global positioning system department of defense developed for navigation standard positioning...

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

POST PROCESSING – 2 TO 5 M

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.

UTM CONVERSION PROGRAM