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=                             Topography                             =
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                            Introduction
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Topography is the study of the shape and features of land surfaces.
The topography of an area could refer to the surface shapes and
features themselves, or a description (especially their depiction in
maps).

Topography is a field of geoscience and planetary science and is
concerned with local detail in general, including not only relief but
also natural and artificial features, and even local history and
culture. This meaning is less common in the United States, where
topographic maps with elevation contours have made "topography"
synonymous with relief.

Topography in a narrow sense involves the recording of relief or
terrain, the three-dimensional quality of the surface, and the
identification of specific landforms. This is also known as
geomorphometry. In modern usage, this involves generation of elevation
data in digital form (DEM). It is often considered to include the
graphic representation of the landform on a map by a variety of
techniques, including contour lines, hypsometric tints, and relief
shading.


                             Etymology
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The term 'topography' originated in ancient Greece and continued in
ancient Rome, as the detailed description of a place. The word comes
from the Greek   ('topos',  "place") and  ('-graphia', "writing"). In
classical literature this refers to writing about a place or places,
what is now largely called 'local history'. In Britain and in Europe
in general, the word topography is still sometimes used in its
original sense.

Detailed military surveys in Britain (beginning in the late eighteenth
century) were called Ordnance Surveys, and this term was used into the
20th century as generic for topographic surveys and maps. The earliest
scientific surveys in France were called the Cassini maps after the
family who produced them over four generations. The term "topographic
surveys" appears to be American in origin. The earliest detailed
surveys in the United States were made by the �Topographical Bureau of
the Army,� formed during the War of 1812, which became the Corps of
Topographical Engineers in 1838. After the work of national mapping
was assumed by the U.S. Geological Survey in 1878, the term
topographical remained as a general term for detailed surveys and
mapping programs, and has been adopted by most other nations as
standard.

In the 20th century, the term topography started to be used to
describe surface description in other fields where mapping in a
broader sense is used, particularly in medical fields such as
neurology.


                             Objectives
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An objective of topography is to determine the position of any feature
or more generally any point in terms of both a horizontal coordinate
system such as latitude, longitude, and altitude. Identifying (naming)
features, and recognizing typical landform patterns are also part of
the field.

A topographic study may be made for a variety of reasons: military
planning and geological exploration have been primary motivators to
start survey programs, but detailed information about terrain and
surface features is essential for the planning and construction of any
major civil engineering, public works, or reclamation projects.


                             Techniques
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There are a variety of approaches to studying topography. Which
method(s) to use depends on the scale and size of the area under
study, its accessibility, and the quality of existing surveys.


Direct survey
===============
Surveying helps determine accurately the terrestrial or
three-dimensional space position of points and the distances and
angles between them using leveling instruments such as theodolites,
dumpy levels and clinometers.

Work on one of the first topographic maps was begun in France by
Giovanni Domenico Cassini, the great Italian astronomer.

Even though remote sensing has greatly sped up the process of
gathering information, and has allowed greater accuracy control over
long distances, the direct survey still provides the basic control
points and framework for all topographic work, whether manual or
GIS-based.

In areas where there has been an extensive direct survey and mapping
program (most of Europe and the Continental U.S., for example), the
compiled data forms the basis of basic digital elevation datasets such
as USGS DEM data. This data must often be "cleaned" to eliminate
discrepancies between surveys, but it still forms a valuable set of
information for large-scale analysis.

The original American topographic surveys (or the British "Ordnance"
surveys) involved not only recording of relief, but identification of
landmark features and vegetative land cover.


Remote sensing
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Remote sensing is a general term for geodata collection at a distance
from the subject area.


Passive sensor methodologies
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Besides their role in photogrammetry, aerial and satellite imagery can
be used to identify and delineate terrain features and more general
land-cover features. Certainly they have become more and more a part
of geovisualization, whether maps or GIS systems. False-color and
non-visible spectra imaging can also help determine the lie of the
land by delineating vegetation and other land-use information more
clearly. Images can be in visible colours and in other spectrum.


Photogrammetry
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Photogrammetry is a measurement technique for which the co-ordinates
of the points in 3D of an object are determined by the measurements
made in two photographic images (or more) taken starting from
different positions, usually from different passes of an aerial
photography flight. In this technique, the common points are
identified on each image. A line of sight (or ray) can be built from
the camera location to the point on the object. It is the intersection
of its rays (triangulation) which determines the relative
three-dimensional position of the point. Known control points can be
used to give these relative positions absolute values. More
sophisticated algorithms can exploit other information on the scene
known a priori (for example, symmetries in certain cases allowing the
rebuilding of three-dimensional co-ordinates starting from one only
position of the camera).


