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CHAPTER 2

LITERATURE REVIEW

________________________________________________________________________

In the era of virtual reality, physical models of the earth’s terrain may seem irrelevant.

However, physical models’ capability to represent third dimension in “easy to

understand” manner, suitability for mass-visualization and ability to provide haptic

experience to its viewer make them very appealing and more relevant today. This chapter

presents a critical review of the existing literature in the field of terrain modeling and

related areas. The chapter starts with a review of physical terrain modeling techniques. It

also discusses benefits of the three-dimensional models over two-dimensional maps.

There is a vast range of the data formats related to Geographical Information Systems

(GIS), Computer-Aided Design (CAD) and Additive Manufacturing (AM) available in the

market. The chapter contains a brief discussion on relevant data formats. Many

researchers have used AM technology for fabrication of physical scale terrain models

using digital data of the terrain. The chapter presents a dedicated section reviewing the

applications of AM technologies for fabrication of physical scale terrain models. The next

section explores the application of AM in the arena of architecture and construction. Then

the conclusions from the literature review are drawn. The scope and objectives of this

thesis work are presented at the end of the chapter.

2.1 Physical Modeling of Terrains

The tradition of building physical models is very old. Physical modeling of the

architectural structures is very popular among architects since last several years. The

military has also been making use of physical models of terrains for strategic and battle

field planning since past couple of centuries [Faulkner (2006), Terrain Models (2014)].

Historically, the first primitive physical terrain model of the Eastern Alps was created

during 1500–1540. The Pope Clemens VII used a cork model of Florence created by

Benvenuto di Lorenzo della Volpaia and Niccolò Tribolo to plan his siege of 1529-30.

The city of Venice was the first centre of relief modeling in the middle of the 16th century

[Terrain Models (2014)]. Initially, physical modeling of the terrain did not receive much

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attention by the cartographers because of limited availability of the spatial information,

time consuming and expensive manual methods and limited portability of the bulky

models [Faulkner (2006)]. Later on, with the development of simple and better model-

making techniques by different model-makers, physical terrain modeling became popular

among architects, cartographers and land developers.

Two-dimensional cartographic maps are another popular tool among the cartographers to

represent spatial information. However, the physical models often prove advantageous

over two dimensional maps because the information in third dimension is not lost. In

cartography, the third dimension is represented by color, shading, texture, size, shape, etc.

The users of the maps may find difficulty in understanding visual variables without a

proper training and imagination. This is especially true for representation of mountainous

regions with steep slopes and rocky surfaces in conventional maps. The paintings or

photographs of the same are only able to provide the view from a single point. A physical

terrain model can dynamically depict the third dimension and can be easily understood by

common people with no formal training. The physical models have the advantage over

other two-dimensional drawings that slight movements of the head or body suffice to look

at parts of the model covered by obstacles in the line of sight [Rase (2002), Jacobs

(2003)]. Chua et al. (1998) carried some case studies and recognized the need for physical

models for ergonomic and tactile evaluations. A physical terrain model provides better

information flow and more effective communication of the design intent [LGM (2014)].

In recent years, computer monitor based virtual visualization techniques such as

photorealistic images over stereoscopic views or flights over virtual landscape have begun

to be used in architecture and spatial planning. The stereoscopic display makes use of the

ability of the human brain to construct a 3D mental model from two slightly different

images, one corresponding to each eye [Rase (2009)]. Ghawana and Zlatanova (2013)

presented a comparison of three-dimensional digital visualization with physical 3D

printed models on the basis of different criteria such as visual perception, possibilities to

use scales and resolutions, suitability for a large group, ease of understanding and

analysis, etc. and concluded that 3D printed models can provide enhanced physical

visualization and prove to be very useful in urban planning. Clark et al. (1998) proposed

an innovative approach by combining physical model and computer-based data to

enhance the visual experience. At first, they fabricated a physical model of the Prince of

Wales Island, Alaska using USGS DEM data. Then they used this model to make a mold.

