Maps with gps co-ordinates

1. Using Maps with GPS
GPS for Fire Management - 2004

2. 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

3. Projecting a Sphere Onto a Plane
Three-dimensional sphere to two-dimensional flat map.

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

5. Projections and Datums
Meade Ranch (Clarke 1866)

6. Datum Shift
700m
4789
541
4790
542
4788
543
Datum
corner
NAD27
275m
1000m

7. Datum Shift
Datum
corner
NAD83
600m
4789
541
4790
542
4788
543
1000m
350m

8. Datum Shift
A set of coordinates
can yield different
positions due to
different datums.
NAD27 (true
position)
NAD83/WGS84
WGS72
Bermuda 1957
NAD27 Greenland

9. GPS 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

10. 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).

11.  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

12. 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
Example of a USGS Map Legend

13.  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

14. Coordinate 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

15. Latitude & Longitude

16.  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

17. 0º
0º
Point of Origin
Prime Meridian
Equator
(Longitude)
(Latitude)
Latitude & Longitude

18. Latitude & Longitude
0º, 0º
Prime Meridian
Equator
10º
20º
W 30º 10º 20º 30º E
10º
20º
30º
10º
20º
30º
S
N
+

19.  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

20. Parallels of Latitude
10º
10º
10º
690 miles
690 miles
690 miles
10º S
0º N
10º N
20º N

21.  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

22. Meridians of Longitude
10º
10º
110º W
120º W
690 miles
460 miles
240 mi
10º
Equator
To North Pole
To South Pole

23. Determining Latitude & Longitude
30º N
50º W
Equator (0º)
Prime Meridian
(0º)
30ºN, 50ºW

24. 44º 15’ 00”
17’ 30”
2.5
min
Latitude of
red square =
44º 16’ 30”
Latitude
Line
Latitude
Line
Determining Latitude
L
A
T
I
T
U
D
E
LONGITUDE
7.5
min.
scale
1:24,000

25. 115º 17’ 30”
20’
2.5 min
Meridian
Line
Meridian
Line
Determining Longitude
Longitude of
red square =
115º 19’ 00”

26.  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

27. 11T 0541450
UTM Zone Number
UTM Latitude
Band Letter
4789650
Easting Coordinate
Northing Coordinate
UTM Coordinates

28. 1450
9650
100,000 meter
digit(s)
10,000 meter digit
11T 05
47
4
8
UTM 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.

29. 1 60
UTM Grid Overlay
60 Zones, and 20 Latitude Bands
21
G
M
W
X
80º S
84º N
D
C
E
F
H
J
K
L
N
P
Q
R
S
T
U
V
Latitude
Bands
21 T
T
Zones
Equator

30. UTM Zones in the Contiguous U.S.
1260 1200 1140 1080 1020
960 900 840 780
720 660
10
11
12
13
14 15 16 17
18
19
UTM Zones
Longitude

31. UTM Zones - Side by Side
Equator
840 N
800 S
60 60 60 60 60 60
Zones: 11 12 13 14 15 16

32. UTM Uses a Cartesian Grid
542 543
4790
4791
x
y
Increasing
Increasing

33. Plotting UTM Coordinates
House coordinates = 0541450mE
4789650mN
UTM grid reader
Place the corner of
the UTM grid
reader on the point
to be plotted
542 543
4790
4791
1,000 m
541
4789
5
5
9
9
Each tic = 100 meters
on this grid reader (your
grid reader has 20 meter tics)
0

34.  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 Word
Precision vs Accuracy

35. 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