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. 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).

ON/OFF PAGE Data Entry/Selector MENU ENTER The Buttons QUIT NAV igation ZOOM

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

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

To Add a Waypoint—Hold ENTER 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. Waypoint Name Symbol Comment Location as UTM or Lat/Lon

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.

Navigating to a Waypoint

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

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.

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!

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

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

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 m added to the coordinate to prevent negative coordinates Source: The Geographer’s Art

UTM Zones are 6 o 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 8 o sections of latitude (same projections). Source: Wikipedia

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!

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.

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

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