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A translucent model is made using this mold. Now, deforestation data from the island was

color mapped and rear-projected onto the translucent model within a light box.

Despite the several advantages of virtual reality technology, it is remarkable that physical

models are still important and needed for effective communication of spatial information

in architecture, land planning and other areas. The physical models are unlikely to replace

cartographic 2D maps as well as virtual reality techniques [Rase (2002), Ryder et al.

(2002), Faulkner (2006), Rase (2009), Ghawana and Zlatanova (2013)]. However,

physical models prove a better choice in several key applications. Virtual terrain models

are usually meant for a single user and prove very expensive for mass-visualization. The

immersive environment in virtual reality may lead the user to a feeling of separation from

the real world and nausea on the part of the user [Mitasova et al. (2006)]. The virtual

reality equipments are also not easily transportable. On the other hand, physical models

are suitable for mass visualization and a group of people can simultaneously grasps the

spatial information at no added cost. The estimation of distance and height within the

model is easier with a physical model [Rase (2009)]. The architects and land planners

have inclination towards the physical models not only because they provide haptic

experience to their customers but also the cost of physical model is marginal in

comparison to cost of losing in competition or even cost of real building [Rase (2002),

Ryder et al. (2002), Rase (2009), Caldwell (2013)]. An accurate physical 3D model of a

terrain may help reduce costs and misunderstanding and shorten the project completion

time in major projects. A saving as high as 3% to 8% of total cost of the project can be

achieved by the use of physical 3D terrain models [True3D (2014)]. The physical models

also prove very handy in comparing and visualizing the change in terrain of a mountain

before and after any natural calamity like landslide or volcanic eruption [White (2014)].

Traditionally, a number of techniques discussed in Section 1.1.2 of Chapter 1 have been

used by military and other users to build physical models of the earth’s terrain. The most

manual methods were rudimentary and used nails, wood, plywood sheets, cardboard etc.

to make a model of the terrain. These techniques used either information obtained from

the contour maps or measured values of elevation at several points on the terrain. These

methods were unable to produce very fine details on models, hence cannot produce good

quality portable models. Besides, they were very time consuming and not suitable for

making very accurate scale models of the terrain [Terrain Models (2014)]. The

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Pantograph technique was a quick and cheap method to build a physical terrain model. It

employed a pantograph to guide a milling cutter to cut contours on a block of plaster

using a contour map [Pearson (2002)]. This technique produces a fairly accurate model of

the terrain but fails to cut an undercut in the physical model of the terrain [Mueller

(2004)]. A Pantograph is also used to enlarge the size of the sculptures by the Sculptors

[Keropian Sculpture (2014)]. Vacuum forming method makes use of a vinyl plastic sheet

to make a model. The models made by this technique have good aesthetic appeal but are

not very durable and robust [Terrain Models (2014)]. Meanwhile, in the middle of 20th

century with the advent of computers, computer-aided subtractive manufacturing

techniques began to be used to create physical scale terrain models with the help of a

digital elevation models [Noma and Misulia (1959), Kershner (2005), Aitcheson et al.

(2005), Sun et al. (2008), Terrain Models (2014)]. The range of materials for making

models became wider with CNC techniques. Though complex free-form surfaces were

difficult to cut, these techniques were relatively faster and more accurate in comparison

with their traditional counterparts. However, even these techniques were unable to

produce an undercut in a terrain model [Mueller (2004)]. In 1980s, rapid prototyping

technology now known as AM attracted the attention of model makers because of its

unique advantages over its subtractive counterparts. According to Sheerin (2003), AM can

make physical models of terrains very accurately in relatively less time and cost. These

technologies are a natural fit for three dimensional topographic or terrain modeling

[RapidToday (2014)]. Today, AM has become a preferred choice of the architects and

model makers for fabricating accurate scaled down physical models of architectural

structures and terrains on the earth [Kershner (2005), Terrain Models (2014)].

