Conceptual Foundations Flashcards
Distance
how far the object is from the reference object
Examples: at, nearby, in the vicinity, far away
Euclidean Distance
exact distance between two coordinates, straight line sqrt((x1-x2)^2 + (y1+y2)^2))
Manhattan Distance
vertical plus horizontal distance, x and y plane abs(x1-x2) + abs(y1-y2)
Internal Direction
where an object is located inside a reference object
Examples: left, on the back, athwart, abaft
External Direction
where the object is located outside of the reference object
Examples: on the right of, behind, in front of, abeam, astern
Spatial Model
Basic Properties and process for a set of spatial features
Vector
points, lines, polygons
Raster
composed of rectangular arrays of regularly spaced square grid cells and each cell has a value (attribute)
Examples: elevation, soil pH, salinity of a body of water
Pixel
smallest resolvable piece of scanned image. All pixels are cells
Geodatabase
object oriented spatial model (feature class, feature dataset, nonspatial tables, topology, relationship classes, geometric networks
Temporal
Time. the world is constantly changing. Multiple Maps can be created to show the changes or somehow superimpose multiple time period pieces of data on a map
Imprecision
all data is taken from a 3D globe and transferred to a 2D surface through spatial transformations (projections and datums) which causes distortions to the data
Uncertainty
The GIS data was created/collected at a certain point of time, may already be out of date
Geoid
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 tides and winds.
Used to reference heights, by registering ocean’s water level at coastal places using tide gauges, this is how mean sea level is determined
Reference Ellipsoid
a mathematically defined surface that approximates the geoid, the truer figure of the Earth or other planetary body. preferred surface on which geodetic network computations are performed such as longitude, latitude and elevation
Oblate Ellipsoid
fits the geoid to a first order approximation- formed when an ellipse is rotated about its minor axis
Sphere
As can be seen from the dimensions of the Earth ellipsoid, the semi-major axis a and the semi-minor axis b differ only by a bit more than 21 kilometres
International Terrestrial Reference System (ITRS)
three-dimensional coordinate system with a well defined origin ( center of the mass of the Earth) and three orthogonal coordinate axes (X,Y,Z)
X axis - oriented toward Greenwich meridian
Z axis - oriented toward Earth’s North Pole
Geodetic Datums
geocentrically positioned with respect to the center of mass of the Earth – WGS84, ITRF2000
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 its position relative to the Earth
Vertical Datum
reference point for elevations of surfaces and features on the Earth - could be based on tidal, seas levels, gravimetric, based on a geoid
Map Projection
Transforming coordinates from a curved earth to a flat map
NAVD88
gravity based geodetic datum in North America
WGS 84
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
Distortions, Distance,Direction,Shape,Area
Mercator
Preserves shape and direction, area gets distorted - projecting earth onto a cylinder tangent to a meridian
Azimuthal Equidistant
planar tangent - used for air route distances - distances measured from the center are true - distortion of other properties increases away from the center point
Cylindrical equal-area projections
preserves area, shape and distance gets 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 Projections
preserves directions and areas in limited areas - distorts distances and scale except along standard parallels - generated by projecting a spherical surface onto a cone
Latitude
low-latitude areas ( near equator) use a conical projection; Polar regions use a azimuthal planar projection
Extent
Broad in East-West (USA) use a conical projection; Broad in North-South (Africa) use a transverse-case cylindrical projection
Thematic
If you are doing an analysis that compares different values in different locations, typically an equal-area projection will be used
Spatial Reference System
coordinate-based local, regional or global system used to locate geographical areas
