ubiquitous location tracking for context-specific information delivery

Automation in Construction 17 (2008) 737–748

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Automation in Construction

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Ubiquitous location tracking for context-specific information delivery onconstruction sites

Amir H. Behzadan a,1, Zeeshan Aziz b,2, Chimay J. Anumba b,2, Vineet R. Kamat a,⁎a Department of Civil and Environmental Engineering, University of Michigan, 2340 G.G. Brown, 2350 Hayward, Ann Arbor, MI 48109, USAb Department of Civil and Building Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK

a r t i c l e i n f o

⁎ Corresponding author. Tel.: +1 734 764 4325; fax: +E-mail addresses: [email protected] (A.H. Behzad

(Z. Aziz), [email protected] (C.J. Anumba), vkamat1 Tel.: +1 734 764 4325; fax: +1 734 764 4292.2 Tel.: +44 1509 222615; fax: +44 1509 223982.

0926-5805/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.autcon.2008.02.002

a b s t r a c t

Article history:Accepted 11 February 2008

Construction projects are information-intensive in nature and require site personnel to have continuous on-demand access to information such as project plans, drawings, schedules, and budgets. Awareness of auser's context (such as user profile, role, preferences, task, and existing project conditions) can enhance theconstruction project delivery process by providing a mechanism to determine information relevant to aparticular context. Context awareness can also be used to improve security, logistics and health and safetypractices on construction sites. Location is an important aspect of context awareness. A location awareapplication can utilize the knowledge of the user/object location to provide relevant information andservices. This paper argues that a successful and reliable location tracking system must be able to track auser's spatial context and deliver contextual data continuously in both outdoor and indoor environments toeffectively support construction projects. Research describing the use of Wireless Local Area Network(WLAN) for indoor tracking and Global Positioning System (GPS) for outdoor spatial context tracking ispresented, and an integrated tracking technique using WLAN and GPS for ubiquitous location sensing isintroduced. The key benefits and technical challenges of such an integrated approach are also highlighted.The presented tracking techniques have been validated in both indoor and outdoor environments to ensuretheir practical implementation on real construction jobsites.

© 2008 Elsevier B.V. All rights reserved.

Keywords:Augmented realityConstructionContext awarenessInformation deliveryLocation tracking

1. Introduction

The information-intensive nature of construction projects requiresthe site staff to have on-demand access to construction project datasuch as plans, drawings, schedules, and budgets. The unprepared anddynamic nature of a construction site, and the hazards and difficultiespresented by the on-site work, also necessitate the use of intelligentways to support on-site construction staff and personnel. Contextaware information delivery provides the ability to intelligently captureand interpret the user context, and delivering data and services to themobile worker based on the user's context. In this way, it is possible toeliminate distractions for mobile workers, related to the volume andlevel of information. Also, user interaction with the system can bereduced by using context as a filtering mechanism to deliver onlycontext relevant information to users. This has the potential to increaseusability, by decreasing the level of interaction required between themobile devices and the end users. The emergence of complementary

1 734 764 4292.an), [email protected]@umich.edu (V.R. Kamat).

l rights reserved.

technologies such as user profiling, ubiquitous computing and sensornetworking enables the capture of many other context parameters.

Location is an important andprobably themostwidely usedaspectofcontext awareness. Location aware applications utilize the knowledge ofthe user/object location to provide relevant information and services,control labor inputs, and measure project productivity [1]. Accurateand timely identification and tracking of construction components arecritical to operating a well managed and cost efficient constructionproject [2]. Location tracking technologies are often classified as indoor(i.e. location tracking in indoor environments) and outdoor (i.e. locationtracking in outdoor environments). A variety of indoor and outdoorlocation tracking technologies exist with significantly different char-acteristics, infrastructure, and device requirements. Although someresearchers have demonstrated the potential of various indoor andoutdoor positioning technologies for location tracking [3–5], the ben-efits of integrating the two categories to develop a robust positiontracking platform capable of delivering context aware information havenot been widely investigated in the construction industry.

This paper argues that both indoor and outdoor positioning tech-nologies are important to support construction projects. It presentscurrent research on outdoor as well as indoor position tracking forcontext aware information delivery on construction sites. It describesthe rationale behind the integration of these two methods and howthis adds value to current practice of position tracking. The keybenefitsand technical challenges are also highlighted.