The literature review discussed above reveals that a physical model of the terrain is still

relevant and needed in the era of virtual reality. Traditional methods of model making are

becoming obsolete due to their limitations in terms of speed, accuracy and ability to

produce complex freeform and undercut surfaces. Besides, physical models made by these

techniques lacks in aesthetic appeal, robustness, portability and making fine details neatly.

Additive manufacturing overcomes the above limitations of traditional methods and offers

an excellent alternative to fabricate physical models of the terrain.

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2.2 Data Formats for GIS, DEM, CAD and AM Applications

The geographic information systems, computer-aided design and additive manufacturing

data are available in several proprietary (or native) as well as open (or neutral) formats in

the market. The proprietary file formats are native to a particular computer program, e.g.

PRT (Pro/ENGINEER part file) format is native to Pro/ENGINEER and not recognized

by any other program. On the other hand, open file formats are neutral data formats that

allow the digital exchange of information among different computer programs, e.g. Initial

Graphics Exchange Specification (IGES) format.

2.2.1 GIS Data Formats

The GIS data represents spatial data and are available in several formats. The formats are

primarily classified into two categories:

(1) Raster data formats

(2) Vector data formats

The common raster and vector GIS file formats are listed in Table 1.1 and Table 1.2 in

Chapter 1. In the case of raster formats, the geographic location of each cell is implied by

its position in the cell matrix. Accordingly, other than the origin point, e.g. bottom left

corner, no geographic coordinates are stored. However, in vector formats the location of

each vertex needs to be stored explicitly. In raster formats, the cell size determines the

resolution of the data being represented; whereas, in vector formats data can be

represented at its original resolution and form without generalization. Linear features are

difficult to represent and the graphic output depends on the cell resolution in the case of

raster formats. However, the graphic output is usually aesthetically more pleasing

(traditional cartographic representation) in the case of vector formats. Most available

input data, e.g. hard copy maps are in vector form, so data must undergo vector-to-raster

conversion. In case of vector formats no such conversion is required. Due to the nature of

the data storage technique, the data analysis in raster formats is usually easy to program

and quick to perform. The inherent nature of raster maps, e.g. one attribute maps, is

ideally suited for mathematical modeling and quantitative analysis. However, for effective

analysis, the vector data must be converted into a topological structure. This is often

processing intensive and usually requires extensive data cleaning. Besides, algorithms for

manipulative and analysis functions are also complex and processing intensive. Usually

substantial data generalization or interpolation is required for these data layers. Grid-cell

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systems are very compatible with raster-based output devices, e.g. electrostatic plotters

and graphic terminals. Processing of associated attribute data may be cumbersome in

raster formats, if large amounts of data exist [Aronoff (1989), Davis (2001), Ian et al.

(2010), Lloyd (2010), GIS resources (2014)].

2.2.2 Digital Elevation Model (DEM) File Format

Digital Terrain Modeling, being an important utility of GIS, provides an opportunity to

model, analyze and display phenomena related to topography or other surfaces. The GIS

elevation data are usually known as digital elevation model (DEM), digital terrain model

(DTM) and digital surface model (DSM) in scientific literature [Sharma et al. (2006),

Gomarasca (2009), Shippert (2011), Intermap (2014)]. However there are slight

differences in the meanings of the terms as explained below.

The term DSM represents the earth's surface including vegetation and cultural features

such as buildings. The term DTM is a bare-earth model that contains elevations of natural

terrain features. It does not include elevations of vegetation and cultural features, such as

buildings and roads. The term DEM in geometrical terms, is equivalent to DTM. It can be

defined as a regular grid of elevation data organized in raster format [Collins and Moon

(1981), Li et al. (2004), Sharma et al. (2006), National Research Council (2007),

Gomarasca (2009)].

According to US Geological Survey (USGS), “A DEM is the digital cartographic

representation of the elevation of the terrain at regularly spaced intervals in x and y

directions, using z-values referenced to a common vertical datum” [Sharma et al. (2006)].

