Equal area or equivalent projection: Accurate transformation of areas
Equidistant projections: Accurate transformation of distances
Azimuthal and gnomonic projection: Accurate transformation of directions
What are some light source positions?
Gnomonic: Light source is at the center of the globe
Stereographic: Light source is at the exact opposite of the tangency of the projection surface
Orthographic: At a considerable distance (infinite point). Light rays are parallel.
What flat surfaces are used to project the earth?
Azimuthal: Projection onto a plane
Cylindrical: Projection onto a cylinder
Conic: projection onto a cone
Where does distortion occur in the 3 types of projection?
Azimuthal: Distortions increase from standard point or line
• increase outward in concentric bands
• conformal azimuthal projections: area exaggeration increases outward
• equal area azimuthal projections: shape distortion increases outward
• can be varied to preserve area, angle, distance, or direction
Cylindrical: Distortions increase away from the line(s) or
tangency
Conic: Distortions increase away from the line(s) of tangency
UTM system
Universal transverse mercator system
The world is divided into 60 zones, 6 degrees of longitude each, from
84°N to 80°S latitude. Zone 1 is at the international date line