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http://dx.doi.org/10.1016/j.autcon.2008.02.002

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Table 1Comparison of various indoor tracking technologies

Indoortrackingtechnology

RFID-basedtracking

WLAN-basedtracking

Bluetooth-basedtracking

Dedicatedspectrum basedtracking

Description The location ofthe movingRFID tag isdeducted fromthe location ofthe reader

Measures thesignal strengthdata, which isthen correlatedwith location

Works on thesame principleas WLAN-basedpositioningusing Bluetoothtechnology

Works on sameprinciple asWLAN-basedpositioningusing protocol,to minimizepowerconsumption

Accuracy Relatively low —

depends onsensing of tagsby RFID readers

Up to 1 m,provided threeor more accesspoints areprovided

Up to 1 m,provided thereare a number ofaccess pointsavailable

Very highaccuracy up to0.3 m

Ability tosend andreceivedata fromthe tag

Poor — RFID-based systemprimarilysupports onewaycommunicationi.e. from tag toinfrastructure

Supports 2 waydata e.g. fromWLAN enabledmobile device tobackendinfrastructureand vice versa

Supports 2 waydata (e.g. fromPDA toinfrastructureand vice versa)

Does notsupport datatransfer to tag

Range About 30 cm (forpassive tags) and90 cm (for activetags)

100 m 50 m 50–100 m

Tag costs Very cheap(less than 5$)

Relativelyexpensive(about 60$)

~10$ ~10$

738 A.H. Behzadan et al. / Automation in Construction 17 (2008) 737–748

2. Related work

2.1. Context aware computing

Context aware computing is defined as the use of environmentalcharacteristics such as the user's location, time, identity, profile andactivity to inform the computing device so that it may provide in-formation to the user that is relevant to the current context [6]. Contextaware computing enables a mobile application to leverage knowledgeabout various context parameters such aswho the user is, what the useris doing, where the user is and what mobile device (if any) the useris using. The application adapts services to the interpreted context,therebyensuring that the busyuser getshighly specific data and services[7]. Pashtan [8] describes four key partitions of context parameters,including user static context (i.e. user profile, user interests, and userpreferences), user dynamic context (i.e. user location, user current task,and proximity to other people or objects), network connectivity (i.e.network characteristics, mobile terminal capabilities, available band-width, and quality of service), and environmental context (i.e. time ofday, noise, andweather). These various context types can be tracked andused to filter the delivery of information and services tomobileworkersin a variety of industry sectors.

The use of context awareness for mobile users has been demon-strated in a large number of applications, including fieldwork [9,10],museums [11,12], route planning [13], libraries [14], and tourism[15,16]. Other projects that have specifically focused on location-baseddata delivery include the GUIDE project [17] and the Mobile ShadowProject (MSP) [18]. The MSP approach is based on the application ofagents to map the physical context to the virtual context. Contextaware applications are also being investigated in other fields of com-puter science research includingmobile computing,wearable comput-ing, augmented reality, ubiquitous computing, and human–computerinteraction.

2.2. Location tracking for context aware computing

A context aware data delivery system retrieves and displays data tothe users based on their latest position on a site and the level of detailthey request. As a result, position tracking is a crucial task in almost allapplications designed to obtain, maintain, and deliver context awareinformation on a continuous basis. Good examples of such informa-tion are material, labor, and equipment tracking on a site. Accurateand effective data delivery to the site personnel is the key for higherproductivity and faster service. Different tracking technologies withdifferent implementations and hardware installation requirementsare currently available in the market. Radio Frequency Identification(RFID) technology, Wireless Local Area Network (WLAN), GlobalPositioning System (GPS), and ZigBee are amongst the most commontracking technologies. Their application has been investigated indifferent fields and with different levels of detail and functionality.

For example, Ergen et al. [19] studied the use of RFID technology totrack the status of different facility components during operation andmaintenance for an extended period of time. They connected RFID tagsto a number of fire valves in a facility to conduct a longevity test forsixty consecutive days by simulating tag identification, data access, andentry in real life conditions. Song et al. [20,21] developed an RFID-basedmethod to automate the task of tracking, delivery, and receipt offabricated pipe spools in lay downyards and under shipping portals. Inanother study, Caldas et al. [22] investigated the use of GPS integratedwith a handheld device to track the position of pipe spools on lay downyards in order to improve the process and reduce the number of lostitems. Jang et al. [23] developed an Automated Material Trackingsystem (AMTRACK) based on ZigBee localization technology with twodifferent types of query and response pulses. They installed ZigBeerouters at different locations on a construction site to detect the eventsassociated with the movements of distributed sensors.

Although some of the above tracking techniques provide satisfactorylevel of accuracy, there are two major drawbacks that limit their use inwide range applications such as a real construction site. First, except forthe GPS-based tracking techniques all others are either dependent onpre-installed infrastructure (e.g. routers) or controlled environmentalconditions. Second, their data transmission capability is usually limitedto a small range compared to awidearea inwhich a large scale operation(e.g. a construction project) typically takes place [24].

