1. Using GPS with Maps

2. GPS’ Own Internal System
GPS Ellipsoid: GRS-80
(Geodetic Reference System 1980)
GPS Datum: WGS-84
(World Geodetic System 1984)
(equivalent to NAD-83)
GPS Coordinate System: ECEF
(Earth Centered Earth Fixed)

3. Modeling the Earth Ellipsoids and Geoids Projections Datums
Unprojected
Projected
Coordinate
Systems
UTM
Lat/Long
Other

4. Ellipsoid & Geoid
Both are mathematical formulas used to represent the earth’s surface so that it can be projected onto maps.
An ellipsoid treats the earth as a smooth, featureless sphere, and approximates the shape of the earth at sea level without regard to land masses. GPS uses ellipsoid.
A geoid is an imaginary representation of the earth characterized by constant gravity, which corresponds to the average level of the oceans (mean sea level), and to vertical locations on land masses that have the same constant gravity as mean sea level. USGS maps use geoid to generate height above MSL.

5. Ellipsoid & Geoid Earth Topographic Surface Ellipsoid (GPS)
Geoid (USGS Maps)
Earth

6. Projecting a Sphere Onto a Plane

7. Types of Projections

8. Two Different Projections of the U.S.

9. Maps Datums, & Coordinate Systems

10. Datum
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.

11. North American Datums NAD27 CONUS (Continental US) NAD27 Alaska
NAD27 Canada
NAD83 CONUS (Continental US)
NAD83 Alaska
NAD83 Canada
WGS72
WGS84 (GPS internal datum)

12. Datum Shift 4790 4789 Map Corner NAD27 541 542 Map Corner NAD83 350m

13. 2 Datums = 2 Sets of Coordinates
Map Corner
NAD83
NAD83 Coordinates = 150m, 610m
610m
150m
4789
541
4790
542
NAD27 Coordinates = 350m, 650m
Map Corner
NAD27
4789
541
4790
542
650m
350m

14. Maps 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.

15. UTM GRID AND 1971 MAGNETIC NORTH DECLINATION AT CENTER OF SHEET
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 Aerial photographs taken 1951
Polyconic projection 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 MN GN
UTM GRID AND 1971 MAGNETIC NORTH
DECLINATION AT CENTER OF SHEET

16. Coordinate Systems
All coordinate systems reference some particular set of numbers for the size and shape of the earth (the datum).
Coordinates designate locations within a spatial reference system (datum).
There are two types of global coordinate systems:
Angular Coordinates
Rectangular (Cartesian) Coordinates
Latitude and longitude, and Universal Transverse Mercator are two global coordinate systems used by GPS.
Many other coordinate systems exist worldwide.

17. Coordinate Systems
Different coordinates representing the same location:
hddd0 mm’ ss.s”: N ’ 55.8” X W ’ 14.1”
(55.8” / 60 = .93’)
hddd0 mm.mmm’: N ’ X W ’
(40.93’ / 60 = )
hddd.ddddd0 : N X W
UTM/UPS: 11T m E m N

18. Precision vs Accuracy Precision and accuracy are not the same.
Precision refers to how small an area coordinates can be defined.
GPS lat/long coordinates can be defined to 1/10 of a second on most C/A Code, GPS receivers.
UTM coordinates can be defined down to one meter.
Accuracy refers to how close GPS can place a receiver to its true location.
Accuracy can vary from a few centimeters to several kilometers.

19. Latitude & Longitude

20. Latitude & 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.

21. Latitude & Longitude Prime Meridian (Longitude) Equator (Latitude)0º 0º
10º S
Equator
(Latitude)
Point of Origin

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

23. Latitude
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.

24. Parallels of Latitude
10º

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

26. Longitude
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.

27. Meridians of Longitude
10º

28. Meridians of Longitude
10º
240 mi
10º
460 miles
10º
690 miles
120º W
110º W

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

30. Universal Transverse Mercator
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.

31. 60 Zones, and 20 Latitude Bands
UTM Grid Overlay
60 Zones, and 20 Latitude Bands
Zones 1 21 60 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

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

33. UTM Zones - Side by Side
840 N 60 60 60 60 60 60
Equator
800 S

34. UTM Zone Grid Overlay Meridian Central 6º of Longitude Zero East

35. Anatomy of a UTM Zone 840 N 60 800 S increasing central meridian
m N
Equator
m N
m E
decreasing
800 S
decreasing
increasing

36. UTM Coordinates Similar coordinates, but two different locations.
increasing
Similar coordinates, but two different locations.
m N
Equator
m N S
decreasing
800 S

37. Zone to Zone Easting Changes
East Edge of Zone 11
West Edge of Zone 12
750000m E
265000m E
600000m E
140000m E

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

39. 11T 05 47 4 8 1450 9650 UTM Coordinates 10,000 meter digit
1,000 meter digits
100,000 meter digit(s)
11T 05 47 4 8
1450
9650

40. UTM Coordinates Red square is located at 54 E, and 478 N
100,000 Meter Grid
10,000 Meter Grid N
500000E
550000
600000
400000
500000
600000
Red square is located at 54 E, and 478 N
(Red square = 10,000 sq m)

41. UTM Coordinates 1,000 meter square 10,000 meter square
10,000 meter square
540000E
545000
550000
Red square is located at 541 E, and 4789 N

42. UTM Coordinates 1,000 m Red house coordinates = 0541450mE 4789650mN
4791
1,000 m
4790 5 9
4789
541
542
543
Red house coordinates = mE mN

43. UTM uses a Cartesian Grid
Increasing
4791
4790 x
Increasing
542
543
Coordinates are calculated from lower left corner. y

44. UTM Principle Digits 90 89 41 42 Principle Digits 47 350m 650m 47
Map Corner
(Neatline) 5 41 5 42
Principle Digits

45. Using UTM to Determine Direction
“You are at E, N” W
increasing E + +
300m
500m
“I am at E, N”
decreasing S

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

48. Determining LongitudeA T I U D E
LONGITUDE
7.5 min. scale 1:24,000
Longitude of red square =
115º 19’ 00”
Meridian
Lines
2.5 min
20’
115º 17’ 30”