Global Relief Model
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A global relief model, sometimes also denoted as global topography model or composite model, combines
digital elevation model A digital elevation model (DEM) or digital surface model (DSM) is a 3D computer graphics representation of elevation data to represent terrain or overlaying objects, commonly of a planet, moon, or asteroid. A "global DEM" refers to a discrete gl ...
(DEM) data over land with digital
bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water de ...
model (DBM) data over water-covered areas (oceans, lakes) to describe Earth's relief. A relief model thus shows how Earth's surface would look like in the absence of water or ice masses. The relief is represented by a set of heights (elevations or depths) that refer to some height reference surface, often the mean
sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...
or the
geoid The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended ...
. Global relief models are used for a variety of applications including
geovisualization Geovisualization or geovisualisation (short for geographic visualization), also known as cartographic visualization, refers to a set of tools and techniques supporting the analysis of geospatial data through the use of interactive visualization. Li ...
,
geologic Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Eart ...
,
geomorphologic Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or n ...
and
geophysical Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' som ...
analyses,
gravity field In physics, a gravitational field is a model used to explain the influences that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenome ...
modelling as well as geo-statistics.


Measurement

Global relief models are always based on combinations of data sets from different
remote sensing Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth ...
techniques. This is because there is no single remote sensing technique that would allow measurement of the relief both over dry and water-covered areas. Elevation data over land is often obtained from
LIDAR Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
or
inSAR Interferometric synthetic aperture radar, abbreviated InSAR (or deprecated IfSAR), is a radar technique used in geodesy and remote sensing. This geodetic method uses two or more synthetic aperture radar (SAR) images to generate maps of surface defo ...
measurements, while
bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water de ...
is acquired based on
SONAR Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigation, navigate, measure distances (ranging), communicate with or detect o ...
and altimetry. Global relief models may also contain elevations of the bedrock (sub-ice-
topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...
) below the ice shields of
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
and
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland is t ...
. Ice sheet thickness, mostly measured through ice-penetrating
RADAR Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
, is subtracted from the ice surface heights to reveal the bedrock.


Spatial resolution

While digital elevation models describe Earth's land topography often with 1 to 3 arc-second resolution (e.g., from the
SRTM The Shuttle Radar Topography Mission (SRTM) is an international research effort that obtained digital elevation models on a near-global scale from 56°S to 60°N, to generate the most complete high-resolution digital topographic database of Ea ...
or
ASTER Aster or ASTER may refer to: Biology * ''Aster'' (genus), a genus of flowering plants ** List of ''Aster'' synonyms, other genera formerly included in ''Aster'' and still called asters in English * Aster (cell biology), a cellular structure shap ...
missions), the global bathymetry (e.g., SRTM30_PLUS) is known to a much lesser spatial resolution in the kilometre-range. The same holds true for models of the bedrock of Antarctica and Greenland. Therefore, global relief models are often constructed at 1 arc-minute resolution (corresponding to about 1.8 km postings). Some products such as the 30 and 15 arc-second resolution SRTM30_PLUS/ SRTM15_PLUS grids offer higher resolution to adequately represent SONAR depth measurements where available. Although grid cells are spaced at 15 or 30 arc-seconds, when SONAR measurements are unavailable, the resolution is much worse (~20-12 km) depending on factors such as water depth.


Public data sets

Data sets produced and released to the public include Earth2014, SRTM30_PLUS and ETOPO1.


ETOPO 2022


Earth2014 (2015)

The most recent global relief model is Earth2014, developed at Curtin University (Western Australia) and TU Munich (Germany). Earth2014 provides sets of 1 arc-min resolution global grids (about 1.8 km postings) of Earth's relief in different representations based on the 2013 releases of bedrock and ice-sheet data over Antarctica (Bedmap2) and Greenland (Greenland Bedrock Topography), the 2013 SRTM_30PLUS bathymetry and 2008 SRTM V4.1 SRTM land topography. Earth2014 provides five different layers of height data, including Earth's surface (lower interface of the atmosphere), topography and bathymetry of the oceans and major lakes, topography, bathymetry and bedrock, ice-sheet thicknesses and rock-equivalent topography. The Earth2014 global grids are provided as heights relative to the
EGM96 The Earth Gravitational Models (EGM) are a series of geopotential models of the Earth published by the National Geospatial-Intelligence Agency (NGA). They are used as the geoid reference in the World Geodetic System. The NGA provides the model ...
mean sea level There are several kinds of mean in mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. ...
for the conventional relief model, and as planetary radii relative to the centre of Earth to show the shape of the Earth.


SRTM30_PLUS (2014)

SRTM30_PLUS is a combined bathymetry and topography model produced by
Scripps Institution of Oceanography The Scripps Institution of Oceanography (sometimes referred to as SIO, Scripps Oceanography, or Scripps) in San Diego, California, US founded in 1903, is one of the oldest and largest centers for oceanography, ocean and Earth science research ...
(California). The version 15_PLUS comes at 0.25 arc-min resolution (about 450 m postings), while the 30_PLUS version offers 0.5 arc-min (900 m) resolution. The bathymetric data in SRTM30_PLUS stems from depth soundings (SONAR) and from satellite altimetry. The bathymetric component of SRTM30_PLUS gets regularly updated with new or improved data sets in order to continuously improve and refine the description of the sea floor geometry. Over land areas, SRTM30 data from the USGS is included. SRTM30_PLUS provides background information for
Google Earth Google Earth is a computer program that renders a 3D computer graphics, 3D representation of Earth based primarily on satellite imagery. The program maps the Earth by superimposition, superimposing satellite images, aerial photography, and geog ...
and
Google Maps Google Maps is a web mapping platform and consumer application offered by Google. It offers satellite imagery, aerial photography, street maps, 360° interactive panoramic views of streets ( Street View), real-time traffic conditions, and rou ...
.


ETOPO1 (2009)

The ETOPO1 1-arcmin global relief model, produced by the
National Geophysical Data Center The United States National Geophysical Data Center (NGDC) provided scientific stewardship, products and services for geophysical data describing the solid earth, marine, and solar-terrestrial environment, as well as earth observations from spac ...
(Colorado), provides two layers of relief information. One layer represents the global relief including bedrock over Antarctica and Greenland, and another layer the global relief including ice surface heights. Both layers include
bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water de ...
over the oceans and some of Earth's major lakes. ETOPO1 land topography and ocean bathymetry relies on SRTM30 topography and a multitude of bathymetric surveys that have been merged. Historic versions of ETOPO1 are the ETOPO2 and ETOPO5 relief models (2 and 5 arc-min resolution). The ETOPO1 global relief model is based on the 2001 Bedmap1 model of bedrock over Antarctica, which is now superseded by the significantly improved Bedmap2 bedrock data. The ETOPO1-contained information on ocean depths is superseded through several updates of the SRTM30_PLUS bathymetry.


References

{{Reflist


External links


Earth2014 homepage



ETOPO1 homepage
Cartography Digital elevation models