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Wdowinski, S., and S. Eriksson (2009), Geodesy in the 21st century,
Eos Trans. AGU, 90(18), 153–155

Geodesy in the 21st Century
>From flat Earth, to round Earth, to a rough
and oblate Earth, people’s understanding of
the shape of our planet and its landscapes
has changed dramatically over the course
of history. These advances in geodesy—
the study of Earth’s size, shape, orientation,
and gravitational field, and the variations
of these quantities over time—developed
because of humans’ curiosity about the
Earth and because of geodesy’s application
to navigation, surveying, and mapping, all
of which were very practical areas that benefited
society.
Today, geodesy is no different. The field is
now concerned with changes in the shape
of Earth’s surface, because small detectable
changes are associated with issues of great
societal impact such as ice melting, sea level
rise, land subsidence, and aquifer depletion.
For example, the rate of polar ice melt may
be estimated from combined satellite gravity
and ground Global Positioning System (GPS)
measurements. Global estimates of sea level
changes are measured by altimetry satellites
within entire ocean basins. Human- induced
depletion of aquifers is reflected in subsidence
measured by synthetic aperture radar
(SAR) satellites.
Twenty- first- century geodetic studies are
dominated by geodetic measurements from
space. Space geodesy uses a set of techniques
relying on precise distance or phase
measurements transmitted or reflected from
extraterrestrial objects, such as quasars, the
Moon, or artificial satellites. Early space geodetic
measurements, beginning in the 1980s,
had accuracy levels between 5 and 10 centimeters.
These measurements were conducted
across the entire globe and yielded
the first direct observations of tectonic plate
motion. Further improvements to space geodetic
technologies have increased the accuracy
to subcentimeter levels.
Today, space geodetic observations can
detect small movements of the Earth’s solid
and fluid surfaces as well as changes in the
atmosphere and ionosphere. Hence, geodesy
has many applications in a variety of fields
extending well beyond its traditional role in
solid Earth sciences (Figures 1 and 2).
Geodesy, like many scientific fields, is
technology driven. Over the centuries, it
has developed as an engineering discipline
because of its practical applications. By the
early 1900s, scientists and cartographers
began to use triangulation and leveling measurements
to record surface deformation
associated with earthquakes and volcanoes.
For example, one of the most important geophysical discoveries, the
basic understanding
of earthquake mechanics known
as the “elastic rebound theory” [Reid, 1910],
was established by analyzing geodetic measurements
before and after the 1906 San
Francisco earthquakes.
In 1957, the Soviet Union launched the
artificial satellite Sputnik, ushering the world
into the space era. During the first 5 decades
of the space era, space geodetic technologies
developed rapidly. The idea behind
space geodetic measurements is simple: Distance
or phase measurements conducted
between Earth’s surface and objects in space (satellites, the Moon, or
quasars) are
very precise, easily repeated, and obtainable
for almost any surface location. Consequently,
positioning, surface elevation, and
gravity field and their changes with time
can be determined precisely with global
coverage.
The first generation of space geodetic
observations relied on existing astronomical
equipment such as the radio telescope, and
on analysis of early satellite orbits. These
observations yielded the first space- based
measurements of tectonic plate motion
and Earth’s gravity field. Dedicated satellites
for geodetic measurements were soon developed. In parallel, clever
utilization of
observables from nongeodetic missions
such as GPS or SAR satellites resulted in
very precise positioning or change measurements—
scientists could now detect earthquake
and magma- induced crustal deformation,
subsidence, glacial movements, and
wetland surface water level changes.
During the short history of the space geodetic
era, innovative development of space
technologies has yielded numerous geodetic
methods (see Table S1 in the electronic
supplement to this Eos issue (http://
www . agu . org/ eos _elec/)). A quick look
into the main space geodetic technologies shows how far and how
rapidly space geodesy
has advanced since its beginnings about
50 years ago.