It is worth noting that the term "DEM" can refer either to a specific elevation file format

or to gridded elevation data in general [Sharma et al. (2006), VTerrain (2014)].

Modeling terrain relief via DEM is a powerful tool in GIS analysis and visualization. The

DEM data can be stored and arranged in three possible structures in GIS database as

shown in Figure 2.1 [Moore et al. (1991), Weibel and Heller (1991)]:

1. Grid based network - a regularly-spaced square or rectangular mesh or a regular

angular grid as in 3 arc-second grid

2. Triangular Irregular Network (TIN) - irregularly spaced set of points connected as

triangles, and

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3. Contour-based network

(1) (2) (3)

Figure 2.1: Three data structures of DEM (1) Grid (2) TIN and (3) Contour based

The choice of the structure largely depends on data availability, nature of the surface,

scale and resolution of the data, application techniques used to analyze and manipulate

model, etc. However, the most widely used data structures consist of square-grid networks

because of their ease of computer implementation and computational efficiency [Collins

and Moon (1981)]. Some popular DEM grid file formats are given below [Blue Marbles

(2014), Wikipedia (2014a)]:

DEM ASCII XYZ

ARC/INFO ASCII Grid

USGS Digital Elevation Model (DEM)

Surfer Grid (ASCII and Binary) Format Files

Spatial Data Transfer Standard Format (SDTS)

Global 30-arc-second Digital Elevation Data (GTOPO30)

Digital Terrain Elevation Data (DTED)

Shuttle Radar Topography Mission (SRTM)

Global Land 1-km Base Elevation (GLOBE)

2.2.3 Data Formats Relevant to CAD and AM

The CAD model represents the geometry of any part very precisely and accurately. Some

popular schemes to create precise solid CAD models are Constructive Solid Geometry

(CSG), Boundary representation (B-rep) and Sweeping. The B-rep technique uses a

combination of precise geometry and boundary topology to represent objects such as

solids, surfaces and wires. On the other hand, the CSG uses the Boolean operations of a

set of primitives such as boxes, cones, spheres, etc. to make a CAD model. The B-rep

technique is considered better than CSG for free-form modeling. It is because in the B-rep

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the faces and edges are defined by the equations of the precise curves, whereas in CSG

there is a natural limit to create free-form features by Boolean operations of primitives

[Zeid (1991), Kamrani and Nasr (2010)].

The graphics processor in a computer cannot interpret precise B-rep model. Therefore, to

visualize the model on the screen every precise CAD model is converted to an

approximate simplified graphical representation, usually called a tessellated model. These

“graphics”, “tessellated” or “mesh” models are generated on-the-fly every time we

modify our design, and most CAD systems store the graphics in their files along with the

precise data. These tessellated models are created by approximating the surface of the

precise model with polygonal facets [Jadhav (2013), Courter (2013)].

Additive Manufacturing (AM) can make physical models of terrains very accurately in

relatively less time and cost [Sheerin (2003)]. According to Chua et al. (2010) savings in

time and cost in the fabrication of a model could range from 50% to 90%. Initially, AM

technology was capable of producing parts in single color only. Many researchers tried

fabricating colored parts using processes such as Laminated Object Manufacturing

(LOM) and StereoLithography (SL) [Ming and Gibson (1999)]. However, Z

Corporation’s 3D printer Z406 based on ink-jet printing technology is known to be the

first full- color printer commercially available in the market [Gibson and Ming (2001),

Dimitrov et al. (2006), Wang and Leng (2007), 3D Systems (2014)]. Current models

ZPrinter 450 and ZPrinter 650 can print parts in full, 24-bit color.

