Conceptual Foundations Flashcards
Georeferencing
Associating a map (such as a pdf without spatial information), or image (such as an aerial image without spatial information), with spatial locations.
Control Points
Consisting of multiple points, points come in pairs that match the spatial location with a point on an unreferenced image or map.
Spatial Reference System (SRS)/ Coordinate Reference System (CRS)
A coordinate-based local, regional, or global system is used to locate geographical entities.
International Terrestrial Reference System (ITRS)
It’s a three-dimensional coordinate system with a well-defined origin (the center of mass of the Earth) and three orthogonal coordinate axes (X, Y, Z).
Map Projection
transforming coordinates from a curved earth to a flat map
UTM
Universal Transverse Mercator - a global coordinate system. UTM zones are 6 degrees
Horizontal Datum
Model of the Earth as a spheroid ( 2 components, reference ellipsoid and a set of survey points, both the shape of the spheroid and the features on the Earth)
NAVD88
Gravity-based geodetic datum in North America
Geodetic Datum
A set of control points whose geometric relationships are known, either through measurement or calculation.
Vertical Datum
reference point for elevations of surfaces and features on the Earth
WGS84
World Geodetic System, reference coordinate system used by the Global Positioning System (GPS).
SRID Integer
spatial reference system id numbers, including EPSG codes defined by the International Association of Oil and Gas Producers
Four Distortions
Angle, Area, Direction, Shape (A SAD)
Mercator Projection
Preserve shape and direction, the area gets distorted, projecting earth onto a cylinder tangent to a meridian.
Azimuthal Equidistant
planar (tangent), used for air route distances; Distance measured from the center is true; Distortion of other properties increases away from the center point
Cylindrical Equal-area Projection
preserves area; shape and distance get distorted near the upper and lower regions of the map; straight meridians and parallels; meridians are equally spaced and the parallels are unequally spaced
Conic Projection
preserves directions and the area in limited areas; Distorts distances and scales except along standard parallels; Generated by projecting a spherical surface onto a cone
Choosing projections: based on latitude
Latitude: low -> conical projection; polar regions –> azimuthal planar projection
Choosing projections: based on the extent
Extent: Broad in East-West –> conical; Broad in North-South –> Cylindrical projection
Choosing projection: based on thematic
If you are doing an analysis that compares the different values in a different location, typically an equal-area projection will be used.
Discrete features
a feature that has a definable boundary, begins and ends, for example a highway or a lake
Continuous phenomena
Each location is a measure of something, including the measure of concentration level, and the measure of a value in terms of a fixed point, such as elevation
Geoid
the shape that the surface of the oceans would take under the influence of Earth’s gravitation and rotation alone, in the absence of other influences such as wind or tides; Used to reference heights, by registering the ocean’s water level as coastal places using the tide gauges (how the sea level is decided).
Reference Ellipsoid
A mathematically defined surface that approximates the geoid, the truer figure of the Earth, or other planetary body
