Using GPS with Maps

Ellipsoid & GeoidEllipsoid & Geoid

Geoid (USGS Maps)

Ellipsoid (GPS)Topographic Surface

Earth

Projecting a Sphere Onto a PlaneProjecting a Sphere Onto a Plane

Types of ProjectionsTypes of Projections

Maps Datums,& Coordinate Systems

A datum specifies the earth-model (ellipsoid), and the origin associated with a particular set of coordinates.

It’s a function of a projection. Datums provide the link between the earth and coordinate

systems. Without a datum, coordinates have no meaning. There are many datums used worldwide. Only one datum, WGS-84, is global, the rest are regional.

Datum

NAD27 CONUS (Continental US)NAD27 AlaskaNAD27 CanadaNAD83 CONUS (Continental US)NAD83 AlaskaNAD83 CanadaWGS72WGS84 (GPS internal datum)

North American Datums

4789

541

Map CornerNAD27

650m

350m

Map CornerNAD83

610m

150m

Datum ShiftDatum Shift

4790

542

2 Datums = 2 Sets of Coordinates2 Datums = 2 Sets of Coordinates

NAD27 Coordinates = 350m, 650m

Map CornerNAD27

4789

541

4790

542

650m

350m

Map CornerNAD83

NAD83 Coordinates = 150m, 610m

610m

150m

4789

541

4790

542

A map is a two-dimensional representation of the earth. Maps incorporate projections and datums for accuracy. All maps distort the earth to some extent. Many different types of maps can be used with GPS. When using a map with a GPS receiver, the datum and

coordinate system must match.

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

Map LegendMap Legend

Latitude & LongitudeLatitude & Longitude

A spherical coordinate system that is unprojected. Angular coordinates are perfectly suited to the spherical

surface 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

Latitude & LongitudeLatitude & 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

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 circles around the earth paralleling the equator.

Parallels are rays that originate at the center of the earth. They are designated by their angle north or south of the equator.

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

The linear distance between parallel lines never changes, regardless of their position on earth. That distance is approximately 69 miles per degree anywhere on earth.

Latitude

Parallels of LatitudeParallels of Latitude

10º10º

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 rays which originate at the center of the earth. 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 about 26 statute miles.

Longitude

Meridians of LongitudeMeridians of Longitude

10º10º

Meridians of LongitudeMeridians of Longitude

10º10º

10º10º

110º W110º W120º W120º W

690 miles690 miles

460 miles460 miles

240 mi240 mi10º10º

Determining Latitude & LongitudeDetermining Latitude & Longitude

30º N

50º W

Equator (0º) Equator (0º)

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

Is a rectangular coordinate system derived from latitude and longitude.

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 derive

coordinates. Coordinates are expressed in meters.

Universal Transverse Mercator

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

Zones

1260 1200 1140 1080 1020 960 900 840 780 720 660

UTM Zones Covering the Contiguous U.S.UTM Zones Covering the Contiguous U.S.

1011

1213

14 15 16 1718

19

UTM Zones

Longitude

Equator

840 N

800 S

60

UTM Zones - Side by SideUTM Zones - Side by Side

60 60 60 60 60

UTM Zone Grid OverlayUTM Zone Grid Overlay

Cen

tral

Mer

idia

n

6º of Longitude

Zero East 500,000m E <1,000,000 m E

11T 0541450

UTM Zone Number

UTM Latitude Band Letter

4789650

Easting Coordinate

Northing Coordinate

UTM CoordinatesUTM Coordinates

541 542 5434789

4790

4791

1,000 m

5

5

9

9

UTM CoordinatesUTM Coordinates

Red house coordinates = 0541450mE4789650mN

4789

541

Map Corner(Neatline)

4790

542

Principle Digits

650m

350m

UTM Principle DigitsUTM Principle Digits

Using UTM to Determine DirectionUsing UTM to Determine Direction

““I am at I am at 565400E, 565400E, 4500500N”4500500N”

WW EE

SS

NN

+

dec

reas

ing

increasing

““You are at You are at 564900E, 564900E, 4500200N”4500200N”

+

300m300m

500m500m

Determining LatitudeDetermining Latitude

LL

AA

TT

II

TT

UU

DD

EE

LO

NG

ITU

DE

7.5 min. scale 1:24,000

44º 15’ 00”

17’ 30”

2.5

minLatitude of

red square =

44º 16’ 30”

Parallel

Lines

Determining LongitudeDetermining Longitude

Longitude of red square =

115º 19’ 00”

115º 17’ 30”20’

2.5 minMeridian

Lines

LL

AA

TT

II

TT

UU

DD

EE

LONGITUDE

7.5 min. scale 1:24,000