The STL, PLY, VRML, DXF, 3DS, OBJ, MSH are the popular tessellated file formats

available in the market. However, STL (Standard Triangulation Language or

STereoLithography) is the de facto geometric information transfer standard for additive

manufacturing and often called a “bucket of triangles” [Jurrens (1993), Cao and

Miyamoto (2003), Bailey (2005), Chowdary et al. (2007), Udroiu and Nedelcu (2011]. It

consists of connected, three-dimensional triangles representing the part shape. The

vertices of the triangles are ordered to indicate which side of the triangle contains the part

mass [Wah (1999), Nagy and Matyasi (2003)]. From a mesh generation point of view,

triangular meshes are easy to generate than quadrilateral meshes. This is because a

triangle is a simplex while a quadrilateral is not [Zeid (1991)]. An STL file can be written

either in an ASCII or in a binary format [Chua et al. (1998), Zhao and Luc (2000)]. An

STL file does not contain color information.

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However, some researchers attempted to incorporate color information in the STL file

format [Gibson and Ming (2001), Nagy and Matyasi (2003)]. Wu and Cheng (2006)

proposed an enhanced version of STL format, wherein additional geometric feature codes

were proposed to define a tetrahedron based on triangular facets for better surface

approximation. They have also included attributes to incorporate color information. The

Center for Visualization of Prototypes (CVP) at Oregon State University also added its

own extension to STL file to incorporate color information [Bailey (2005)]. The PLY

format also approximates the surface by polygonal facets and was originally developed

for storing data from 3D scanners. Virtual Reality Modeling Language (VRML) is an

ISO/IEC language developed for describing 3D scenes on the web. The documents have

WRL extensions and can be viewed by a web browser with an appropriate plug-in

application. WRL is the popular VRML file format for interactive scenes, and refers to

“world”. VRL is the executable file of VRML. Some researchers have used VRML and

PLY file formats for producing color models [Gibson and Ming (2001), Ming and Gibson

(2002), Bailey (2005), Wang et al. (2005), Rase (2009), Franic et al. (2009)]. However, as

the terrain data such as DEM contain no information other than elevation values; the

standard STL format can very well be used to store the information contained in the

DEM. Additive Manufacturing File format (AMF) is an open standard for describing

objects for additive manufacturing processes [ASTM (2013)]. It is an XML-based format

designed by ISO/ASTM to allow any CAD software to describe the shape and

composition of any 3D object to be fabricated by any AM technology. The AMF format

has native support for color, materials, lattices, and constellations [Wikipedia (2014b)].

2.3 Terrain Modeling by Additive Manufacturing

In the beginning of the 21st century, AM technology became popular among the

researchers [Rase (2002), Jacobs (2003), Mueller (2004), Agrawal et al. (2006), Bechwar

and McGreen (2009)] to fabricate physical models of terrain and architectural objects

using GIS data. Rase (2002) fabricated physical models of 3D choropleth maps as shown

in Figure 2.1 using Z Corporation’s 3D color printer Z406. He wrote his own software

package to obtain VRML files of the 3D maps from the GIS data files. He discussed the

VRML format in brief. However, he has not mentioned the GIS file format used. Jacobs

(2003) created a terrain model using GIS data with four different programs. She first

converted SDTS format into DEM format using a GIS data converter. Then she loaded

DEM data into Land Desktop as shown in Figure 2.2.

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Figure 2.2: 3D choropleth map representing inhabitants and population density

The conversion from SDTS to DEM was necessary because Land Desktop can interpret

the DEM format only. Now the vertical and horizontal scales were adjusted to exaggerate

the vertical scale, so that the changes in elevation are visible. The file was saved into DXF

format. Now the DXF file was open into AutoCAD for further editing, e.g. taking off the

rough edges, making it rectangular and breaking the terrain into small pieces, if necessary.

Once all the editing is

Figure 2.3: 7.5-Minute map in Land Desktop

completed it was again saves into DXF format. Now the DXF file is imported into RP

Magics, where it was further rescaled. The actual and rescaled terrains are shown in

Figure 2.3 and 2.4 respectively. The physical model was then fabricated on two AM

machines, viz. Z Corporation’s Z406 system and Laminated Object Manufacturing.