3. Indoor tracking with WLAN-based positioning

3.1. Overview

There are various indoor location tracking technologies includingRFID, WLAN, Bluetooth and those based on a Dedicated Spectrum. Keyfeatures of these technologies are summarized in Table 1. From thisTable, it is apparent that there is a clear tradeoff between the accuracyand operational range expected from an indoor tracking technology andthe cost of the required components to achieve that level of accuracy. Forexample, while a relatively cheaper RFID-based system can only cover asub-meter range in terms of operational space and provide a lowaccuracy, application of a more accurate WLAN-based system whichcan work in a wider area (up to 100 m) requires a more expensiveinfrastructure setup. In addition, the decision to use a specific indoorpositioning techniquedepends, to a largedegree, on the interaction level(i.e. oneway or twoways) between the user and the system. If a onewaycommunication (i.e. from the user to the infrastructure) is desirable,RFID-based techniques can be applied whereas if a two way commu-nication is essential, WLAN-based and Bluetooth-based techniques be-come more attractive options to the system developer.

3.2. Technical approach

A context aware system using a WLAN-based positioning technol-ogy was developed and tested at Loughborough University (UK). A keyreason for choosing WLAN-based positioning technology was that it

739A.H. Behzadan et al. / Automation in Construction 17 (2008) 737–748

does not require additional infrastructure provided good accuracycould be achieved through existing WLAN deployment. Also, WLANprovided considerably higher bandwidth of up to 54 mbps enough tosupport data, voice communications and tracking features. In theimplementation, user context was used as a filtering mechanism todeliver relevant applications and services to users. The WLAN-basedtracking system from Ekahau [25] was used to track user location. Thissystem makes use of the signal strength to determine the actualposition of the target device, and then reports the tag coordinates aswell as area, direction, and speed within the WLAN coverage area. Itconsists of the following software components:

• A client component which is a small program that runs on a WLANenabled client device (PC laptop, PDAs, Wi-Fi Tag, etc.) and sendpositioning information to the server.

• The positioning engine that runs on a server and calculates the clientdevice location. It provides location coordinates and relevantinformation to other applications through a Java-based API.

• A manager application for recording the calibration data for a po-sitioning model, tracking client devices on a map, and analyzing thepositioning accuracy.

4. Outdoor tracking with GPS-based positioning

4.1. Overview

For indoor environments, the area in which the user moves is wellconfined. This situation which is often referred to as a preparedenvironment, simplifies the task of position tracking. An extensive listof popular indoor position trackingmethodswas previously presentedin Table 1. Additional approaches already studied by researchers in-clude mechanical, electromagnetic, optical, wireless signal andinfrared tracking. In a mechanical tracking system the user standsinside an articulated frame consisting of a number of mechanical armsall connected together. The user grabs the grippers by two hands andstartsmoving them to adjust the position of the display as s/he changesposition or direction of view. Based on the latest arm configuration (i.e.angles and lengths) the final position of the user is calculated. In anoptical tracking system, the position of an object (including the user) inthe scene is calculated based on the visibility and relative distancebetween a number of pre-installed optical markers on the site.

Using wireless signal tracking, the position of an object (includingthe user) on the site is calculated based on the absolute position of anumber of pre-installed benchmarks. Each benchmark is usuallyequipped with a signal transmitter that continuously sends signals tothe surrounding space. Objects are equipped with signal receivers andas they move on-site, their absolute position is calculated based onthe signal strength received from the benchmarks. Although all thesemethodsprovideacceptable level of accuracy, their application is limitedto relatively small environments. Thus far, significant research effort hasbeen devoted to improving performance, precision, robustness, andaffordability of these tracking methods. What makes most of thesemethods inappropriate for a construction site is the fact that they alldepend on pre-installed infrastructure (e.g. articulated frame, opticalmarkers, and signal transmitters). Construction projects usually takeplace in a dynamic environment where equipment and materials arebeingmoved, attached together, or taken apart, and the terrain itself canchange shape [26]. Such an environment does not present ideal con-ditions for the installationof trackingequipment, particularly if theworkis primarily outdoors and spans large areas. Hence, field personnel in anoutdoor unprepared environment such as a construction site must beprovided with a positioning system capable of being set up and runrapidly and without any dependence on pre-installed infrastructure.

