ANDSCAPE L INES

11

Global Positioning Systems INTRODUCTION

In little more than a decade, the Global Positioning System (GPS) has

revolutionized the surveying and mapping sciences. Until the early

1980s, the surveyor’s transit had been the primary tool for accurately

locating one’s position on the earth’s surface. This equipment takes

months to learn to use properly, and years are required to master the

technique sufficiently to become a licensed surveyor. With relatively

little training a novice GPS user can now record his or her location to

within a few meters.

GPS was developed by the United States Department of Defense

(DOD) for military purposes. Although DOD believed that there

would be civilian use of the system, it was never anticipated that

nonmilitary applications would quickly surpass defense uses in volume

and diversity. The system’s foundation is a constellation of satellites

known as NAVSTAR. These satellites continually send out signals that

are picked up by GPS receivers and used to calculate a receiver’s

location in three-dimensional space. This information is then translated

to any of several geographic coordinate systems, such as latitude and

longitude or Universal Transverse Mercator (UTM).

There are several types of GPS receivers that are useful for different

applications. (See Figure 1.) Military grade receivers, which can

decode a special noncivilian signal called precise positioning system

(PPS), are very helpful for navigating (within about 10 meters) to

known sites. Normal civilian GPS receivers, which use coarse

acquisition (CA) signals, are better suited to mapping applications.

After data is collected in the field with these receivers, accuracy can

be greatly improved (to approximately two to seven meters)

through a process called differential correction (discussed later in this

section). Some GPS receivers are capable of collecting data in either
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PPS or CA mode. There are also receivers

specifically designed for extremely high-accuracy

surveying. Most of these units can deliver

coordinates accurate to within a centimeter. The

most precise GPS receivers have even been

used to detect motion in the earth’s crust to

within a millimeter.

APPLYING GPS TO CULTURAL

LANDSCAPE MANAGEMENT

In association with a Geographic Information

System (GIS), GPS is an expedient and accurate

tool for verifying the location of specific landscape characteristics and associated features to

identify UTM coordinates. The coordinates

can, in turn, be used to verify georeferenced

spatial data in GIS and aerial photographs. A

GPS unit with an accuracy to within nine feet

horizontally will most often be used to locate

cultural landscape characteristics and associated

features on a 1 inch = 20 feet base map. A

survey grade GPS unit is more likely to be used

when locating features on a 1 inch = 20 feet

base map.

TECHNICAL CONSIDERATIONS

When a GPS receiver picks up signals from four

satellites simultaneously, it can calculate a

three-dimensional position for the user. When

GPS first came into use, the NAVSTAR satellite

system was incomplete. As a result, it was

always necessary to plan data collection times

to coincide with maximum local satellite visibility to ensure that four satellite signals would be

detected by the receiver (often referred to as

“mission planning”). Now satellite coverage is

available nearly continuously throughout the

world, and data collection can be performed

almost anywhere at any time. However, there

can still be problems receiving a sufficient

number of satellite signals in deep canyons or

under heavy canopy. Under these conditions,

mission planning may still be advisable.

Accounting for Signal Error

Because DOD wants to retain a strategic advantage over other countries’ military organizations, it

introduces a deliberate random error into the

civilian satellite signals called selective availability,

Figure 1. Catalog photograph of a GPS rover unit. (Photograph

courtesy of Trimble Navigation, Inc., 1995)
G L O B A L P O S I T I O N I N G S Y S T E M S 3

or SA. This error makes it necessary to take extra

measures to obtain coordinate locations more

accurate than 100 meters.

The most common method of eliminating SAintroduced error is known as post-processed

differential correction. This technique requires that

a second GPS receiver, known as a base station,

be established at a known location, preferably over

a survey control monument or other such site.

The base station must be collecting data from the

same satellites at the same time as the rover unit.

As the base station collects data recording its

location, the coordinates recorded wander randomly due to the SA error. When field collection is

complete, software is used to apply a set of

corrections to the base station’s data, based on its

known, true location. These same corrections are

then applied to the corresponding rover data. In

this manner, data that was no more accurate than

100 meters can be made as close as two meters

or even better. To ensure that the base station and

rover unit “see” the same satellites, it is usually

recommended that the rover unit be no farther

than 500 kilometers from the base station, and that

no signals from satellites lower than a certain level

above the horizon (usually 15 degrees) be used in

calculating rover positions.

Another method of eliminating SA error is real-time

differential correction. The process is similar to postprocessed differential correction, except that the

base station has a radio beacon that transmits its

corrections to the rover unit as data is collected. The

rover unit is equipped with a radio receiver to pick

up the correction signals, and an internal computer

to calculate the corrected coordinates.