Types of Modern Geodetic Missions
Space- based geodetic observations can
be categorized into four basic techniques:
positioning, altimetry, interferometric synthetic
aperture radar ( InSAR), and gravity
studies.
Precise positioning is the fundamental
geodetic observation required for surveying
and mapping. Instead of the traditional
triangulation and leveling networks
that require line of sight (LOS) between
measurement points, space geodetic methods
use LOS between the measurement
points and celestial objects or satellites.
For example, to measure changes in distance
across the San Andreas Fault, scientists
used radio telescopes at Vandenberg
(California) and Yuma (Arizona) to detect
the phase delay in the arrival of quasar signals
between the two sites. Similar distance
changes have also been determined by satellite
laser ranging (SLR) and more recently
and more precisely by GPS. As a result of
these techniques, relative positioning can
be achieved over very large distances in
which the precision is almost independent
of the distance between the two measurement
points.
Building on this idea, scientists have
developed advanced positioning techniques
through Global Navigation Satellite Systems
(GNSS). GNSS encompasses the various satellite
navigation systems, such as the United
States’ GPS, Russia’s Globalnaya Navigatsionnaya
Sputnikovaya Sistema ( GLONASS),
and Europe’s Galileo. Although these satellite
systems were designed mainly for navigation,
they were found to be very useful for
precise positioning, with accuracy levels of
less than a centimeter. GNSS also provides
very high temporal resolution measurements
(second by second, or even faster), yielding
key observations of time- dependent processes
in the lithosphere, atmosphere, and
ionosphere.
By contrast, rather than measuring three
dimensional (3-D) changes by positioning
techniques, altimetry involves only changes
in surface elevation. Space- based radar and
laser altimetry techniques accurately measure
the satellite’s height above the Earth’s
surface, which is then converted to the surface’s
height above a reference ellipsoid.
Altimetry measurements are conducted by
releasing pulses toward the Earth’s surface
every several milliseconds, resulting in circular
ground measurements (footprints)
along the satellite track. Because of the
large footprint (diameter > 75 meters), altimetry
measurements are useful for measuring
flat surfaces. Radar altimetry was actually
designed for measuring the height of
ocean water surfaces but also was found
to be useful in measuring changes in ice
cap elevation and water levels in rivers and
lakes.
Similar types of data, not measured from
satellites, involve airborne light detection
and ranging (lidar) and terrestrial laser
scanning (TLS), both of which measure 3-D
positions of a large number of points located
on surfaces facing the instrument. Airborne
lidar is widely used to measure elevation,
whereas TLS measures small- scale structures
and detailed surface features.
A powerful method to detect surface
change is InSAR. This method compares
pixel- by- pixel SAR phase observations of
the same area acquired from roughly the
same location in space to produce digital
elevation models (DEMs) or surface displacement
maps with high spatial resolution
(typically from 5 to 100 meters). Such maps,
termed interferograms, are obtained from
repeat orbit observations and can reach
centimeter- level precision.
In addition, satellite orbits are very sensitive
to lateral variations in the Earth’s gravity
field. Precise measurements of satellite
orbits by ranging (distance) and other technologies
yield accurate determination of the
geoid shape and its variations over time. The
geoid is defined as the equipotential surface
of Earth’s gravity field that best fits global
mean sea level. It describes the mass distribution
within the Earth, from which one can
infer Earth’s dynamic structure. The new
generation of gravity satellites are also very
sensitive to short- term changes in the geoid
reflecting near- surface mass redistribution
such as ice melt or large- scale seasonal
changes in water budget. Geoid measurements
are also crucial for calibrating GPS
determined height with a standard mean sea
level datum.
Space geodesy provides observations at
various spatial and temporal scales with a
corresponding variety of applications. High
spatial resolution measurements and global
coverage provide the means to investigate
localized, continental, and global- scale processes.
The precise and repetitive nature
of satellite orbits enables reliable, repeated
data acquisition, with temporal resolution
of seconds (GNSS techniques) to tens
of days (altimetry, InSAR, and gravity techniques).