GPS is an effective tracking tool on construction sites since asignificant amount of work takes place in outdoors environmentswhere there is a clear line of sight (LOS) to the sky [20–22,27,28]. The

requirement of having an adequate view of the sky is a vital part of aGPS-based tracking system since, for uninterrupted GPS data commu-nication betweenGPS satellites and the receivers, the data signals haveto be continuously received and interpretedwith an acceptable level ofaccuracy. In order for a GPS receiver to obtain accurate positioning data(i.e. longitude, latitude, and altitude), it has to be visible by a certainminimumnumber of GPS satellites orbiting the earth. This number andthe location of the satellites relative to the earth is a function of wherethe receiver is being used.


A GPS-based position tracking systemwas developed and success-fully tested at the University of Michigan [27,29]. GPS data signalsfollow certain data transmission standards, themost common of thembeing the National Marine Electronics Association (NMEA) standard.Under this standard, each sample of GPS data is encoded as a sentencestarting with a data type indicator which defines the interpretation ofthe rest of the sentence. Each data type indicator has its own uniqueinterpretation and is defined in the NMEA standard. Different sen-tenceswith different data type indicatorsmay repeat someof the sameinformation but will also supply new data. Depending on what dataelements are needed, a GPS-based positioning application can receiveand process appropriate data sentences and ignore other sentences.The sentence that contains all three pieces of positional data (i.e.longitude, latitude, and altitude) for a location starts with indicator$GPGGA. A set of string manipulation statements are used inside thetracking application to extract the necessary parts of a GPS sentenceand store the values in a usable format. A sample sentence startingwith this type of indicator as well as the algorithm used in thepresented research to obtain and extract theGPS data is shown in Fig.1.

5. Head orientation tracking

5.1. Overview

In all context aware information delivery applications developedthus far (both indoor and outdoor), the spatial context is defined solelyby the location (position) of the user. Another major attribute, the 3Dhead orientation, is ignored in the computations. As depicted in Fig. 2,a user's three dimensional head orientation is defined by three angles(yaw, pitch, and roll) and fully describes the user's line of sight (i.e. thedirection in which the user is looking).

Together with position, 3D orientation can define a user's spatialcontext with much greater precision than is possible with positionalone. For example, tracking only an engineer's position on aconstruction site might help determine which floor of a building theengineer is located on [30]. However, this information is not sufficientto conclude which part or section of the room, or what particularcomponent or object in that room the engineer is currently looking ator is interested in.

While the global position of the user is being tracked by a set ofhardware devices (e.g. WLAN, GPS) and designed software imple-mentation, head orientation data is an essential piece of informationthat also needs to be continuously tracked. Only by knowing both theexact position and head orientation, the context aware application candeliver precise information to the user. Head orientation is usuallycaptured using a 3D head orientation tracker connected directly abovethe user's head which continuously sends rotational values of thehead in the form of the three angles shown in Fig. 2.


From the different head tracking technologies available in themarket, magnetic head trackers are widely used since they are easilyapplicable to both indoor and outdoor applications. Fig. 3 illustrates

Fig. 2. Definition of yaw, pitch, and roll angles.

Fig. 1. Sample GPS sentence and data extraction algorithm.


the six degrees-of-freedom (DOF) parameters required by a GPS-basedcontext aware information delivery system to precisely locate a user inan outdoor environment.

In order to track the user's head orientation in a 3D space, a TCM5magnetic head orientation tracker was used in the presented research[29]. The orientation data coming through different head trackers followdifferent data transmission standards based on the brand and model ofthe tracker. In the presented work, a binary data transmission protocolwas used to obtain and extract the tracker data. Each data packet con-tains a Frame Type IDwhich describes the contents of the packet. Basedon this ID, the packet may contain each of the 3D rotational angles aswell as the current temperature. These values are stored in the packetPayload. Although the protocol takes advantage of fast data transmissiondue to the fact that all data is in binary format, it introducesmore risk ofdefective data due to signal interferences and unwanted environmentalnoise signals. As a result, a mechanism called Cyclic Redundancy Check(CRC) is used to distinguish between useful and corrupted binary datapackets. In general, a CRC is a mathematical transformation applied to aseries of bytes that produces an integer result that can be used for errordetection. Upon receiving data from the orientation tracker, the trackingapplication calculates the CRC value of the packet using its actual con-tents and then compares this value to the received CRC. If the two donotmatch, thepacket is concluded to be corrupted andno longer reliable foruse. Hence, the packet is ignored and the applicationwaits for the nextincoming data packet.

Otherwise, the data stream is error-free and will be extracted intoits components. A set of binary data manipulation statements areused inside the tracking application to convert, extract, and store thenecessary parts of a head orientation data packet. A sample head

orientation tracker binary data packet as well as the data extractionalgorithm used in this research is shown in Fig. 4.

