Civil Engineering Reference
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and ultrasonic systems. The biggest problems with magnetic systems, however, are
distortions caused by metal objects, and a very rapid decrease in accuracy and
resolution with distance (Nixon
et al
., 1998). Magnetic trackers are subject to large
amounts of error and jitter. Despite their lack of accuracy, magnetic trackers are
popular because they are robust and place minimal constraints on user motion
(Baratoff and Blanksteen, 2001).
Inertial orientation trackers were also investigated. They are truly source-less,
and are unaffected by almost any environmental interference (Foxlin
et al
., 1998).
They can be used in large workspaces because they do not need a transmitting
source to perform tracking and there is no hardware or cabling between computer
and tracker. The user is free to move around in the real world with no restrictions.
Inertial orientation trackers are also known for their low latency. They use
accelerometers and gyroscopes to instantaneously derive orientation changes.
Although comparatively cheap, fast and accurate, inertial trackers suffer from
drift error accumulation (3-10 degrees per min) following rapid motion or long
tracking periods (Baratoff and Blanksteen, 2001). Moreover, the effect of gravity on
accelerometers and gyroscopes induces an erroneous downward acceleration force
on the tracking device.
It was thus decided to use a magnetic orientation tracker, specifically the TCM5
magnetic orientation tracker. This tracker includes a built-in compass, thereby
avoiding manual calibration and error accumulation (Behzadan and Kamat, 2006).
The TCM5 orientation tracker employs solid state magnetic field sensors which
measure compass heading through a full 360
of rotation. The tracking device is
placed at the highest point inside the user's helmet, directly above the top of their
head, and parallel to their forward line of sight.
In subsequent sections of this chapter, the technical characteristics of the
WLAN, UWB, and Indoor GPS indoor wireless technologies introduced above
are explained. Additionally, the extent to which each technology can be used to
accurately calculate the positional context of a user in congested dynamic
environments, such as those found on construction sites, is highlighted. Also
presented are an infrastructure-independent inertial navigation-based posi-
tioning system and the latest research advances in integrating infrastructure-
based positioning systems with the infrastructure-independent positioning
system.
6.3 User tracking in construction environments
Considering the dynamic nature of typical construction projects, mobile users
(e.g. construction engineers, inspectors, etc.) need to be constantly tracked
outdoors as well as indoors. By capitalizing on the ability to accurately track
mobile users in any indoor and/or outdoor environment, dynamic spatial context-
sensing frameworks that allow the identification of construction entities and
artifacts visible in a user's field of view at any given time and location, as well
as contextual
information retrieval, can be designed and implemented. A
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