Using Maps with GPS

GPS for Fire Management - 2004

Objectives:

Explain the purpose of datums. Identify the two “global” coordinate systems most

commonly used with GPS. Describe “datum shift,” and the relevance it has when using

GPS in the field. Describe the four components that make up UTM

coordinates. Identify the three ways that lat/long coordinates can be

expressed.

Using Maps with GPS

Projecting a Sphere Onto a PlaneProjecting a Sphere Onto a Plane

Three-dimensional sphere to two-dimensional flat map.

Examples of Several ProjectionsExamples of Several Projections

Depending on the projection, a certain amount of distortion occurs when portraying the earth on paper.

Projections and DatumsProjections and Datums

Meade Ranch (Clarke 1866)

Datum ShiftDatum Shift

700m

4789

541

4790

5424788

543

Datum cornerNAD27

275m

1000m

Datum ShiftDatum Shift

Datum cornerNAD83

600m

4789

541

4790

5424788

543

1000m

350m

Datum ShiftDatum ShiftA set of coordinates can yield different positions due to different datums.

NAD27 (true position)

NAD83/WGS84

WGS72

Bermuda 1957

NAD27 Greenland

GPS Works in WGS84 & ECEFGPS Works in WGS84 & ECEF

The receiver’s processor always works in datum WGS84 and coordinate system ECEF.

The user can only change the way coordinates are displayed by setting datum and coordinate system.

Datum Displays

Coordinate System Displays

GPS Uses WGS84 & ECEFGPS Uses WGS84 & ECEF

User selects the datum and coordinate system for display only.

Receiver’s processor always performs calculations in WGS84 and Earth Centered Earth Fixed (ECEF).

Datum

Coordinate System

A map is a two-dimensional representation of the earth. Maps incorporate projections and datums to provide a way to

reference locations on the map to features on the ground (via coordinate systems).

All maps distort the earth to some extent. Many different types of maps can be used with GPS. When using a GPS receiver with a map, the datum and

coordinate system in the receiver must match the map datum.

Maps

Mapped, edited, and published by the Geological Survey

Control by USGS USC&GS

Topography from aerial photographs by multiplex methods

and by plane-table surveys 1953. Aerial photographs taken 1951

Polyconic projection. 1927 North American Datum

10,000 foot grid based on Idaho coordinate system, west zone

1000-meter Universal Transverse Mercator grid ticks,

1000-meter Universal Transverse Mercator grid ticks, zone 11, shown in blue

To place on the predicted North American Datum 1983 move the projection lines 15 meters north and 77 meters east as shown by dashed corner ticks

UTM GRID AND 1971 MAGNETIC NORTH

DECLINATION AT CENTER OF SHEET

00 28’

8 MILS

18 1/20

329 MILS

MNGN

Example of a USGS Map LegendExample of a USGS Map Legend

All coordinate systems reference some particular set of numbers for the size and shape of the earth (the datum).

Coordinate systems are used to designate locations within a datum.

There are two types of global coordinate systems:Angular coordinate system (lat/long is one)Rectangular (Cartesian) coordinate system (UTM is one)

Latitude and longitude, and Universal Transverse Mercator are two global coordinate systems commonly used by GPS users.

Many other coordinate systems exist worldwide.

Coordinate Systems

Coordinate SystemsCoordinate Systems

hddd0 mm’ ss.s”: N 430 40’ 55.8” X W 1160 17’ 14.1”

(55.8” / 60 = .93’)

Different coordinates representing the same location:

hddd0 mm.mmm’: N 430 40.93’ X W 1160 17.235’(40.93’ / 60 = .682160)

hddd.ddddd0 : N 43.682160 X W 116.287250

UTM/UPS: 11T 0557442m E 4836621m N

Latitude & LongitudeLatitude & Longitude

A geographic (spherical) coordinate system. Are angular coordinates are perfectly suited to the ellipsoidal

shape of the earth. Coordinates are expressed in degrees, minutes and seconds

(and variations of that). Position coordinates are based on an angular distance from a

known reference point. That reference point is where the Prime Meridian and Equator

intersect. Lat/long is the predominant coordinate system used for nautical

and aeronautical navigation.

Latitude & Longitude

0º0º

0º0º

Point of OriginPoint of Origin

Prime MeridianPrime Meridian

EquatorEquator

(Longitude)(Longitude)

(Latitude)(Latitude)

10º N10º N

30º N30º N

10º S10º S

Latitude & LongitudeLatitude & Longitude

Latitude & LongitudeLatitude & Longitude

0º, 0º0º, 0º

Prime MeridianPrime Meridian

EquatorEquator

10º10º20º20ºW 30ºW 30º 10º10º 20º20º 30º E30º E10º10º

20º20º

30º30º

10º10º

20º20º

30º30º

SS

NN

+

Latitude is comprised of parallels, which are equally spaced circles around the earth paralleling the Equator.

