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GPS and GIS Flashcards

Global Positioning System (GPS) and Geographic Information System (GIS)

Introduction to GPS

  • Global Positioning System (GPS) emerged during the 1970s.
  • The system originated from space programs and was funded by the military for global navigation and guidance.
  • Many countries are developing their own systems.
  • Global Navigation Satellite Systems (GNSS) is the broad term for all satellite systems used in positioning.
  • GNSS receivers utilize GPS satellites and other satellite systems developed by various countries.
  • Performance:
    • Provides precise timing and positioning globally with high reliability and low cost.
    • Operates day or night, rain or shine, without needing clear lines of sight between survey stations.

Overview of GPS

  • Receiver positions are computed using precise distances from satellites derived from timing and signal information.
  • Satellites act as reference or control stations in satellite surveying.
  • Ranges (distances) to satellites are used to compute receiver positions.
  • This is conceptually similar to resection in traditional ground surveying, where distances and/or angles are observed from an unknown ground station to control points of known position.

GPS Segments

  • The Global Positioning System consists of three segments:
    • Space Segment
    • Control Segment
    • User Segment
  • Space Segment:
    • Nominally consists of 24 satellites operating in six orbital planes spaced at 60° intervals around the equator.
    • Includes four additional satellites as spares.
  • Control Segment:
    • Monitoring stations track satellite signals and positions over time.
  • User Segment:
    • Two categories of receivers are classified by access to two services.
      • Standard Position Service (SPS)
      • Precise Positioning Service (PPS).

The Geodetic Coordinate System

  • Geocentric vs. Geodetic Coordinate Systems:
    • Both systems describe locations on Earth using different reference frames and latitude definitions.
    • Geocentric Systems:
      • Use a three-dimensional Cartesian coordinate system with the Earth's center as the origin.
    • Geodetic Systems:
      • Use latitude and longitude, defined relative to a reference ellipsoid (spheroid or ellipsoid) and the local vertical.
  • Positions in satellite surveys are computed in the geocentric coordinate system but are inconvenient for surveyors because:
    • Geocentric coordinates have extremely large values due to their origin at the Earth’s center.
    • The X-Y plane aligns with the equator, unrelated to conventional north-south or east-west directions on the Earth's surface.
    • Geocentric coordinates do not indicate relative elevations between points.

What is GIS

  • GIS is an information management system that can:
    • Collect, store, and retrieve information based on its spatial location.
    • Identify locations within a targeted environment that meet specific criteria.
    • Explore relationships among data sets within that environment.
    • Analyze related data spatially to aid in decision-making about the environment.
    • Facilitate selecting and passing data to application-specific analytical models for assessing the impact of alternatives.
    • Display the selected environment graphically and numerically before or after analysis.

Layers in GIS

  • Maps represent different layers of spatially related information digitally recorded and incorporated into a GIS database.
  • Examples:
    • A: Parcels of different landownership
    • B: Zoning
    • C: Floodplains
    • D: Wetlands
    • E: Land Cover
    • F: Soil Types
    • G: Geodetic reference framework (network of survey control points).
  • Control points are found in each layer, providing a means for spatially locating all data in a common reference system.
  • Composite maps merge two or more different data sets.
  • H: Composite overlay, composite of all layers.

Spatial Elements in GIS

  • Points:
    • Define single geometric locations.
    • Used to locate features such as houses, wells, mines, or bridges.
    • Coordinates give spatial locations, commonly in state plane or UTM systems.
  • Lines and Strings:
    • Obtained by connecting points.
    • A line connects two points; a string is a sequence of two or more connected lines.
    • Used to represent roads, streams, fences, property lines, etc.
  • Interior Areas:
    • Consist of continuous space within three or more connected lines or strings forming a closed loop.
    • Used to represent governmental jurisdictions, parcels of landownership, land cover types, or large buildings.

Spatial Elements in GIS (Continued)

  • Pixels:
    • Tiny squares representing the smallest elements in a digital image.
    • Arranged in rows and columns to enter data from aerial photos, orthophotos, satellite images, etc.
    • Numerical values assigned to each pixel specify color or tone distributions.
    • Pixel size is variable, specified by dots per inch (dpi).
    • Example: 100 dpi corresponds to squares of \frac{1}{100} in. on each side, yielding 10,000 pixels per square inch.
  • Grid Cells:
    • Single elements (usually square) within a continuous geographic variable.
    • Sizes can be varied; smaller cells improve resolution.
    • Used to represent slopes, soil types, land cover, water table depths, land values, population density, etc.
    • Numerical values assigned to each cell indicate data distribution; e.g., soil types: 2 for sand, 5 for loam, 9 for clay.

Data Models of GIS

  • Spatial elements in GIS create two formats for storing and manipulating spatial data: vector and raster.
  • Vector Format:
    • Uses a combination of points, lines, strings, and interior areas.
  • Raster Format:
    • Uses pixels and grid cells.
  • Raster formats are often preferred because:
    • Many data sources are available in raster format (aerial photos, orthophotos, satellite images).
    • Enables computer-based data collection, storage, and manipulation.
    • Available image processing software can refine raster images.
    • Boundary locations for data sets like wetlands and soil types are often vague, so raster format doesn't significantly affect accuracy.

Vector to Raster Data Conversion

  • Vector-to-raster conversion (coding) can be accomplished in several ways:
    • Predominant Coding:
      • Each grid cell is assigned the value corresponding to the predominant characteristic of the area it covers.
    • Precedence Coding:
      • Each category in the vector data is ranked by importance or “precedence” relative to other categories.
    • Center-Point Coding:
      • A cell is assigned the category value at the vector location corresponding to its center point.

GIS Applications

  • Common application areas include:
    • Land-use planning
    • Natural resource mapping and management
    • Environmental impact assessment
    • Census, population distribution, and demographic analyses
    • Route selection for highways, rapid-transit systems, pipelines, transmission lines, etc.
    • Displaying geographic distributions of events such as automobile accidents, fires, crimes, or facility failures
    • Routing buses or trucks in a fleet
    • Tax mapping and mapping for surveying and engineering purposes
    • Subdivision design
    • Infrastructure and utility mapping and management
    • Urban and regional planning