Sources of Geographic Information

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1 Sources of Geographic Information Data properties: Spatial data, i.e. data that are associated with geographic locations Data format: digital (analog data for traditional paper maps) Data Inputs: sampled from the real world Using GPS? DIY! digitizing from paper maps produced by government agencies, e.g. census bureau, USGS, USFS, state government, etc. space or airborne remote sensing (NASA, NOAA, commercial, etc.) Approximately 80% of the duration of many large scale GIS projects is concerned with data input and management

2 Global Positioning System (GPS) A space-based 3-dimensional measurement and positioning system that operates using radio signals from satellites orbiting the earth Created and maintained by the US Dept. of Defense and the US Air Force The system as a whole consists of three segments: satellites (space segment) receivers (user segment) ground stations (control segment) Note: Russia and a European consortium are implementing similar systems.

3 GPS Space Segment (Satellites) 24 NAVSTAR satellites in the GPS constellation orbit the Earth every 12 hours ~11,000 miles altitude (a very high orbit) positioned in 6 orbital planes (4 per plane) orbital period & planes are designed to keep 4-6 satellites above the horizon at any time everywhere on the planet controlled and monitored by five ground stations around the globe

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5 GPS User Segment (Receivers) Ground-based devices that can read and interpret the radio signals from several of the NAVSTAR satellites at once Use timing of radio signals to calculate the receiver s position on the Earth's surface Calculations result in varying degrees of accuracy that depend on: quality of the receiver user operation of the receiver local & atmospheric conditions current status of system

6 GPS Control Segment (Ground Stations) Five control stations master station at Falcon (Schriever) AFB, Colorado monitor satellite orbits & clocks broadcast orbital data and clock corrections to satellites

7 GPS - How Does it Work? GPS allows us to determine a position by calculating the distance between a receiver and multiple satellites Distance is determined by timing how long it takes the signal to travel from satellite to receiver Radio signals travel at speed of light: 186,000 mi / sec Satellites and receivers generate exactly the same signal at exactly the same time Signal travel time = delay of satellite signal relative to the receiver signal 1µsec Satellite signal Receiver signal

8 GPS - Satellite Signals Satellites have accurate atomic clocks and all 24 satellites are transmitting the same time signal at the same time The satellite signals contains information that includes Satellite number Time of transmission Receivers use an almanac that includes The position of all satellites every second This is updated monthly from control stations The satellite signal is received, compared with the receiver s internal clock, and used to calculate the distance from that satellite Trilateration (similar to triangulation) is used to determine location from multiple satellite signals

9 GPS - Trilateration Start by determining the distance between a single GPS satellite and your position (a sphere) Adding a second distance measurement to another satellite narrows down your possible positions to a circle where the spheres intersect

10 GPS - Trilateration Cont. Adding a third satellite narrows down the position to two points where the three sphere intersect, and usually only one point is a reasonable answer The intersection of four spheres occurs at one point, but the 4th measurement is not needed, and is used for timing purposes instead

11 GPS - Using the 4th Signal How do we know that satellites and receivers generate the same signal at the same time? The satellites have atomic clocks, so we know they are accurate But receivers do not -- so can we ensure they are exactly accurate? No! If the receiver's timing is off, the location in 3-D space will be off slightly... So: We use the 4th satellite to resolve any signal timing error instead by determining a correction factor using the 4th satellite (like solving multiple equations... there will only be one solution that satisfies all equations)

12 GPS - Sources of Error Satellite errors satellite position / orbit error satellite clock error Atmospheric errors Speed of electromagnetic waves in the atmosphere Path taken by the signal Multi-path distortion errors Receiver errors (Selective availability)

13 GPS - Sources of Error Satellite Errors Although the satellites are in high orbits to minimize their deviations, sometimes there is a slight wobble due to local gravitational forces While the atomic clocks used in the satellites are extremely accurate (and quadruple redundant), sometimes clock errors can occur These can contribute up to 1-5 meters of error

14 GPS - Sources of Error Atmospheric Delays/Bending The speed of light is only precisely 186,000 miles per second in a vacuum, and is slightly slower in the atmosphere, varying by composition The signal can be bent as it moves through the atmosphere (sphere size based on a straight path) Up to 30m of error

15 GPS - Sources of Error Multi Path Interference The signal can bounce off of buildings, trees, etc. and this again distorts the time and distance between the receiver and the satellite Up to 1m of error