Active sensor methodologies
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Satellite RADAR mapping is one of the major techniques of generating
Digital Elevation Models (see below). Similar techniques are applied
in bathymetric surveys using sonar to determine the terrain of the
ocean floor. In recent years, LIDAR (LIght Detection And Ranging), a
remote sensing technique that uses a laser instead of radio waves, has
increasingly been employed for complex mapping needs such as charting
canopies and monitoring glaciers.


                     Forms of topographic data
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Terrain is commonly modelled either using vector (triangulated
irregular network or TIN) or gridded (Raster image) mathematical
models. In the most applications in environmental sciences, land
surface is represented and modelled using gridded models. In civil
engineering and entertainment businesses, the most representations of
land surface employ some variant of TIN models. In geostatistics, land
surface is commonly modelled as a combination of the two signals - the
smooth (spatially correlated) and the rough (noise) signal.

In practice, surveyors first sample heights in an area, then use these
to produce a Digital Land Surface Model in the form of a TIN. The DLSM
can then be used to visualize terrain, drape remote sensing images,
quantify ecological properties of a surface or extract land surface
objects. Note that the contour data or any other sampled elevation
datasets are not a DLSM. A DLSM implies that elevation is available
continuously at each location in the study area, i.e. that the map
represents a complete surface. Digital Land Surface Models should not
be confused with Digital Surface Models, which can be surfaces of the
canopy, buildings and similar objects. For example, in the case of
surface models produces using the lidar technology, one can have
several surfaces - starting from the top of the canopy to the actual
solid earth. The difference between the two surface models can then be
used to derive volumetric measures (height of trees etc.).


Raw survey data
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Topographic survey information is historically based upon the notes of
surveyors. They may derive naming and cultural information from other
local sources (for example, boundary delineation may be derived from
local cadastral mapping). While of historical interest, these field
notes inherently include errors and contradictions that later stages
in map production resolve.


Remote sensing data
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As with field notes, remote sensing data (aerial and satellite
photography, for example), is raw and uninterpreted. It may contain
holes (due to cloud cover for example) or inconsistencies (due to the
timing of specific image captures). Most modern topographic mapping
includes a large component of remotely sensed data in its compilation
process.


Topographic mapping
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In its contemporary definition, topographic mapping shows relief. In
the United States, USGS topographic maps show relief using contour
lines. The USGS calls maps based on topographic surveys, but without
contours, "planimetric maps."

These maps show not only the contours, but also any significant
streams or other bodies of water, forest cover, built-up areas or
individual buildings (depending on scale), and other features and
points of interest.

While not officially "topographic" maps, the national surveys of other
nations share many of the same features, and so they are often called
"topographic maps."

Existing topographic survey maps, because of their comprehensive and
encyclopedic coverage, form the basis for much derived topographic
work. Digital Elevation Models, for example, have often been created
not from new remote sensing data but from existing paper topographic
maps. Many government and private publishers use the artwork
(especially the contour lines) from existing topographic map sheets as
the basis for their own specialized or updated topographic maps.

Topographic mapping should not be confused with geologic mapping. The
latter is concerned with underlying structures and processes to the
surface, rather than with identifiable surface features.


Digital elevation modeling
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The digital elevation model (DEM) is a raster-based digital dataset of
the topography (hypsometry and/or bathymetry) of all or part of the
Earth (or a telluric planet). The pixels of the dataset are each
assigned an elevation value, and a header portion of the dataset
defines the area of coverage, the units each pixel covers, and the
units of elevation (and the zero-point). DEMs may be derived from
existing paper maps and survey data, or they may be generated from new
satellite or other remotely sensed radar or sonar data.


Topological modeling
======================
A geographic information system (GIS) can recognize and analyze the
spatial relationships that exist within digitally stored spatial data.
These topological relationships allow complex spatial modelling and
analysis to be performed. Topological relationships between geometric
entities traditionally include adjacency (what adjoins what),
containment (what encloses what), and proximity (how close something
is to something else).
* reconstitute a sight in synthesized images of the ground,
* determine a trajectory of overflight of the ground,
* calculate surfaces or volumes,
* trace topographic profiles,


                     Topography in other fields
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Topography has been applied to different science fields. In
neuroscience, the neuroimaging discipline uses techniques such as EEG
topography for brain mapping. In ophthalmology, corneal topography is
used as a technique for mapping the surface curvature of the cornea.
In tissue engineering, atomic force microscopy is used to map
nanotopography.

In human anatomy, topography is superficial human anatomy.

In mathematics the concept of topography is used to indicate the
patterns or general organization of features on a map or as a term
referring to the pattern in which variables (or their values) are
distributed in a space.


                              See also
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* Cartography
* Digital elevation model
* Fall line (topography)
* Geomorphology
* Global Relief Model
* Hypsography
* Marine topography
* Topographic map


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