6. Indoor and outdoor tracking systems integration rationale

A construction site typically consists of a relatively wide areaoccupied byworkers, rawmaterial, and equipment. Existing structures

Fig. 4. Sample head orientation tracker binary packet and data extraction algorithm.

Fig. 3. Six necessary DOF parameters for outdoor user tracking.


such as trailers, temporary on-site inventory, and emergency shelters,as well as natural obstacles such as water ponds, and treesmay also bepart of a construction site. Hence, depending on the nature of theoperation, site personnel may beworking in various indoor or outdoorlocations aswell as continuously changing their location. As a result, anubiquitous tracking systemwith reliable position and head orientationtracking capability should be able to continuously deliver contextualdata in both indoor and outdoor environments. For example, an en-gineer who is in charge of monitoring the inventory level of a certaintype of raw material, and consequently controlling how it is beinghandled and delivered to the installation area may periodically walkindoors and outdoors. As a result, the corresponding position trackingsystem should be capable of identifying the nature of the environment(i.e. indoors or outdoors) the engineer is located in, and accordinglyswitching to the appropriate tracking method in order to immediatelyprovide positional information to the system. Fig. 5 shows an exampleof a timber construction project in which the engineer has to controlthe inventory level, monitor the actual construction operation on-site,and conduct the final inspection when the work is complete.

At each stage and depending on where the engineer is located,appropriate contextual data has to be presented through the mobilecomputer. For example, when the engineer is inside the inventoryroom, there may be a need to know the exact location of certainwoodsections. As the engineer walks outside to monitor the actual progressof the work, the data delivery system may be asked to displayinformation about the crew and equipment, and how they should bestaged on the site. At the final inspection stage, the engineer mayrequire detailed information about a wood connection such as thenumber and type of joints, and connection angles so that it can becompared to what has been specified in the drawings and what hasbeen actually done at the jobsite.

In each step, the environment the engineer is located in is different.In the particular example of Fig. 5, the engineer starts the job from anindoor material inventory, moves outdoor to monitor the actualoperations, and again walks inside the completed structure to do thefinal inspection. Other examples are excavation and tunnel boringoperations in which the workers and material are in constant tran-sition between indoor and outdoor environments. As discussedearlier, GPS-based tracking systems are only functional where thereis a clear line of sight to the sky (i.e. outdoor conditions) while indoorpositioning techniques can be effectively used in environments thatare deployed with tracking infrastructure(e.g. optical markers, wire-

less routers, etc.). As a result, an integrated hybrid tracking systemequipped with both GPS and indoor positioning system hardware is apromising approach to the problem.

7. Example applications

7.1. Indoor applications

The implementation of the indoor position tracking technologyintroduced in Section 3was conducted on a simulated construction site.Four logical areas were defined within the simulated construction site,including a site office, a site warehouse, a walking track, and site opera-tions area. The positioning engine was first calibrated. This involvedwalking around a particular point on the floormap and recording signalstrengths for the point. Measurements were taken at every two steps. Asimilar procedure was repeated for a number of points. When all thepoints were recorded, the calibration data was stored in the positioningmodel. The positioning engine compared the measurements madeduring runtimewith those stored in the positioningmodel to determinethe real timeposition of the user. The object's locationwas updated aftera fixed time interval. Once the location was calculated the positionwasshown on the map. Fig. 6 shows the tracking of a notebook (local-host)and a WLAN tag (IP Address 192.168.1.101). During trials conducted in astationary office environment an accuracy of up to 1 mwas achieved in90% of the total 44 readings taken. However, in the trials conducted inthe actual construction site, themeasurements were not stable enough.As WLAN-based tracking is based on calibration and signal strengthmeasurement, any change in site conditions because of the ongoingwork (such as changes in soil, structure, plant and equipment, site-layout, etc), will require calibration after regular intervals to maintainhigh level of accuracy. Such regular calibration requirements may makethe system difficult to manage. During trials WLAN-based trackingsystem has shown its capability for location determination in indoor

Fig. 5. Sample contextual data delivery cycle in a timber construction project.


stationary environments but further development is needed before it ispossible to use it in dynamic construction site environments.

A key advantage of using aWLAN-based positioning engine is thatit has considerably less infrastructure requirements, compared toother real time indoor location tracking systems. This makes itaffordable for deployment in a site environment. Also, it is appro-

Fig. 6. Tracking a notebook d

priate for use both during the construction process and within theconstructed facilities.