16 GPS - Sources of Error Receiver Errors (Timing/Rounding) Satellites have quadruple redundant atomic clocks that are accurate to nanoseconds (about $800,000 of clock hardware on each satellite), e.g. the time is 2:02: Receivers are powered by 4 AA batteries worth about $2.99, generate their clock signal with an oscillating crystal that is sensitive to battery current, e.g. the time is 2:02: Up to 10 meters of error

17 GPS - Sources of Error Satellite Coverage in Sky Position Dilution of Precision (PDOP) Poor Ideal

18 GPS - Selective Availability A former significant source of error Error intentionally introduced into the satellite signal by the U.S. Dept. of Defense for national security reasons Officially turned off on May 1, 2000

19 GPS - Error Budget Here s an example of some typical observed errors associated with finding a position using a consumer GPS receiver: Satellite Clocks Orbit Error Receivers Atmosphere Meters Typical observed errors satellite clocks 0.6 meters orbit error 0.6 meters receiver errors 1.2 meters atmosphere 3.7 meters Total 6.1 meters Multiplied by PDOP (1-6) meters

20 GPS - Error Correction It is possible to take steps to minimize the error in GPS derived positions, and to obtain positional accuracies that are better than those produced by a single position measurement with a consumer receiver We can use: Point Averaging Differential Correction

21 GPS - Point Averaging Point averaging is one of the simplest ways to correct obtain more accurate GPS position locations Collect points for a period of time, and then average them to get one point This newly averaged point should have greater accuracy than if you collected a single point Accuracy varies with this method but you usually should have a position that is within 5 meters of its true location (i.e. ~95% of the time)

22 GPS - Point Averaging This figure shows a successive series of positions taken using a receiver kept at the same location, and then averaged Averaged Location Note how the apparent position shifts systematically through time, moving around the true position

23 GPS - Differential Correction Differential correction collects points using a receiver at a known location (known as a base station) while you collect points in the field at the same time (known as a rover receiver) Any errors in a GPS signal are likely to be the same among all receivers within 300 miles of each other ~ 300 miles (~ 480 km) or less Base station (known location) Rover receiver

24 GPS - Differential Correction The base station records positions determined using the GPS satellite signals and determines the error present in the signal by comparing them to the known position Base Station File Time GPS Lat GPS Long Delta Lat Delta Long 3:12.5 3:13.0 3:13.5 3:14.0 3:14.5 3:

25 GPS - Differential Correction A person in the field with a receiver collects their points at the same time as the base station is collecting theirs and recording the error at those times that applies to that vicinity Rover Points Time GPS Lat GPS Long 3:12.5 3:13.0 3:13.5 3:14.0 3:14.5 3:

26 GPS - Differential Correction The positional errors found at the base station at those times can then by used to correct the positions produced by the rover receiver, assuming that the errors in the GPS satellite signals within 300 miles of the base station are relatively invariant By correcting the rover receiver positions in this way, we can substantially increase their accuracy (can be within 1-3 meters of true location) A national network of differential base stations (National Differential GPS Network - NDGPS) is being developed so this capability will be available everywhere in the USA

27 Base Station File Time GPS Lat GPS Long Delta Lat Delta Long 3:12.5 3:13.0 3:13.5 3:14.0 3:14.5 3: Rover Points Time GPS Lat GPS Long 3:12.5 3:13.0 3:13.5 3:14.0 3:14.5 3: Corr. GPS Lat Corr.GPS Long

28 GPS - Differential Correction To use differential correction requires A base station at a known location Using a rover receiver, within 300 miles of the base station (the National network is not yet operational, and there are many places that do not have a permanent differential correction base station, although temporary ones can be established for research projects) Software to calculate the errors in the base station files and then apply it to the rover files, potentially in real time

29 GPS - Differential Correction There are two ways that differential correction can be applied: Post-processing differential correction Does the error calculations after the rover has collected the points Real-time differential correction Done in real time by receiving a broadcasted correction signal (usually expensive), requiring other hardware (not just a consumer GPS receiver)

30 GPS Applications Generating mapped data for GIS databases traditional GIS analysts & data developers travel to field and capture location & attribute information cheaply (instead of surveying) Other uses (many in real time): E911/firefighter/police/ambulance dispatch car navigation roadside assistance business vehicle/fleet management mineral/resource exploration wildlife tracking boat navigation recreational

31 Comparing Soil Moisture and TMI Sites Theta TMI Compare Vol. Soil Moisture (V/V) Pond Branch - 6/26/02 - Average TMI

32 Pond Branch Catchment Control Color Infrared Digital Orthophotography

33 Glyndon Catchment Urbanizing Color Infrared Digital Orthophotography

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