Some measurements began in the
1980s, providing scientists with data sets of
repeated observations that span decades.
Global- Scale Applications
The different geodetic measuring techniques,
along with varying spatial and temporal
accuracies, allow for geodetic insight
on a global scale.
In addition to broadly monitoring plate
motion (Figure 1a), independent geodetic
measurements have revealed congruence
of short- term plate motions with those on
geological timescales and provide better
constraints for quantitative plate motion
models. The same positioning techniques
have been used to monitor Earth’s rotation,
including variation in rotation rates affecting
the length of each day and pole motion
(Chandler wobble). These properties are
essential for precise determination of satellite
orbits used in a variety of applications,
such as weather forecasting and GPS
navigation.
The global coverage of altimetry satellites
allows decadal observations of sea level
change over entire ocean basins, complementing
tide gauge records, many of which
span the past 100 years, acquired only along
coastlines. In addition, GPS measurements
help improve the relative sea level change
record by monitoring the subsidence or
uplift of gauge stations, which can decouple
the relative movement of land from sea.
Accurate determination of the Earth’s
shape by measuring the geoid (Figure 1b)
is another important contribution using precise
tracking of altimetry and gravity mission
satellites. Altimetry measurements over
oceans combined with gravity models can
yield detailed information on the bathymetry
of the ocean floor. For example, they
can help detect the location of unknown
seamounts (Figure 1c), which show up as
localized gravity anomalies reflecting lateral
mass differences between seamounts and
ocean water.
Applications on a Continental Scale
Relative motion of tectonic plates produces
deformation along plate boundaries through
associated volcanoes and earthquakes. Geodetic
studies can determine the details of
earthquake- and magma- induced deformation
on a local scale. Integrating these details
for a larger geographic area reveals the full
picture of plate boundary deformation.
For example, the geodetic component of
EarthScope—the Plate Boundary Observatory,
funded by the U.S. National Science
Foundation—has expanded on existing
local GPS networks to provide continental
scale coverage of the western United States.
The broad and dense geographic coverage
of EarthScope, with its high- quality, continuous
data, is ideal for measuring volcanic and
plate tectonic motions, strain accumulation
on faults, earthquake surface displacement,
and postseismic deformation on timescales
of seconds to decades.
Geodetically observed vertical movements
by GPS in Europe and North America
reflect glacial isostatic adjustment (GIA, previously
known as postglacial rebound; see
Figure 1d) due to the melt of ice caps following
the last glacial period (~110,000 to
10,000 years ago) and corresponding mass
adjustment in the newly unloaded mantle.
These observations provide excellent constraints
on mantle viscosity. GIA also has
a global- scale contribution as it changes
the Earth’s dynamic oblateness (J2 ), which
reflects changes in the latitudinal distribution
of mass within the Earth.
Shorter- timescale seasonal and multiyear
redistribution of water and ice mass are
expressed by small but detectable changes
in the geoid shape. The high precision of
the Gravity Recovery and Climate Experiment
(GRACE) satellite enables estimation
of large- scale regional water budget
changes (Figure 1e) and changes in polar
ice mass due to glacier melting. Changes in
ice cap elevation are also monitored by the
polar- orbiting Ice, Cloud, and Land Elevation
Satellite (ICESat) and other altimetry
satellites. Geoid and elevation changes over
polar ice caps include both ice changes and
GIA response to current and past melting.
Ground- based GPS measurements are essential
for determining mass changes due to GIA
induced crustal uplift, allowing improved estimates
of polar ice cap melt rate.
In addition to monitoring changes to the
surface of Earth, many continental- scale
atmospheric and ionospheric phenomena
can be measured through satellite geodesy.
GPS and other GNSS techniques can monitor
changes in atmospheric precipitable water
(Figure 1f) and ionospheric total electron
content (TEC). Such changes can be monitored
because signals transmitted from the
GNSS satellites are sensitive to the water content
in the atmosphere and to TEC in the ionosphere.
