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Principles of Photogrammetry:
Stereoscopic Parallax
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Stereophotography
Adjacent but
overlapping aerial
photos are calledstereo-pairs and are
needed to determine
parallax and stereo/3D
viewing
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Overlapping Stereophotography
Overlapping
photography
Endlap - ~60%
Sidelap - ~20-30%
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Relief Displacement
Even from great flying
heights, tall objects can
exhibit image displacement.
In this example from aQuickbird satellite image,
the Washington Monument
appears to lean outwards
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Radial Displacement
Objects will tend tolean outward, i.e. be
radially displaced.
The greater the object
is from the principalpoint, the greater the
radial displacement.
Example: storagetanks towards the edge
of photo show greater
radial displacement.
Center of
photo
Edge of
photo
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Maps vs. Aerial Photos
Maps: Scale is constant
No relief displacement
Photos: Scale varies with elevation
Relief displacement
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Stereoscopic Parallax
The displacement ofan object caused by a
change in the point of
observation is called
parallax.
Stereoscopic parallax
is caused by taking
photographs of thesame object but from
different points of
observation.
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Stereoscopic parallax
Line of Flight
Note the displacement between the top and base ofthe storage towers in this photo stereo-pair
top
bottom
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Stereoscopic Plotting Instruments
Stereoplotters - precision
instruments designed to
duplicate the exact relative
position and orientation of
the aerial camera at thetime of photo acquisition
to recreate the stereo-
model. A floating mark
can be used trace specificelevations. Relief
displacement is removed
creating a planimetric
map.
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Stereoscopic Plotting Instruments
Soft-copyphotogrammetry
workstations - computer
software recreates the
stereomodel and allowsfor digital mapping
Soft-copy
photogrammtery has
largely replaced optical-mechanical systems
Digital
scanner
Soft copy
workstation
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Simulated 3-D Stereo viewing
One view displayed in red; the otherperspective view in blue spatially shifted
The spatial shift is a
function of thedifferential parallax
To visualize, use
red-blue glasses
NASA Mars Lander
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Electronic Distance Measurement
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Got its start in 1948 with Swiss device using
visible light (range=40km only at night)http://www.gmat.unsw.edu.au
Microwaves used in 1957 (range=80km)http://www.gmat.unsw.edu.au
Infra-red, IR, devices are common todayhttp://www.pentaxcanada.ca
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Electro-magnetic energy travels through the
atmosphere according to the following:
Where:
c = speed of light in vacuum
n = atmospheric index of refraction, 1.003 for STP
f = frequency of the electro-magnetic energy
l= wavelength of the energy
c/n=fl
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So called total-station instruments package a
digital theodolite (for measuring azimuth andaltitude) with an EDM, data storage device, and
often a modem for transmitting data from the
field. The most common instruments use a pulse of 2
to 4 AM frequencies ranging from 150kHz to
15MHz. This range of frequencies have corresponding
half-wavelengths of 1.0km to 10m,
respectively.
http://www.nikon.co.jp/survey-e
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12/19/2013
It is a combination of an electronic theodolite(transit), an electronic distance meter (EDM) andsoftware running on an external computer known as adata collector.
A total station is an optical instrument used inmodern surveying and archaeology as well as by
police, crime scene investigators, private accidentreconstructionists and insurance companies to takemeasurements of scenes.
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Salient features of modern TS
TS is a fully integrated equipment that captures all the spatial
data necessary for a three-dimensional position fix. The angles anddistances are displayed on a digital readout and can be recorded at the
press of a button. Total station is usually operated by a surveyor assisted
by a labourer or geodesist who carries the target pole to the points of
detail to be surveyed. Various components of a typical TS are shown inFig.6.
Fig. 6 Parts of Total Station
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Electronic transitReads andstores horizontal and vertical
angles
Uses EDM to measure and store
the distance of points
Points can be stored in the
instrument using computer
fig.7Total station
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EDM Electronic theodolite
On-Board Micro-processor
Data Collector
Data Storage
Prisms
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Distance units are in metres.
IR mode has high accuracy, long range,
and measures to a specific point.
Personal access is required to the targetpoint.
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Reflector less moderequires no prism.
Range up to 160m.
Distance units are in metres
Reflector less mode can measure to inaccessiblepoints, but be careful about pointing and beam
interruptions!
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HD = SD * Sin(VA)
VD = SD * Cos(VA)
SDVA
IH
TH
The inbuilt software computes the HD and VD from
the SD and the VA
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Application pre-settings
F1 = Name of jobF2 = Name of setup station
F3 = Orientation
F4 = Begin
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Advantages of Total Station over
Conventional instruments:
Traditional survey methods are laborious and time
consuming
Fully automatic electronic measurement
Digital display of staff reading and distance
Data storage in instrument possible
Direct transfer to personal computer of data stored in
instruments
Online operation through integrated interface to computer
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Disadvantages
Total stations are dependent on batteries and electronics. The
LCD screen does not work well when it is cold.
Battery life is also short, batteries and electronics both do not
work well when wet.
Loss of data is an important consideration.
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