Photogrammetry
and Remote Sensing Applications
Topographic
Mapping
Photogrammetry is most commonly associated with the production of topographic
mapping generally from conventional aerial stereo photography although digital
and satellite imagery is increasingly being used.
Photogrammetry
using imagery obtained from fixed wing aircraft helicopters and from satellites
is used to produce mapping at a variety of scales. Recent improvements in
camera design and photogrammetric instrumentation have led to increases in
accuracies enabling mapping at scales as large as 1:200 to be produced from
conventional aerial photography.
Photogrammetrically
derived three dimensional information produced from analytical or digital
stereo plotting instruments is used to produce maps and plans and provide
the source data for ground modelling packages, orthophotos and geographical
information systems.
Photogrammetry
is used to provide national mapping and map revision at small and medium scales.
At larger scales photogrammetric data form the basis for three-dimensional
modelling in a wide variety of applications including highway design, floodplain
studies and pipeline routing.
Architectural
Applications
As the term 'photogrammetry' was devised by an architect, it is not surprising
that the greatest use of the technique, outside of mapping, has been in the
arena of architectural recording. Photogrammetry is now established as a standard
technique for the survey of building elevations and is employed by English
Heritage, Cadw, and the National Trust to name but a few.
The
technique is applied in the recording of historic buildings and monuments
to produce elevation drawings and sections normally at scales of 1:20, 1:50
and 1:100.
Imagery
can be obtained from a variety of cameras ranging from large format metric
cameras, where high accuracy and archival value are important, to smaller
format 35mm cameras which are useful where access is restricted. Use is often
made of aerial platforms and scaffolding to obtain the most economic coverage
of a façade.
Architects
are becoming increasingly aware of the benefits of accurate photogrammetric
surveys particularly the advantage of using a totally remote measurement system
with superb archival qualities.
The
products available range from rectified images, orthophotos or precise 3D
data formatted for use in a CAD package.
Engineering
Applications
Over the years photogrammetry has been used in a wide variety of engineering
situations including ship building, aircraft manufacture, chemical plant surveys,
'as builts', tunnel profiling and many others.
The
non-contact nature of photogrammetry and the relatively short time required
on site are important advantages particularly in areas of a hazardous nature.
The accuracies attainable compare favourably with other systems that may require
longer site time and more intensive use of manpower.
As
equipment and cameras develop, real and near real-time monitoring of components
and structures is becoming a reality. The phrase "if you can photograph
it you can measure it" sums this up succinctly.
Laser
Scanning
A
relatively new technique complementing photogrammetry and remote sensing is
that of laser scanning. The core technology utilised in terrestrial laser
scanning systems will be familiar to any engineering surveyor who has used
a total station. Terrestrial scanners for use in engineering surveying applications
generally calculate the range to a surface in one of two ways; by measuring
the time between the emission of a pulse and the detection of the reflected
energy or by measuring the phase difference of the returned signal relative
to the emitted signal.
The laser pulse is automatically scanned (generally horizontally and
vertically) to create a fan shaped pattern over the surface. This is
performed using a rotating mirror, or servo motors fitted to the ranging
device, that physically redirects the laser. Using a known distance and a
horizontal and vertical angle, a XYZ position in the scanners
arbitrary coordinate system can be derived for the point illuminated on the
surface. The result is a 'point cloud' of data that describes the
shape of the object and can be manipulated by the user. If the location and
orientation of the scanner is known then real world coordinates can be calculated.
Airborne laser scanning systems can create a three dimensional model
of the earth's surface in a similar fashion. The aircraft's heading defines
one scan direction and the laser emitted from the ranging device onboard the
aircraft is redirected perpendicular to this. Onboard GPS and inertial measurement
instruments calculate the position and attitude of the aircraft and the range
and angle measurements are then used to determine the precise position of
the point on the ground.
Other
Applications
The remote measurement aspect of photogrammetry makes it ideal for certain
applications such as body modelling, road traffic accident and scene of crime
recording where intrusion in the subject area should be kept to a minimum.
In
the case of body modelling the data acquired has been used for clothing design,
the manufacturing of false limbs and for animal husbandry.
Forensic
applications have been developed extensively throughout Europe although less
so here in the United Kingdom. Currently a number of UK police forces are
employing photogrammetry both from the air and the ground to ensure that site
time is kept to an absolute minimum.