Fig. 7 shows the overall deployment architecturewhich is based onthree tiers, context capture (captures user location and context), con-text-inference (reasons about the captured context) and context-in-tegration (retrieves information based on the captured location and

evice and a WLAN tag.

Fig. 7. Overall deployment architecture.


context). User context was drawn from indoor location trackingsystem and other sources. Examples include obtaining user profilethrough the unique IP address of user device and log-in information,user device type through RDF-based Capability/Preference Profile (CC/PP) [31], user activity through integration with the project manage-ment application, and time through computer clock. Based on the userprofile, a set of services were pushed to the user device. Context-inference tier provided the ability to reason about the capturedcontext using a Semantic-Web based model to describe a knowledgemodel for a corresponding context domain, thereby helping con-text description and knowledge access (by supporting informationretrieval, extraction and processing) based on the inferred context.The understanding of semantics (i.e. meanings of data) enables thecreation of a relationship between the context parameters and avail-able data and services. Output from the context-inference tier ispassed into applications to make them aware of events on the site.The context adapter converts the captured context (e.g. user id, user

Fig. 8. View of team

location, time, etc.) into semantic associations. Different levels ofsemantic mapping included:

• User profile to project data:Mapping of data, based on the role of theuser on-site;

• Location to project data: Mapping user location to project data (e.g.if an electrician is on floor 3, the user probably requires floor 3drawings and services);

• User task to project data: Mapping the task the user is currentlyinvolved in order to provide appropriate data.

The project database acted as a shared repository for all projectrelated data (e.g. project documents and drawings) which could beaccessed by all project partners. Semantic annotation using ontologyas shown in Fig. 8 were developed for all project documents anddrawings, and were used to develop the project repository. Theseannotations facilitated indexing and searching. It also enabledimproved ways of information submission and retrieval, by describing

profile ontology.

Fig. 9. Delivering information to users based on their context.


resources, and links between them. Such semantic description enabledsynthesis of content frommultiple information sources based on usercontext.

Based on the captured context, context-integration tier helps inservice discovery and integration. Changes in the context prompt thecontext broker to trigger the preprogrammed eventswhichmay includepushing certain information to users or an exchange of informationwith other applications using Web Services, to make them aware of theevents on the site.Web-services standards are used to allowapplicationsto share context data and dynamically invoke the capabilities of otherapplications in a remote collaboration environment. As the user context(i.e. location, task) changes, services available to users are calculated inreal time. Context information is then used to support both pull (e.g. anexchange of information including project documents with the backend

Fig. 10. An illustrative snapshot

system) and push-based (e.g. health and safety warnings) informationdelivery. Fig. 9 shows an example of how the information delivered to auser changes based on the user's context.

7.2. Outdoor applications

One of the emerging fields which uses position tracking as animportant building block is interactive Augmented Reality (AR) vi-sualization of simulated construction processes. AR is a technique inwhich computer generated data (i.e. graphics, text, diagrams, etc.) areembedded into the views of the real environment. Compared to VirtualReality (VR) in which the synthetic world completely generated bycomputer plays the dominant role, AR takes advantage of the existingelements of the surrounding 3D space as the real background. This

of an AR-based animation.

Fig. 12. Screenshots of a sample outdoor case study in ARVISCOPE.

Fig. 11. Profile of an AR system user equipped with position tracking devices.


reduces the amount of time and effort required for modeling andrendering the graphical contents of the scene. At the same time, thefact that a mixed environment consisting of both real and virtualobjects ismuchmore difficult to compose compared to a fully syntheticworld introduces new challenges into AR-based visualizationapplications.

A well designed and fully functional AR-based application mustcreate and maintain both spatial and contextual links betweensuperimposed computer generated data and the views of the realworld as observed by the user. This link is often called registration.Accurate and consistent registration between computer generatedinformation and the real world as seen by the user is one of the mostimportant challenges in AR. For example, if the AR system is designedin a way that it allows the user to walk in the augmented world withlittle physical constraints, then it should be capable to continuouslyobtain the user's position and orientation within the environment inorder to display appropriate superimposed data in real time [27,29].The precise, fast, and robust tracking of the user as well as virtual andreal objects in the scene is a critical task for creating convincing ARapplications.

As discussed in Section 4, GPS-based tracking is an attractiveoption to track a user's outdoor position because it does not rely onany pre-installed infrastructure and instead depends on direct satellitecommunication. Thus it can be set up and used immediately in almostany outdoor location. Fig. 10 is an illustrative AR scene in whichcomputer generated models of construction equipment are super-imposed on top of live scenes of an airport construction site.

