Georeferencing - Part 1 Flashcards
Geodesy
Branch of mathematics dealing with the shape and area of the Earth or large portions of it.
Datum (3 parts)
Refers to a reference or foundation surface against which accurate position measurements are made.
Defines how a coordinate system is seathed over the ellipsoid
A “connected” unified network of measurements,
Examples of different datums
North American Datum 1927 (NAD27)
New North American Datum, 1983 (NAD28)
Tokyo 97
NAD83,
WGS 84
Ellipsoid
An ellipsoid is a three-dimensional geometric figure that resembles a sphere, but whose equatorial axis is slightly longer than its polar axis
Geoid
the hypothetical shape of the earth, coinciding with mean sea level and its imagined extension under (or over) land areas.
a ‘lumpy’ ellipsoid.
DEM
Digital Elevation Model
The Earth’s Shape (3 parts)
“Lumpy” Geoid
Imperfect Oblate Ellipsoid (Spheroid)
Slightly flattened at the North and South poles
Ellipsoid vs Geoid
Ellipsoids are commonly used as surrogates for geoids so as to simplify the mathematics involved in relating a coordinate system grid with a model of the Earth’s shape.
Ellipsoids and Geodesy
Ellipsoids provide a practical solution to measuring earth’s ‘lumpy’ surface.
Ellipsoids are used as surrogates for geoids, simplying the mathematics involved in relating a coordinate system grid with a model of the Earth’s shape.
Digital Elevation Model (DEM)
Generic term used to describe a continuous digital elevation surface above sea level.
Datum Types
Horizontal
Vertical
horizontal datum
A collection of points on the Earth that have been identified according to their precise northerly or southerly location (latitude) and easterly or westerly location (longitude)
Vertical datum
A collection of spatially distributed points on the Earth with known heights either above or below mean sea level.
How are HORIZONTAL DATUMS created?
Surveyors mark each of the positions identified on the Earth with a brass, bronze or aluminum disk monument.
Elevation
Height above a fixed reference point, usually a refrence geoid (model of the Earth’s sea level).
DTM
Digital Terrain Models
Digital Elevation Model (DEM
Generic term used to describe a continuous digital elevation surface above sea level.
Digital Surface Model (DSM)
Includes the elevation of all the features in the terrain above sea level such as bare ground, buildings, trees, powerlines
DIgital Terrain Model (DTM)
Contains only elevations of the bare ground above sea level without any buildings, trees, etc.
How is mean seal level determined near coastal areas?
Tide Gauge.
How is mean sea level determined far away from the shore?
Determined by the shape of the geoid.
Types of Coordinate Systems
Cartesian
Longitude & Latitude
Cartesian Coordinate System
A system that assigns two coordinates (x and y) to every point on a flat surface.
Latitude and Longitude System
Measured from a theoretical point at the center of the Earth
Latitude
The angular distance measured North or South of the equator from a point at the centre of Earth.
A line connecting all points of the same latitudinal angle is a parallel. (Parallel)
Longitude
The angular distance measured East or West of a prime meridian from a point at the centre of Earth. (Meridians)
Distance in curvature of a degree of latitude is always equal to…
aprox. 111 km.
Distance in curvature of a degree of longitude is…
Not constant.
Longitude converges at the poles.
If you wanted to know the distance of one degree of longitude at 49° of latitude, what equation would you use?
1°longitude = 111° * cos (latitude)
Where is the world’s largest globe located?
Yarmouth, Maine.
If you wanted to know the distance of one degree of longitude at 49° of latitude, what equation would you use?
1°longitude = 111 * cos(latitude)
Accurate 3D globe depicts true:
SHAPE
DIRECTION
DISTANCES
AREA
GRATICULE
Consists of the spherical coordinate system based on lines of latitude and longitude.
Only apply to geographic coordinate systems.
Great Circle
Any circle drawn on the globe with its center coinciding with the center of the globe, bisecting the earth into two equal halves.
Only parallel that is a great circle
The equator
The are of a great circle can be used to…
measure the shortest distance between any two points on the surface of the Earth.
Small Circle
A circle on a globe’s surface that does not share Earth’s centre—for example, all parallels of latitude other than the equator.
Map
A map is a representation of what is usually some portion of Earth’s surface as seen from above at a greatly reduced size.
2-D maps distort one or more: (5)
DISTANCE
DIRECTION
AREA
SHAPE
PROXIMITY
Disadvantages of a globe when using GIS (3)
- Very small scales (therefore little detail)
- Expensive to: create, update, transport, store.
- Only approximately one-half is viewable at a time.
Map Projection
A systematic transformation of spherical Earth into a flat map.
Involves the transfer of distinctive global patterns of parallels of latitude and meridians of longitude onto a developable surface (e.g plane, cylinder, cone)
Developable Surfaces (3)
Plane
Cylinder
Cone
Tangent
The line of contact between the earth and the developable surface is called a tangent. Only one standard parallel.
Secant
The developable surface touches the sphere at two points, creating two standard parallels.
Tangents and Secants line characteristics
zero distortion along these lines.
As you move farther away from the tangent or secant lines, distortion increases.
Characteristics normally considered in choosing a map projection are as follows:
Area
Shape
Scale
Direction
Special Characteristics
Equal-Area Map (2)
A map projection is equal area if every part, as well as the whole, has the same area as the corresponding part on Earth, at the same reduced scale.
No flat map can be both equal-area and conformal.
Conformal Map (4)
A map projection is conformal when at any point the scale is the same in every direction.
Meridians and parallels intersect at right angles.
Local shapes, and the shapes of very small areas and angles with very short sides are preserved.
The size of most areas, however, is distorted.
Equidistant map
Show true distances only from the center of the projection or along a special set of lines.
No flat map can be both equidistant and equal area
Types of projections (4)
Conformal
Equidistant
Equal-Area
Azimuthal
Azimuthal map projections
Projections to a plane placed tangent to (just touching) the global at a point. Correctly represent angular relationships.
Scale
A proportion used in determining the dimensional relationship of a representation to that it represents
Rhumb Line
A line of constant compass direction, or constant bearing, that crosses successive meridians at the same angle; appears as a straight line only on the Mercator projection.
Planar Projection
Standard ‘point’ at a pole. Shows great circle routes (shortest distance between two points on Earth) as straight lines.
Standard Line/point
The line/point where the developable surface touches the reference globe, along which there is no distortion.
Tangent to
The sphere touches the developable surface at a single point
Secant to
The sphere intersects the developable surface
Graticule
Spherical coordinate system based on lines of latitude and longitude
A rhumb line is a straight line on…
A mercator projection
Cylindrical projection example
Mercato
