# Shared Flashcard Set

## Details

GPS for GIS Midterm
geodesy, gps theory, gis basics
42
Geography
10/13/2011

Term
 geodesy
Definition
 the science of the measurement and mapping of the earthâ€™s surface
Term
 geodetic survey
Definition
 A survey of a large area of land in which corrections are made to account for the curvature of the earth
Term
 WGS84
Definition
 world geodetic system 1984 - latitude, longitude, and ellipsoid height (X = Greenwich meridian)
Term
 the geoid
Definition
 the real earth; equipotential surface (equal gravity) that best equates to Mean Sea Level; very complicated surface; can be sensed by instruments
Term
 ellipsoid
Definition
 simple geometric surface; can't be sensed by instruments; WGS84 uses best mean fit to the Earth
Term
 geoidal undulation
Definition
 The height difference between the ellipsoid and the geoid
Term
 map projection
Definition
 a system of intersecting lines, such as the grid of a map, on which part or all of the globe or another spherical surface is represented as a plane surface
Term
 reference system
Definition
 Shape, size, position and orientation of a (mathematical) Reference Surface, (eg. Sphere or Sheroid). It normally is defined in a superior, geocentric three dimensional coordinate system (for example WGS84)
Term
 reference surface
Definition
 mathematically (eg. Sphere or Sheroid) or physically (Geoid) defined surface to approximate the shape of the earth for referencing the horizontal and/or vertical position
Term
 reference frame
Definition
 A set of controlpoints to realize a Reference System
Term
 geodetic datum
Definition
Term
 azimuthal projection
Definition
 type of projection that results from projecting a spherical surface onto a plane
Term
 Mercator
Definition
 modified cylindrical projections with all longitudes parallel, and latitudes increasing in size farther North or South
Term
 Transverse Mercator
Definition
 cylindrical projection that intersects globe at two areas; used in U.S. for states that run norht-south
Term
 polyconic projection
Definition
 has a cone for each parallel; very accurate north-south; accuracy distorted east-west
Term
 Robinson projection
Definition
 based on a table of coordinates; projection distorts shape, area, scale, and distance in an attempt to balance the errors of projection properties
Term
 Lambert projection
Definition
 secard conic (cone intersects globe at two areas); used for states that are long east-west
Term
 scale factor
Definition
 the Ratio of a distance on the grid projection to the corresponding distance on the ellipse
Term
 combined scale factor
Definition
 used to scale ground distances to grid distances, or vice versa; a product of both the Elevation Scale Factor and the Projection Scale Factor
Term
 3 components of GPS
Definition
 space (satellites); control (ground control stations); user (us!)
Term
 Space Vehicles (SV)
Definition
 satellites that continuously broadcast position and time in circular orbit around Earth; contain receivers and transmitters
Term
 control segment of GPS
Definition
 tracks satellites & checks & corrects orbits and clocks
Term
 NAVSTAR stands for...
Definition
Term
 three components of GPS
Definition
 1. Space2. Control3. User
Term
 Why is GPS better than traditional surveying...
Definition
 Weather independentDoes not require line of sightGives high geodetic accuracyCan be operated day and nightIn general it is quicker and requires less manpower
Term
 Range =
Definition
 Time Taken x Speed of Light
Term
 Example of error for one-tenth of a second off..
Definition
 1/10 second error = 30,000 Km error
Term
 How GPS works in five steps:
Definition
 1. The basis of GPS is "triangulation" from satellites. 2. To "triangulate," a GPS receiver measures distance using the travel time of radio signals. 3. To measure travel time, GPS needs very accurate timing. 4. Along with distance, you need to know exactly where the satellites are in space.5. Finally you must correct for any delays the signal experiences as it travels through the atmosphere. You must also correct for the clock differences between the GPS receiver and the satellites.
Term
 Number of satellites required for point positioning:
Definition
 3 to determine longitude and latitude4 to determine height
Term
 Triangulation
Definition
 three spheres in 3D space intersect at no more than two points; one is discarded since it is not on the geoid at all
Term
 Trilateration
Definition
 uses the known locations of two or more reference points, and the measured distance between the subject and each reference point; uses geometry of triangles; 3 ref. points needed for trilateration alone
Term
 Measuring distance to a satellite
Definition
 distance = rate x timesatellite transmitter sends a number of unique signals that repeat every millisecondGPS measures how long it takes to recieve signals from satellite transmittertime delay in signal transfer determines distance
Term
 Selective Availability (SA)
Definition
 dep. of defense can degrade signal over specific areas of the world during conflict; disabled May 2000
Term
 Waypoints
Definition
 locations or landmarks worth recording and storing in your GPS. These are locations you may later want to return to.They may be check points on a route or significant ground features. (e.g., camp, the truck, a fork in a trail, or a favorite fishing spot).
Term
 Route
Definition
 a series of waypoints entered in the order that you want to navigate them
Term
 Sources of error for GPS
Definition
 timing; satellites drift from their predicted orbits; waves travel through a number of Earth's layers and can be delayed; signal may be reflected from objects like buildings; mask angle/cut-off elevation if satellites have low angles
Term
 How to minimize GPS errors
Definition
 differential GPS - uses reference point with known coordinateeliminates errors in the satellite and receiver clocksminimizes atmospheric delaysAccuracy 5 mm - 5 m
Term
 GDOP
Definition
 geometrical dilution of precisionincludes latitude, long, height, and time
Term
 PDOP
Definition
 positional dilution of precisionincludes lat, lon, and height
Term
 HDOP
Definition
 horizonal dilution of precisionincludes lat and lon
Term
 VDOP
Definition
 vertical dilution of precisionincludes only height
Term
 GNSS
Definition