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Using Maps with GPS
GPS for ICS - 2003
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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. Successfully plot or locate four points on a map in lat/long,
and four points in UTM.
Using Maps with GPS
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Projecting a Sphere Onto a PlaneProjecting a Sphere Onto a Plane
Three-dimensional sphere to two-dimensional flat map.
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Examples of Several ProjectionsExamples of Several Projections
Depending on the projection, a certain amount of distortion occurs when portraying the earth on paper.
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Projections and DatumsProjections and Datums
Meade Ranch (Clarke 1866)
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Datum ShiftDatum Shift
700m
4789
541
4790
5424788
543
Datum cornerNAD27
275m
1000m
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Datum ShiftDatum Shift
Datum cornerNAD83
600m
4789
541
4790
5424788
543
1000m
350m
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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
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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
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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 systemRectangular (Cartesian) coordinate system
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
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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
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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.
Precision vs Accuracy
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Latitude & LongitudeLatitude & Longitude
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A geographic (spherical) coordinate system. 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
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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
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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
+
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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
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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
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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
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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
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Determining Latitude & LongitudeDetermining Latitude & Longitude
30º N
50º W
Equator (0º) Equator (0º)
Prime MeridianPrime Meridian(0º)(0º)30ºN, 50ºW30ºN, 50ºW
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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
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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
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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
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11T 0541450
UTM Zone Number
UTM Latitude Band Letter
4789650
Easting Coordinate
Northing Coordinate
UTM CoordinatesUTM Coordinates
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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.
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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
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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
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UTM Zones - Side by SideUTM Zones - Side by Side
Equator
840 N
800 S
60 60 60 60 60 60
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UTM Uses a Cartesian GridUTM Uses a Cartesian Grid
542 543
4790
4791
x
y
Increasing
Increasing
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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
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Lesson 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. Successfully plot or locate four points on a map in lat/long,
and four points in UTM.
Using GPS with Maps