Figure 2.4: Terrain in RP Magics Figure 2.5: Scaled Terrain

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Now the terrain surface was given some thickness by offsetting itself by 1/8 inch. For

making a model its bottom surface required to be made even, that was done by extruding

and cutting the extruded portion through a datum plane. This gave the terrain a flat bottom

so that it could rest on a surface. Finally model was rescaled, fixed and saved in the STL

format.

Figure 2.6: Terrain with thickness Figure 2.7: Terrain with an even base

Mueller (2004) presented a software framework for the creation of three-dimensional

physical models of terrain data using AM machines. He used SDTS DEM and DLG-3

formats as input data and used Greedy insertion, a meshing algorithm to get a mesh from

an elevation grid. The surface mesh was then converted into solid object. Finally 3D mesh

data was translated into ASCII STL and PLY formats. Agrawal et al. (2006) converted

DEM ASCII XYZ data into an STL surface using Global Mapper. This STL surface was

then imported into Magics RP to obtain a 3D STL part. They made a physical model of

Table Mountain on EOSINT P380 machine. However, the study concluded that the

sequence of loading the STL surface in Magics RP, offsetting, extruding and then

hollowing do not always give a good STL part and suggested to remove the intermediate

steps to obtain an accurate model. The authors have also discussed popular GIS, CAD and

AM file formats and provided information about the software programs which support

these data in brief.

Bechwar and McGreen (2009) have developed a methodology to create three-dimensional

terrain models of surface and subsurface water flow-data for educational purpose using

additive manufacturing. The ground water data was obtained from GeoCommunity

website in the form of digital elevation model (DEM) data; whereas subsurface water data

was obtained from regional agencies in tabular form. Initially, authors used AutoCAD

Map 3D, a Geographical Information Systems tool from AutoDesk Inc. to convert surface

DEM data into DXF format. They have written their own software program in Visual

Basic to convert the textual tabular subsurface data into DXF format. The z values were

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scaled up twenty times for better visualization. Now DXF file was opened in AutoCAD to

make a 3D solid model out of surface data as AM requires 3D data to fabricate the part.

Other features like township border, highways and lakes, etc. were also added in

AutoCAD. The solid model was exported to STL format. Finally, the STL file was

opened in Magics RP software to check and remove the errors.

Color is an essential graphic variable in cartography and model visualization. Sheerin

(2003) discussed the potential application of AM beyond the mechanical industry such as

transportation, landscape planning, utility planning, watershed planning, architecture,

education and visualization by AM created color models. Bailey (2005) at Center for

Visualization of Prototypes (CVP) at Oregon State University reported creation of

physical terrain model of the Grand Canyon, the map of United States and the Southern

California fires of October 2003 based on the satellite data. The Center has also fabricated

color models of Mars globe, Haemoglobin molecule, interior of human head based on

MRI data and other models based on ultrasound and CAT scan data for scientific

visualization with haptic experience. However, the author has not specified the data

format used as input.

Rase (2009) created multi-color physical models of city of Vienna, Austria and terrain

models of cartographic surfaces generated from demographic and accessibility data using

3D printer Z650 of Z Corporation. Author used VRML97 and PLY file format to print

multi-color models. Antlej et al. (2010) discussed creation of museum exibits using

ZBuilder Ultra machine, a process which employs Digital Light Processing (DLP)

technology instead of laser light for curing of photopolymer. The process is faster than

stereolithography as the entire cross section is scanned at once and solidifies the layer.

Therefore, the building time depends only on build height and not on the number of

models in building area. Ghawana and Zlatanova (2013) discussed applications of the AM

fabricated terrain models to understand and explore surface and subsurface terrain

variations. They also mentioned examples of geological printing of a model which

represents cave geometry produced by replicating a LIDAR point cloud in clear crystal

glass and a color 3D model made from seismic data.

Literature review reveals that creation of the physical models of terrain was not a direct

process. There is no method available to convert GIS surface data directly into 3D faceted

data needed by the AM processes. Researchers used more than one software packages for

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obtaining faceted data from GIS data formats. Besides, it does not give satisfactory results

always. Apart from this, translation of data through multiple file formats may also involve

loss of significant amount of data. And lastly, the user must have knowledge of several

software packages.