As mentioned earlier, there are several ways to track the position ofthe virtual and real objects in anARenvironment. For the specific case ofFig. 10, the position of the aircraft (i.e. real object) can be conceptuallyobtained by reading real time positional data coming through the GPSdevice of the aircraft. Theposition of the virtual excavator and trucks canbe obtained by keeping track of their initial position (i.e. the positionthey were assigned right before they were displayed on the screen) andtheir physicalmovement history in the scene. Vector calculations can be

performed using this data to compute the updated position of eachvirtual object. Obtaining real time positional data of the user is anothervital part of the calculations. This can be done using either outside–in orinside–out approaches. In brief, anoutside–in tracking approach iswhenthe user's motions are tracked by means of a number of pre-installedsensors on the site whereas an inside–out tracking approach usessensors connected to the moving user (e.g. a GPS device) to obtainpositional data in real time. Depending on the availability of the pre-installed infrastructure and the range over which the objects and theuserare expected tomove in the scene, eitherof these approaches canbeadapted and used for tracking purposes. AR has been widely used inseveral engineering as well as non engineering fields ranging frommedical and automotive to military and gaming. However, the ap-plication of AR in construction has been limited to a few numberof systems developed by researchers for very specific purposes. To the


authors' best knowledge, a general purpose AR-based animation tool inconstruction with full user interactivity has not been developed andimplemented. This has led the authors to apply the concepts of positiontracking in an AR-based visualization application which is capable ofanimating simulated construction operations.

The key factor in developing such a system was to obtain a user'sposition and 3D head orientation to construct the augmented viewingfrustum, andupdate its contents in real time. CADobjects are displayedon top of the real background through a HeadMounted Display (HMD)if and only if they are visible to the user based on the current locationand line of sight. A preliminary prototype called UM-AR-GPS-ROVERwas implemented and successfully tested [29]. The results were verypromising and confirmed the ability of GPS-based user tracking toanimate simulated tasks in outdoor environments. The design andimplementation of ARVISCOPE, a more advanced application, is alsoin progress at the University of Michigan [32]. ARVISCOPE is capableof creating real time dynamic augmented scenes of constructionoperations while allowing the user to walk freely on the site withminimum constraints and look at the animation from different per-spectives. It continuously updates the contents of the augmented viewbased on the latest position and orientation data obtained through theGPS receiver and the 3D orientation tracker. Fig.11 shows the profile ofa user in ARVISCOPE equippedwith trackers and themobile computingbackpack.

To validate and verify the capability and reliability of the GPS-based tracking system, the established data communication and ex-traction methods were integrated into ARVISCOPE. The user of thesystem wears an AR backpack which is equipped with a GPS receiver.A hard hat is also provided to the user inside which a 3D headorientation tracker is installed and secured. Other hardware devicesused in ARVISCOPE include a video camera to capture live scenes ofthe construction site, a HMD to view the final augmented display, anda miniature keyboard together with a touch pad for user input. Alaptop computer inside the backpack is the main computation centerof the system. As the animation runs, the user can walk freely on thesite and change head orientation to view the augmented animationfrom different perspectives and locations. A major part of theapplication is a module responsible for tracking the user's movementsin the 3D outdoor space, and using the tracking information to updateand modify what the user sees inside the HMD accordingly. The heart

Fig. 13. Navigation guidan

of this module is the implementation of the algorithms discussed inSections 4 and 5. At each frame, the user's real time global positionand head orientation angles are obtained, extracted, and stored toconstruct an updated augmented viewing frustum in front of theuser's eye.

Several case studies were conducted to test the validity of thepresented tracking algorithms. Each case study consisted of a certainoutdoor construction operation such as steel erection, excavation, andconcrete delivery on a body of water. Fig. 12 shows snapshots of anoutdoor test visualizing a steel erection operation. In each case studyconducted using ARVISCOPE, the user was allowed to walk freely inthe animation and view the animated scenes from different anglesand locations.

The delivered graphical data at each frame was completely afunction of the user 6 DOF spatial context tracked in real time and as aresult, the accuracy level achieved by both the GPS and 3D headorientation tracker was a major concern in conducting these tests. Forproof-of-concept validation tests, free publicly available GPS signalswere usedwhich provided the applicationwith a sub-meter positionalaccuracy. However, the algorithms and methods developed in thisresearch are generic enough that other types of GPS signals withhigher data accuracy can be deployed without any need to modify orchange the existing positional data extraction methods. The headorientation tracking data accuracy was also very high (much less thanone degree in all three directions) which is completely acceptable forthe type of applications introduced in the presented work.

