GPS for Fire Management - 2004
Using Maps with GPS
Using Maps with GPS
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.
Projecting a Sphere Onto a Plane
Three-dimensional sphere to two-dimensional flat map.
Examples of Several Projections
Depending on the projection, a certain amount of
distortion occurs when portraying the earth on paper.
Projections and Datums
Meade Ranch (Clarke 1866)
Datum Shift
4790
1000m
700m
275m
4789
Datum
corner
NAD27
4788
541
542
543
Datum Shift
4790
1000m
600m
350m
4789
Datum
corner
NAD83
4788
541
542
543
Datum Shift
A set of coordinates
can yield different
positions due to
different datums.
WGS72
NAD83/WGS84
NAD27 (true
position)
NAD27 Greenland
Bermuda 1957
GPS Works in WGS84 & ECEF
Datum Displays
Coordinate System Displays
The user can only change
the way coordinates are
displayed by setting datum
and coordinate system.
The receiver’s processor
always works in datum
WGS84 and coordinate
system ECEF.
GPS 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).
Maps
 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.
Example of a USGS Map Legend
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
00 28’
8 MILS
18 1/20
329 MILS
UTM GRID AND 1971 MAGNETIC NORTH
DECLINATION AT CENTER OF SHEET
Coordinate Systems
 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
Different coordinates representing the same location:
hddd0 mm’ ss.s”: N 430 40’ 55.8” X W 1160 17’ 14.1”
(55.8” / 60 = .93’)
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 & Longitude
Latitude & 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
Prime Meridian
(Longitude)
0º
0º
Equator
(Latitude)
Point of Origin
Latitude & Longitude
N
Prime Meridian
30º
20º
W 30º 20º 10º
+
10º
10º 20º 30º E
10º
20º
Equator
30º
S
0º, 0º
Latitude
 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.
Parallels of Latitude
20º N
10º N
0º N
10º S
10º
690 miles
10º
690 miles
10º
690 miles
Longitude
 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.
Meridians of Longitude
To North Pole
10º
240 mi
10º
460 miles
10º
690 miles
Equator
To South Pole
120º W
110º W
Determining Latitude & Longitude
Prime Meridian
(0º)
30ºN, 50ºW
50º W
30º N
Equator (0º)
Determining Latitude
17’ 30”
L
A
T
I
44º 16’ 30”
Latitude
Line
U
D
E
7.5 min. scale 1:24,000
Latitude of
red square =
2.5 min
T
LONGITUDE
Latitude
Line
44º 15’ 00”
Determining Longitude
Meridian
Line
Longitude of
red square =
115º 19’ 00”
Meridian
Line
2.5 min
20’
115º 17’ 30”
Universal Transverse Mercator
 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.
UTM Coordinates
UTM Zone Number
Easting Coordinate
11T 0541450
4789650
UTM Latitude
Band Letter
Northing Coordinate
UTM Coordinates
10,000 meter digit
1,000 meter
digits
100,000 meter
digit(s)
11T
0541450
4789650
You need only plot the black numbers on the map. The rest
of the coordinate values are provided for you by the map.
UTM Grid Overlay
60 Zones, and 20 Latitude Bands
1
Latitude Bands
84º N
80º S
X
W
V
U
T
S
R
Q
P
N
M
L
K
J
H
G
F
E
D
C
Zones
21
60
21 T
Equator
UTM Zones in the Contiguous U.S.
Longitude
1260 1200 1140 1080 1020 960 900 840 780
720 660
19
10
11
12
UTM Zones
13
18
14
15
16 17
UTM Zones - Side by Side
840 N
60
60
60
Zones: 11
12
13
60
60
60
15
16
Equator
800 S
14
UTM Uses a Cartesian Grid
Increasing
4791
4790
x
Increasing
542
y
543
Plotting UTM Coordinates
Place the corner of
the UTM grid
reader on the point
to be plotted
1,000 m
4791
UTM grid reader
4790
9
5
5
0
9
4789
541
Each tic = 100 meters
on this grid reader (your
grid reader has 20 meter tics)
542
543
House coordinates = 0541450mE
4789650mN
A Final Word
Precision vs Accuracy
 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.
Using GPS with Maps
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.