2.4 AM in Architecture and Construction

In the era of digital technology and parametric architectural design, making physical

models with complex geometric features is a real challenge. There is a dire need to adopt

new approaches and applying new techniques [Stavric et al. (2013)].

Additive manufacturing technologies are gaining popularity among architects, builders

and land- use planners for making of models at different design stages. According to

Ryder et al. (2002), there may be three types of scale models in architecture depending on

the stage of the design project; namely 1) Feasibility model 2) Planning model and 3)

Final display model. The first two types of models are usually used by architects in-house

as feedback for the design process and to make the necessary modifications until final

shape is achieved. The final display models are used to promote and sell the project to the

customers. Many researchers have used AM for fabrication of different types of

architectural models for design review and sales promotion of the project.

Gibson et al. (2002) investigated the application of AM technology for architectural

modeling. He fabricated architectural models on FDM, SLS and 3D printer machines.

Initially model was made on FDM machine. But it was unable to stand its own weight and

collapsed after supports were removed. Then it was made using SLS and Z402 3D printer

successfully. Authors found that the models were homogeneous in appearance but were

rough finished and required further treatment in order to obtain an appearance of crafted

work. They concluded that AM technology can be used to creative fabrication of

architectural models.

De Beer et al. (2004) built a 3D scale model of a building in Cape Town on SLS machine

using Duraform material. Two-dimensional architectural drawings and plans were

converted into 3D CAD models. They concluded that the AM technology proves very

beneficial when fine details, accuracy and shorter lead time are key parameters in model

making. Sánchez et al. (2005) explained the application of AM technologies for

conceptual as well as the detailed scale modeling of the architecture structures with the

help of two case studies.

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Franic et al. (2009) made a virtual and a physical model of the city of Zagreb, Croatia

using photogrammetric mapping, Digital Terrain Model (DTM), and true orthophoto as

input data. The physical model was generated based on the photogrammetric mapping of

rooflines and digital terrain model (DTM) using CityGRID, a powerful technology for the

3D city data management. The 3D model was large and hence divided into printable areas

by creating 250 mm × 300 mm pieces. All pieces of 3D CAD model of the city was

exported to VRML format and then fixed for errors in Magics RP software. A physical

model was made using Z510 Spectrum 3D printer. Rase (2009) has reported building of

colored physical models of a city and a landscape with relief. He also constructed physical

models to represent demographic and socio-economic data. He used ESRI ArcGIS

software for preparing the basic data and his own computer program based on

pycnophylactic interpolation, a volume-preserving interpolation technique to generate a

smooth surface from polygon-based cartographic data [Tobler (1979), Rase (2001)].

Campbell et al. (2011) reviewed the use of AM technology for terrain modeling,

architectural modeling and other applications in South Africa. Ghawana and Zlatanova

(2013) presented a case study on fabrication of 3D city model of Dwarka, a subcity of

Delhi, India. They investigated the size and the resolution (Level of Detail or LOD) of

city model that can be printed as physical models. Authors have used satellite image of

Sector-6 of Dwarka available on Google Earth for the case study.

It is quite possible to print components larger than the machine’s build volume by

additive manufacturing technologies. It is usually done by dividing the CAD model into

smaller pieces; printing each piece separately and combining all the printed pieces

together [Dimitrov et al. (2007), True3D (2014)]. However, a few universities and

companies are exploring possibilities to apply additive manufacturing in architecture or

construction for printing large scale models and components such as a room, a building or

even an entire colony [Gardiner (2014)]. Presently there are three such techniques,

namely 1) Contour Crafting 2) D-Shape and 3) Freeform Construction are being

developed by different research groups [Khoshnevis et al. (2006), Dini et al. (2006), De

Kestelier and Buswell (2009)].