7.3. Potential applications of hybrid location tracking

7.3.1. Micro and macro level services delivery to program managersManagers of construction projects spend a considerable amount of

their time generating, managing, sending, collecting, and analyzingproject data. It is, therefore, imperative that only the most relevantinformation ismade available to a projectmanager at anygiven time soas to reduce information overload. Context aware information andservice delivery provides a mechanism to do this by filtering data,information and services based on the construction program man-ager's current context.

It is important that the filtering mechanism takes into accountboth macro and micro perspective in delivering information and

ce in first response.


services as the project manager travels between different jobsites. Themacro level relates to the project management level, which covers awide geographical area (e.g. national or regional territory) and is thelevel at which the project manager is more interested in a portfolio ofprojects than individual projects. At the micro level, the projectmanager is only interested in the local site that is being visited. Fathi etal. [33] briefly described the enabling technologies for such scenarios.The integration of outdoor (GPS-based) and indoor (WLAN-based)tracking is seen as an effective means of providing context awaresupport to project managers. GPS-based tracking could be used totrack the project manager's position at the macro level (as he or shetravels between sites), with control being transferred to a WLAN-based tracking system when the manager arrives at a given projectsite.

7.3.2. Emergency response and managementEmergency response and management is a critical field of research

since the inability to identify and access relevant information is theprimary obstacle that prevents rapid and optimal decision-making byemergency responders (e.g. firefighters, civil engineers) who respondto natural and manmade disasters [34]. A potential application ofubiquitous hybrid position tracking currently being investigated atthe University of Michigan is the development of a self-contained,location aware technology that can automatically provide engineersand first responders with accurate, prioritized contextual informa-tion for making critical, real time decisions in chaotic, post-disasterenvironments.

For such an application, a mobile user context-sensing frameworkcan be developed that will accurately track an engineer's or firstresponder's 3D spatial context in any indoor and/or outdoor environ-mentwithout relying on a pre-installed network of sensors or trackers.Specific information of an engineer's or first responder's interest at agiven time can then be retrieved by interpreting spatial context with alevel of precision sufficiently high to accurately prioritize identifiedcontextual data. An example of such information is a 3D plan of abuilding onfire inwhich the exact locations of emergencyexits andfireextinguishers are marked. If this plan is superimposed on top of whatthe firefighter is observing at the location, the augmented view can bevery helpful in quick navigation through the building,finding potentialvictims, and bringing them out of the fire zone. Fig. 13 shows anexample of contextual data displayed to a first responder during a fireemergency scenario.

8. Discussion and conclusions

Awareness of the user context (such as user profile, role, pref-erences, task, location, and existing project conditions) can enhancethe construction project delivery process by providing amechanism todetermine information relevant to a particular context. Context awareinformation and services delivery offers the following benefits [27]:

• Delivery of relevant data based on the worker's context therebyeliminatingdistractions related to thevolumeand level of information.

• Reduction in user interaction with the system by using context as afiltering mechanism. This has potential to increase usability bymaking mobile devices more responsive to user needs.

Context awareness, through improved sensing and monitoring ofa user's context parameters, can also be used to improve security,logistics and health and safety practices on construction site. A suc-cessful and reliable ubiquitous tracking system with guaranteedtracking capability should be able to track a user's position and deliverposition-based contextual data continuously in both indoor andoutdoor situations. In this paper, the potential and requirements of anintegrated tracking system was introduced which uses WLAN forindoor and GPS for outdoor tracking purposes. A major advantageof such a system is the fact that the tracking application can auto-

matically switch from one technique to the other based on the jobsiteconfigurations. In particular, if the tracking device has clear sight tothe sky, it can conveniently use GPS signals to track a user. If thesesignals seem to fade as the user walks indoor the application im-mediately switches to the WLAN signals in order to avoid any inter-ruption in obtaining the user's position in real time.

There are a number of technical challenges in developing such anintegrated tracking system. These are currently being explored withinthe research projects highlighted in this paper and include but arenot limited to identification of the most appropriate contexts to betracked, seamless transition between outdoor and indoor tracking,determination of the most appropriate transition point, and improv-ing the accuracy of both positioning systems.

Acknowledgments

The presentedwork has been partially supported by the US NationalScience Foundation (NSF) through grant CMS-0448762, and by theUK Engineering and Physical Sciences Research Council (EPSRC). Theauthors gratefully acknowledge the supports of NSF and EPSRC. Anyopinions, findings, conclusions, and recommendations expressed in thispaper are those of the authors and donot necessarily reflect the views ofthe NSF or EPSRC.

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