Precipitable water retrievals have
been shown to improve the representation
of atmospheric water vapor in numerical
weather prediction systems, increasing their
ability to forecast heavy rain and hurricane
intensity. TEC retrievals can help scientists
forecast adverse space weather.
Satellite Geodesy on Local Scales
Changes in the Earth’s surface at local
scales (<100 kilometers) are best measured
by a combination of high temporal resolution
(GPS) and high spatial resolution
(InSAR) observations. In tectonically active
areas, utilization of both techniques has
revealed details of crustal deformation during,
after, and between moderate and large
earthquakes (Figure 2a) as well as magma
induced deformation beneath volcanoes
and continental rifts (Figure 2b). In the cryosphere,
these two methods provide a new
understanding of the kinematics of glacier
flow (Figure 2c).
InSAR is very effective in detecting localized
subsidence and uplift of land surfaces
in response to natural or anthropogenic
causes. Some of the more successful applications
are monitoring subsidence due
to compaction of sediments (Figure 2d),
aquifer- system response to groundwater
pumping and artificial recharge (Figure 2e),
extraction of fluids in oil fields, and excavation
in mines and tunnels. InSAR and GPS
are also powerful tools in studying surficial
processes such as landslides (Figure 2f) and
soil moisture content.
Space geodesy also measures changes in
water surfaces on continents. InSAR combined
with water level gauges provides information
on wetland water level changes (Figure
2g). Altimetry observations are used for
detecting water levels in rivers and lakes
(Figure 2h), especially in remote regions.
The observations are local at points where
ground tracks of satellite orbits intersect rivers
or lakes and can be combined to provide
regional information.
Though not space- based, airborne lidar
and TLS instruments are very effective geodetic
techniques—both are used in geomorphological
studies, such as determining
fault slip rates across offset topographic features.
Airborne lidar can map subtle elevation
changes in extremely flat areas, such as
south Florida, where it is used for determining
flood zones.
Societal Implications
Natural hazard mitigation, the effects of
global warming, and optimum use of water
resources are areas of major concern for
humankind today. The implications of space
geodesy when applied to natural hazards
associated with earthquakes and volcanoes
are well known in the geoscience community,
but space geodesy also has an impact
beyond these traditional solid Earth hazards.
Sea level rise, glacial melting, and hurricane
forecasts are of immediate interest
to communities around the world, particularly
in the context of global climate change.
Geodesy can also reveal the overlapping
threats from multiple hazards—for example,
in areas of coastal subsidence such as Bangkok,
Thailand, the effect of continued sea
level rise amplifies flooding hazards.
One of the greatest global challenges of
the 21st century is securing fresh water for the
increasing worldwide population and for sustaining
natural ecosystems. Geodetic monitoring
of subsidence in depleted aquifers and
water level changes in wetlands, rivers, and
lakes yields important constraints for hydrological
models that can serve as decision support
tools for water resource managers.
The varied scales and high precision
of space geodetic observations are helping
to push the frontiers of knowledge
regarding many Earth processes. Because
space geodetic measurements have many
applications, geodesy today brings scientists
together for interdisciplinary research that
helps mitigate the influence of the forces of
nature on our growing population as well as
the effect of the population on Earth’s fragile
surface.
For more information on space geodetic
techniques and their applications,
please visit http:// www . unavco . org/
geodesy21century.
Reference
Reid, H. F. (1910), The California Earthquake of
April 18, 1906, vol. 2, The Mechanics of the Earthquake,
Report of the State Investigation Commission,
Carnegie Inst. of Wash., Washington, D. C.
Author Information
Shimon Wdowinski, Division of Marine Geology
and Geophysics, University of Miami, Miami, Fla.;
and Susan Eriksson, UNAVCO, Boulder, Colo.;
E-mail: Eriksson@ unavco . org