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Intro to GPS Units

Intro to GPS Units. DB McKinney HWS Geoscience. The GPS Receiver. Receives timing signals from a constellation of overhead satellites Converts timing data to distance, uses distances to triangulate position.

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Intro to GPS Units

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  1. Intro to GPS Units DB McKinney HWS Geoscience

  2. The GPS Receiver • Receives timing signals from a constellation of overhead satellites • Converts timing data to distance, uses distances to triangulate position. • Records position as latitude and longitude or in projected coordinates like Universal Transverse Mercator—UTM. • The units we use cost about $200 and give positions good to plus or minus 5 meters, sometimes better. • Not only measure position, but can also store and display positions. Store both points (waypoints) and tracks (auto recorded points, like a trail of breadcrumbs).

  3. The Buttons Data Entry/Selector ZOOM PAGE NAVigation ON/OFF MENU QUIT ENTER

  4. Setup Press MENU button TWICE to see the MAIN MENU Use Selector button to highlight SETUP Press ENTER to begin setup.

  5. Information Page Simulating GPS 2D GPS Location 3D GPS Location 2D Differential 3D Differential

  6. To Add a Waypoint—Hold ENTER Waypoint Name Symbol Use ENTER key to accept, or use the selector key to highlight other fields, press ENTER and start editing. Highlight OK and press ENTER key to save. Comment Location as UTM or Lat/Lon

  7. Selecting a Waypoint Press MENU button TWICE to see the MAIN MENU Use Selector button to highlight POINTS Press ENTER to see list of waypoints.

  8. Navigating to a Waypoint

  9. Latitude and Longitude Latitude (“Ladder-tude”) 0o at Equator, North (+) and South(-) up to 90o at poles Longitude (“LONG-itude”) 0o arbitrarily set at Greenwich, UK, the Prime Meridian Longitudes are east (+) and west (-) of the PM, up to 180o Prime Meridian, 0o Latitude and Longitude (plus a datum describing the ellipsoidal shape of the earth) uniquely identify any location on the planet.

  10. The Problem with Lat/Lon Hard to work with on 2-D maps! • Distance between meridians changes with latitude • Angles do not scale easily to distances • Using Lat/Lon like normal XY Cartesian coordinates makes very distorted maps—neither map measured distances or angles are accurate! These problems can be solved by using coordinate systems based on well-designed map projections.

  11. Map Projections • Needed to accurately show relations on the ellipsoidal earth surface on a flat piece of “paper” –real or virtual. • Projections necessarily introduce distortions • Scale may vary by direction (conformality) or location in the map • Directions on the map may not be the same as true directions • Areas on the map may not be uniformly proportional to true areas • Projections are compromises!

  12. Universal Transverse Mercator or UTM PROJECTION Cylinder, cuts globe at two meridians 6o apart Unlike a sphere, a cylinder can be cut and then unrolled, producing a 2D surface! Central Meridian Minor distortion along the CM, scale=0.9996, decreasing outward to true, scale=1.0 where the cylinder cuts the globe (3o from CM)

  13. If the Earth were a true sphere…. But the Geoid is flattened at the poles… Standard UTM grids restricted to latitudes < 80o Maptools.com, 10/07

  14. An XY metric grid is overlaid on the projection. The central meridian is given a value 500,000mE (so there are no negative coordinates) For the Northern Hemisphere, the Equator is given a value of 0mN For the Southern Hemisphere, a “false northing” of 1000000m added to the coordinate to prevent negative coordinates Source: The Geographer’s Art

  15. Source: Wikipedia UTM Zones are 6o wide with a CM in the middle. Each is a separate projection. Zones are numbered from west to east starting with 01 at the International Date Line. Geneva is in Zone 18 Zone characters denote 8o sections of latitude (same projections).

  16. UTM coordinates depend on DATUM! • Details of the ellipsoidal model of the earth, known as a datum, are necessary for the calculation of UTM coordinates from lat/lon. • Different datums, different UTM’s! • DON’T MIX DATUMS! For example, don’t use a USGS quad map based on the NAD27 datum with GPS UTM’s based on NAD83 datum. Errors can be 200 meters!

  17. Important Datums • NAD 27 North American Datum 1927 • This datum is used for most printed USGS topographic maps. • NAD 83 North American Datum 1983 • A newer datum now being used on some USGS products, many GIS-based products. • WGS 84 World Geodectic Survey 1983 • Practically the same as NAD83, probably OK to mix with NAD83.

  18. An example of a UTM Coordinate NAD8318T 0337424 4761730 NAD83/18T/0337424/4761730 NAD83 Datum Zone 18 T 337424 mE of CM 4761730 mN of Equator “Read right on up”

  19. Advantages of UTM coordinates • True X-Y coordinates with the map projection built in • Units are meters instead of degrees, minutes, seconds • UTM grids appear on many published maps • Can be used directly in CAD and other plotting programs • Preserves angles, scale distortions minor • Can be used to calculate distances • Can be used to calculate bearings

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