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INTRODUCTION TO GPS AND MAPPING
WHAT IS GPS?
Global Positioning System
HISTORY OF GPS
Department of Defense developed for navigation Standard positioning service (public uses) Precise positioning service
Launches began in 1970s Full operational capability in mid 1990s ‘Selective Availability’ turned off 2000
Degraded accuracy to ~100m
GPS SYSTEM
Space segment – (satellite life = 10 years) Several generations of satellites in use now
Control segment User segment
US spends $400,000,000 yearly Free to you – tax dollars at work again
Other countries operate their own systems too
SPACE SEGMENT 24 satellites with
spares in 6 orbital planes (4 in each)
~12,500 miles elevation
55 degree inclination
Each one circles Earth every 12 hours (7000 mph)
At least 4 visible at every point on the earth at all times
CONTROL SEGMENT Master Control
Station (MCS) in Colorado
5 Monitoring stations
Ground control stations (Ground Antennas) Unmanned Enable MCS to
control the satellites
USER SEGMENT
Antenna Receiver
Base map Record tracks,
waypoints, distance
HOW DOES IT WORK?
Triangulation
Need D+1 satellites to determine position = 2D needs 3 satellites, etc
SOURCES OF ERROR IONOSPHERE
Dual frequency mode of more advanced receivers corrects
SOURCES OF ERROR - SATELLITE GEOMETRY
Geometric Dilution of Precision (GDOP) – the higher the value, the poorer the measurement (very good = <4, bad = >6)
HOW ERROR IS MEASURED: DOP (DILUTION OF PRECISION)
HDOP
VD
OP
PDOP
•The geometry of the satellite constellation can affect the accuracy of the GPS positions.
•DOP is an indicator of quality of the constellation at any given time.
•Lower the DOP, the better the geometry of the constellation and the more accurate the GPS positions.
SOURCES OF ERROR MULTIPATH
HOW ACCURATE?
Recreation grade = ~8m Mapping grade = <1m Survey grade = several cm
Accuracy versus Precision Accurate measurement versus true
location Precise measurements close to each other
GPS ADDENDUM
Most GPS in cell phones do not use satellites.
They triangulate via cell towers whose locations are precisely known.
The signal is not line-of-sight so they can work indoors.
Does not work where limited cell service exists (like many forests).
Accuracy varies (< 10 m to ~100 m).
MAPPING OVERVIEW
SURVEYING METHODOLOGY EFFECTS MAPS
Geodetic surveying (magnetic) (Benchmarks)
MAGNETIC DECLINATION
Declination Calculating Software Online Calculators
WHAT ARE DATUMS?A datum defines an ellipsoid (a three-dimensional ellipse), which is the currently accepted `best fit' for the overall shape of the Earth. When an ellipsoid is fixed at a particular orientation and position with respect to the Earth, it constitutes a so-called `Geodetic Datum'. In other words, a datum describes the model (including the size and shape of the earth as well as the origin and orientation of the coordinate system) that was used to match the location of features on the ground to coordinates and locations on the map.
WGS 84, NAD27, and NAD83 are examples of Horizontal Datum.NAV88 is an example of Vertical Datum.
GEODETIC DATUMS: WHAT ARE THEY? Define the size and
shape of the earth Used as basis for
coordinate systems Variety of models:
Flat earth Spherical Ellipsoidal
WGS 84 defines geoid heights for the entire earth
WHAT ARE ZONES?
Because coordinate systems were designed for detailed calculations and positioning, they are usually divided into different zones to preserve accuracy. The boundaries of UTM zones follow lines of latitude and longitude while State Plane zones generally follow political boundaries.
UTM ZONES
The Earth is divided into 60 UTM Zones following lines of Longitude. The continental US is covered by Zones 10 – 19 with each zone representing 6 degrees of longitude.
STATE PLANE ZONESGenerally, the boundaries between state plane zones follow county lines. Depending on its size each state is represented by anywhere from one to ten zones.
MAP PROJECTIONS
Latitude/LongitudeLambert Conformal Conic
UTM/State Plane
USGS MAP PROJECTIONS
Geographic Coordinate Systems (based on Datums)Latitude, Longitude of curved surface
Projections make math easier
Projected Coordinate SystemsCartesian (X, Y) with designated origin and
distance from origin
PROJECTION AND REGISTRATION
Datum used effects positional errors
UTM COORDINATE SYSTEM
STATE PLANE COORDINATES
UTM is still not accurate enough for small area surveying.
During 1930s, each US state adopt its own projection and coordinate system, generally known as State Plane Coordinates (SPC).
Each state chose its own projection based on its shape to minimize distortion over the area of the state.
Some states have more than one internal zone. The North American Datum 1983 (NAD83) is
commonly used for SPC.
