Environmental Engineering Reference
In-Depth Information
coordinates of points can then be determined, and
interpolated onto a regular grid. Themodel formed
is essentially a DSM. Semi-automatic techniques
are used to remove blunders. The accuracy
achieved depends on the scale of the photography
and the skill of the operator.
Photogrammetric techniques have been used in
the development of the Ordnance Survey (OS)
Landform Profile Plus DTM. There is extensive
aerial photography of theUK, andUKPerspectives
has created a DTM of the UK using photogram-
metry applied to 1:10,000- and 1:25,000-scale im-
agery. This has an approximate vertical accuracy
of
1m and a 10-m grid spacing.
System (GPS). The spot heights measured have to
be interpolated onto a grid to produce a DTM.
While these techniques provide the greatest accu-
racies currently achievable, they require lengthy
fieldwork, making themmore suitable for provid-
ing validation data for other techniques or for
filling gaps in data than for DTM generation over
large areas. Total stations are electronic theodo-
lites with distance measurement capabilities,
which can position points to better than
0.5 cm
(Kavanagh 2003). GPS is a system that provides
continuous all-weather positioning on the Earth's
surface using a constellation of more than
24 satellites transmitting continuous microwave
signals (see, e.g., Hoffman-Wellenhof et al. 1994).
The two main observing modes used in surveying
are differential static positioning and kinematic
positioning, each of which require as a minimum
a base and a roving receiver. Static positioning can
achieve a positional accuracy of
2 cm, whereas
kinematic positioning, requiring less observation
time, can achieve
5 cm.
Interferometric synthetic aperture radar (InSAR)
A DSM can be generated using InSAR, which uses
two side-looking antennae on board a satellite or
aircraft separatedby a known baseline to image the
terrain (Goldstein et al. 1988; Madsen et al. 1991).
Two main configurations exist: repeat pass inter-
ferometry, where the data are acquired from two
passes of a (usually satellite) sensor in similar or-
bits; and single pass interferometry,where the data
are acquired in a single pass using two antennae
separated by a fixed baseline on the same platform,
which to date has been an aircraft or the Space
Shuttle. The height of a point can be determined
by trigonometry, using knowledge of the locations
andorientationsof thetwoantennae(fromGPSand
each sensor's INS), their baseline separation, and
the path difference between the signals from each
antenna.Thesurfaceelevationmeasuredforapixel
may consist of a combined signal from different
scatterers in the pixel. For pixels containing vege-
tation, volume scattering will occur and there will
be some penetration into the canopy, so that the
heightmeasuredwill not be that of thefirst surface.
Other limitationsarethatperformancecandegrade
inurbanareasdue tobright targetsandshadow, and
that artefacts may appear in the DSMdue to atmo-
spheric propagation and hilly terrain. However,
InSAR is all-weather and day-night, and large areas
can be mapped rapidly.
A near-global high-resolution DSM of the
Earth's topography was acquired using InSAR by
Digital aerial photogrammetry
Digital Terrain Models can be produced using
stereo-photogrammetry applied to overlapping
pairs of aerial photographs (Wolf andDewitt 2000).
Photographs are usually acquired in strips with
adjacent photographs having 60% overlap in the
flight direction and 20-30%overlap perpendicular
to this. They are then digitized using a photogram-
metric scanner. Coordinates in the camera's
image coordinate system are defined knowing
the imaging characteristics of the camera and the
location of a set of fiducial marks. A relationship
between the image space coordinates and the
ground space coordinates is determined inmodern
aircraft systems using the onboard GPS to deter-
mine positions and the inertial navigation system
(INS) to determine orientations. In order to gener-
ate ground elevations from a stereo-pair, corre-
sponding image points in the overlapping area of
the pair must be determined. While this image
matching can be performed semi-automatically,
automatic area-, feature- or relation-based match-
ing is less time-consuming. The 3-D ground space
