One of the major issues facing

ecosystem restoration and

management of the Greater

Everglades is the availability and

distribution of clean, fresh water.

The South Florida ecosystem

encompasses an area of

approximately 28,000 square

kilometers and supports a human

population that exceeds 5 million and

is continuing to grow. The natural

systems of the KissimmeeOkeechobee-Everglades watershed

compete for water resources

primarily with the region's human

population and urbanization, and

with the agricultural and tourism

industries. Surface water flow

modeling and ecological modeling

studies are important means of

providing scientific information

needed for ecosystem restoration

planning and modeling. Hydrologic

and ecological models provide muchneeded predictive capabilities for

evaluating management options for

parks, refuges, and land acquisition

and for understanding the impacts of

land management practices in

surrounding areas. These models

require various input data, including

elevation data that very accurately

define the topography of the Florida

Everglades.

Surface water levels and sheet flow

in the Everglades (Figure 1) are very

sensitive to any differences in

topography because of the region's

expansive and extremely flat terrain.

Hydrologic models require very

accurate elevation data to simulate

and predict water flow direction,

depth, velocity, and hydroperiod.

Water resources, ecosystem

restoration, and other land

management decisions will rely in

part on the results of these models, so

it is imperative to use the best

elevation data available to achieve

meaningful simulation results.

Elevation data points are being

collected every 400 meters in a grid

pattern to meet the requirements of

various hydrologic models. The

vertical accuracy specification for

these elevation data is ±15

centimeters (6 inches) as referenced

to the North American Vertical

Datum of 1988 (NAVD88).

Because traditional methods for

collecting elevation data for the

Everglades are impractical or too

costly, the U.S. Geological Survey

(USGS) conducted a feasibility study

to determine if state-of-the-art

techniques using the Global

Positioning System (GPS) could meet

the strict vertical accuracy

specifications of the elevation data.

The feasibility study successfully

demonstrated that differential GPS

techniques, using airboats to navigate

transects (Figure 2), could meet the

vertical accuracy requirement. Also,

the land surface being surveyed in

the Everglades is typically under

water and obscured by vegetation.

This precludes the use of other

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 021-03

March 2003

Measuring and Mapping the Topography of the Florida Everglades for

Ecosystem Restoration

Figure 1. A helicopter view of a part of the Florida Everglades looking south towards Florida Bay.
methods for collecting very accurate

elevation data, such as photogrammetry, Light Detection And Ranging

(LiDAR), Interferometric Synthetic

Aperture Radar (IFSAR), or other

alternative remote sensing technologies. Therefore, topographic

surveys over such a large area of the

Everglades with such a stringent

accuracy specification can only be

accomplished efficiently by using

GPS technology. This is especially

the case in a harsh and inaccessible

wilderness environment with unique

landscape characteristics. 

Because the Everglades is so

expansive and remote and includes

environmentally sensitive areas,

impenetrable vegetation, or other

areas unapproachable by airboat,

access to many places is possible

only by helicopter. To solve this

accessibility problem, the USGS

developed a helicopter-based

instrument, known as the Airborne

Height Finder (AHF) (Figures 3 and

4), which is able to measure the

terrain surface elevation in a

noninvasive, nondestructive manner.

Using an airborne GPS platform and

a high-tech version of the surveyor's

plumb bob, the AHF system

distinguishes itself from remote

sensing technologies in its ability to

physically penetrate vegetation and

murky water, providing reliable

measurement of the underlying

topographic surface.

Accuracy tests have shown that the

AHF system can consistently

measure elevation points at the

subdecimeter level. An accuracy test

of the AHF was conducted in May

2000 when 17 National Geodetic

Survey (NGS) first-order benchmarks

(Figure 5) were measured at two

different helicopter hover heights.

The average difference between the

AHF measured elevations and the

NGS published data sheet values was

3.3 cm. The largest difference was

8.6 cm, and the smallest difference

was 0.2 cm. The root mean square

error was 4.1 cm. These accuracy test

results provide confidence that the

elevation dataset being produced

meets the ±15 cm vertical accuracy

specification.

Elevation data collected with the

AHF have been applied in the USGS- Figure 3. Operation of the Airborne Height Finder is demonstrated. The plumb bob is deployed from the

helicopter to the surface being measured.

Figure 4. The Airborne Height Finder is operated

using the USGS-developed electronic controller

and interface.

Figure 5. A typical GPS reference station set-up

used during Airborne Height Finder operations. All

GPS data are collected within 15 kilometers of the

reference station to ensure data accuracy and

quality.

Figure 2. Airboat configured for collecting GPS data launching for a surveying session.
developed Southern Inland and

Coastal System (SICS) numerical

simulation model, which requires

very accurate data for its

hydrodynamic surface-water

computations. The SICS model was

originally constructed with

topographic estimates based on preexisting contour maps. Although

these contour maps meet national

map accuracy standards, the derived

elevation data are not adequate for

hydrologic modeling purposes for the

Florida Everglades. However, the

distribution of surface-water flows

simulated by the SICS model was

greatly improved when the AHF

elevation data replaced the previously

estimated topography in the model.

The differences between the two

topographic surfaces are shown in

figure 6.

Negative values in Figure 6 indicate

where the AHF elevations are lower

than the previously estimated

topography. Flow and inundation

estimates north of the West Lake area

(southwest part of the SICS model

domain) are most significantly

affected by this elevation difference.

To date, tens of thousands of

elevation data points covering

significant parts of the Florida

Figure 7. A sample plot of the point elevation data overlaid on a USGS digital line graph.

Figure 6. Elevation differences shown in increments of 0.7 feet for the Southern Inland and Coastal System

hydrodynamic model domain.
Everglades (Figures 7 and 8) have

been collected and processed using

differential GPS methods with

airboats and the helicopter-based

AHF system. These data are

organized by USGS 7.5-minute

quadrangle maps and are available

from the South Florida Information

Access Web site at sofia.usgs.gov.

Information

For more information, please contact:

Gregory B. Desmond

U.S. Geological Survey

MS 521 National Center

12201 Sunrise Valley Drive

Reston, VA, 20192

Phone: 703-648-4728

Fax: 703-648-4165

e-mail: gdesmond@usgs.gov.

For information on other USGS

products and services, call 1-888-

ASK-USGS, or visit the general

interest publications Web site on

mapping, geography, and related

topics at mac.usgs.gov/mac/isb/pubs/

pubslists/.

For additional information, visit the

ask.usgs.gov Web site or the USGS

home page at www.usgs.gov.

Figure 8. A hypsographic map generated from the GPS-derived elevation data points detailing the

topography of the Florida Everglades.