Military grade receivers that use PPS to record their

locations avoid SA altogether. To prevent use by

foreign countries, however, PPS is a coded signal

that can only be read by a receiver properly

configured to decode it. Through an agreement

between the departments of Interior and Defense,

the NPS has access to purchase and use receivers

capable of decoding PPS signals. Most of the

receivers of this type procured to date by NPS are

Rockwell PLRGs (nicknamed, “pluggers”).

Satellite Visibility Constraints

Sometimes, even when more than four satellites

are visible, it is not possible for the GPS receiver

to get an accurate reading for its location. This is

due to other sources of error in the system, such

as atmospheric and ionosphere effects on the

satellite signals, and errors in the internal timing

mechanisms in the receivers. Most of this error

can be minimized when the satellites being used

to calculate a position are in a desirable geometrical configuration. However, often the angles

between the satellites are too shallow or too near

180 degrees to help eliminate enough of these

other errors to produce a good positional reading. This problem is known as position dilution of

precision (PDOP), and can be quite vexing in less

than ideal terrain or canopy situations. Often a

GPS user may be having a difficult time just trying

to get enough satellites, only to find that when a

fourth signal kicks in, it results in a reading with

much too high a PDOP to be acceptable for

mapping. Sometimes the GPS user can move to a

more favorable location and record an “offset”

distance to the desired point, but unfortunately

there is often nothing that can be done other than
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to wait for a lower PDOP reading. This is why it

is still important to use mission planning software

when collecting data under difficult satellite

visibility conditions.

Using a Data Dictionary

As data is collected by the user (see Figure 2),

the points, lines, and/or polygons being recorded are stored in the receiver’s data logger.

When collecting data for input into a GIS it is

often useful to have a list of the types of features to be collected, and information about

them, stored in the data logger. This list is

known as a data dictionary. As an example, if

data about a cultural landscape were being

collected with a GPS receiver, the data dictionary might contain a field for a unique feature

identifier number, and another field containing

the type of feature, such as building, fence, or

rock wall. The user can select the appropriate

codes as each feature is collected, saving time

and confusion trying to remember which

feature was which after the data has been

entered into a GIS. (See Landscape Lines 10:

Geographic Information Systems.)

Figure 2. GPS operators and equipment in the field. (Photographs courtesy of Trimble Navigation, Inc., 1995)
G L O B A L P O S I T I O N I N G S Y S T E M S 5

WHERE TO GO FOR ASSISTANCE

Within the NPS there are many sources for

obtaining technical assistance on GPS. Many parks

have staff who are knowledgeable about using

GPS and who can respond to inquiries. In addition, many regions and clusters have field technical support centers that provide GPS services.

There are regional and cluster GIS coordinators

throughout the agency who can provide technical

guidance and direct individuals to appropriate

resources. The coordinators’ names and phone

numbers, along with other agency-related GIS

information, are available on the NPS GIS world

wide web site (www.nps.gov). For general

program information, the national GPS coordinator can be reached at 505-988-6710. Finally, the

NPS Cultural Resource GIS (CRGIS) Facility of

the Heritage Preservation Services Program

provides GPS training and data collection service.

REFERENCES

Ackroyd, N., and R. Lorimer. 1990. Global

Navigation, A GPS User’s Guide. Lloyd’s of

London.

Dahl, B. 1990. The Users Guide to GPS. Chicago: Richardson’s Marine Publishing.

Geodetic Survey Division. 1993. GPS Positioning Guide. Ontario: Natural Resources Canada,

615 Booth Street, Ottawa, Ontario K1A 0E9.

Hofmann-Wellenhof, B., H. Lichtenegger, and J.

Collins. 1992. Global Positioning System,

Theory and Practice. Springer-Verlag.

National Park Service. 1993. Historic Stonemasonry of Great Smoky Mountains National Park,

Open File Report No. 7. Washington, DC:

USDI, NPS, CRGIS Facility.

National Park Service. NBS/NPS Geographic

Information Sourcebook. Washington, DC:

USDI, NPS, CRGIS Facility.

Trimble Navigation, Ltd. 1992. GPS Surveyor’s

Guide—A Field Guidebook for Static Surveying.

Sunnyvale, California: Trimble Navigation Ltd.

Trimble Navigation, Ltd. 1992. GPS Surveyor’s

Guide—A Field Guidebook for Dynamic Surveying. Sunnyvale, California: Trimble Navigation Ltd.
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G L O B A L P O S I T I O N I N G S Y S T E M S 7

The mission of the Department of the Interior is to protect and provide access to our Nation’s natural and

cultural heritage and honor our trust responsibilities to tribes.

U.S. Department of the Interior

National Park Service

Cultural Resources

Park Historic Structures & Cultural Landscapes