Parallels are designated by their angle north or south of the Equator (10º, 20º, etc) .

The Equator is 0º latitude, and the north and south poles are at 90º angles from the Equator.

The linear distance between parallel (latitude) lines never changes, regardless of their position on the earth.

Latitude

Parallels of LatitudeParallels of Latitude

10º10º

10º10º

10º10º

690 miles690 miles

690 miles690 miles

690 miles690 miles

10º S10º S

0º N0º N

10º N10º N

20º N20º N

Longitude is comprised of meridians that form one-half of a circle, or plane.

Meridians are designated by their angle west or east of the prime meridian.

The prime meridian is designated 0º and extends from the north pole to the south pole through Greenwich, England.

Meridians are angled, and do not parallel each other. The linear distance between one degree of longitude at the

Equator is approximately 69 statute miles. The linear distance between one degree of longitude at the

arctic circle is only about 26 statute miles.

Longitude

Meridians of LongitudeMeridians of Longitude

10º10º

10º10º

110º W110º W120º W120º W

690 miles690 miles

460 miles460 miles

240 mi240 mi10º10º

Equator

To North Pole

To South Pole

Determining Latitude & LongitudeDetermining Latitude & Longitude

30º N

50º W

Equator (0º) Equator (0º)

Prime MeridianPrime Meridian(0º)(0º)30ºN, 50ºW30ºN, 50ºW

44º 15’ 00”

17’ 30”

2.5

min

Latitude of red square =

44º 16’ 30”

LatitudeLine

LatitudeLine

Determining LatitudeDetermining Latitude

LL

AA

TT

II

TT

UU

DD

EE

LO

NG

ITU

DE

7.5 min. scale 1:24,000

115º 17’ 30”20’

2.5 minMeridianLine

MeridianLine

Determining LongitudeDetermining Longitude

Longitude of red square =

115º 19’ 00”

LL

AA

TT

II

TT

UU

DD

EE

LONGITUDE

7.5 min. scale 1:24,000

Is a rectangular (planar) coordinate system based on the latitude and longitude (geographic) coordinate system.

The earth is divided into 60 UTM zones. Sixty zones allows the earth to be projected onto maps with

minimal distortion. UTM uses “false” values (easting and northing) to express

coordinates. Coordinates are expressed in meters.

Universal Transverse Mercator

11T 0541450

UTM Zone Number

UTM Latitude Band Letter

4789650

Easting Coordinate

Northing Coordinate

UTM CoordinatesUTM Coordinates

1450

9650

100,000 meter digit(s)

10,000 meter digit

11T 05

47

4

8

UTM CoordinatesUTM Coordinates

1,000 meter digits

You need only plot the black numbers on the map. The rest of the coordinate values are provided for you by the map.

1 60

UTM Grid OverlayUTM Grid Overlay60 Zones, and 20 Latitude Bands

21

G

M

WX

80º S

84º N

D

C

EF

HJ

KL

NP

QRST

UV

Lat

itu

de

Ban

ds

21 T

T

Zones

Equator

UTM Zones in the Contiguous U.S.UTM Zones in the Contiguous U.S.

1260 1200 1140 1080 1020 960 900 840 780 720 660

1011

1213

14 15 16 1718

19

UTM Zones

Longitude

UTM Zones - Side by SideUTM Zones - Side by Side

Equator

840 N

800 S

60 60 60 60 60 60

Zones: 11 12 13 14 15 16

UTM Uses a Cartesian GridUTM Uses a Cartesian Grid

542 543

4790

4791

x

y

Increasing

Increasing

Plotting UTM CoordinatesPlotting UTM Coordinates

House coordinates = 0541450mE4789650mN

UTM grid reader

Place the corner of the UTM grid reader on the point to be plotted

542 543

4790

47911,000 m

5414789

5

5

9

9Each tic = 100 meterson this grid reader (yourgrid reader has 20 meter tics)

0

Precision and accuracy are not the same. Precision refers to how small an area coordinates can be

defined or plotted.GPS lat/long coordinates can be defined to 1/10 of a

second.UTM coordinates can be defined down to one meter.

Accuracy refers to how closely a GPS receiver can calculate its position relative to its true location.GPS accuracy can vary from a few millimeters to several

kilometers.

A Final WordPrecision vs Accuracy

Objectives revisited:

Explain the purpose of a datum. Identify the two “global” coordinate systems most

commonly used with GPS. Describe “datum shift,” and the relevance it has when using

GPS in the field. Describe the four components that make up UTM

coordinates. Identify the three ways that lat/long coordinates can be

expressed.

Using GPS with Maps