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