Contour Crafting, developed at the University of Southern California by Dr. Behrokh

Khoshnevis, is an additive fabrication technique that produces fixed width walls by

depositing an internal and external trowelled skin robotically. The cavity between these

skins is then filled with a bulk material through that same robotic arm [Khoshnevis

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(2004), Khoshnevis et al. (2006)]. Enrico Dini has been developing a large scale

fabrication technology that is similar to the 3D printing technology from Z Corporation

[D-shape (2014)]. It deposits a thin layer of sand over the full bed size of the printer (4 m

x 4 m). This sand has been pre-mixed with a catalyst that chemically hardens when it

comes in contact with an inorganic binder. This binder is jetted onto the sand through a

series of jets. Just as with Z Corp’s 3D printing technology, the sand is used as its own

support structure.

Another freeform construction project was initiated by the Innovative Manufacturing and

Construction Research Centre (IMCRC) at the Loughborough University. An additive

manufacturing machine similar to FDM has been developed and is capable of producing

large (2m x 2m x 2m) parts out of concrete. Instead of plastic here concrete is extruded

through a nozzle at a constant speed. A large computer controlled 3 axis steel gantry

system deposits this concrete with high precision layer by layer. As the parts are being

printed, it is possible that every single printed part can be different and customized [De

Kestelier and Buswell (2009)]. Given the application of AM technologies for

manufacturing of large scale models and components in construction industry, it is very

much possible to print large scale terrain model using AM in the near future.

Physical model of a terrain or a huge architectural structure is also possible by reverse

engineering technique. The point cloud data may be obtained by non-contact type

scanning techniques and then can be converted into 3D scale CAD model by using

available techniques or by writing a separate computer program. A Russian Orthodox

Church has been 3D modeled using point data cloud obtained from 3D laser scanning

technology for quick and accurate measurement of existing conditions for reconstruction

of the church [GIM (2011b)]. This effort greatly slashed the survey time and cost. Sabry

et al. (2007) discussed 3D modeling of a castle, a complex and large-scale architecture by

integration of multiple techniques, including terrestrial and aerial photogrammetry, laser

scanning, survey and GPS. GIM International (2011a) reported creation of accurate 3D

digital model of Stonehenge, UK using short and long range scanner and Geomagic

Studio software. Physical scale models of these digital models can be fabricated easily

using the AM technology. Some researchers have created digital 3D models from the

existing physical model of a city as a reverse engineering process [Chevrier et al. (2010),

Humbert et al. (2011)].

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2.5 Conclusions from the Literature Review

Physical models of terrains on the earth have been used for several years as an effective

communicating tool for variety of purposes in a society. Physical terrain models often

prove advantageous over two-dimensional maps as well as recently introduced virtual

reality techniques. Traditionally several manual as well as machine assisted techniques

have been used by the model makers to build the physical models of the terrains.

However, because of better efficiency and accuracy over other techniques, AM

technology attracted the attention of the architects and model makers in the beginning of

the 21st century.

STL format is a de facto standard for all additive manufacturing processes. However it

does not support color information. The PLY, OBJ and VRML are other popular faceted

formats used in AM industry. All these formats support color information. There are

several types of GIS, CAD and AM data formats available in the market. AM

technologies use GIS surface data of a terrain in vector formats or raster DEM formats to

fabricate a physical model of the terrain. However, raster DEM formats have been found

to be more suitable for the purpose.

Literature review reveals that several researchers have attempted to convert DEM surface

data into faceted models for use in AM technologies. Researchers converted the DEM

formats into different AM file formats such as STL, PLY or VRML. However, the

conversion of DEM data into faceted model was obtained in several steps like conversion

of DEM data into CAD surface in DXF format, surface data is then converted into a 3D

CAD model, etc. using different software packages. The methods used by the researchers

are not direct and there is a data loss associated with translation to intermediate file

formats.

The fabrication of large scale architectural, construction and terrain models is being

explored by some researchers with newly developed additive manufacturing technologies

such as contour crafting and D-shape. Some researchers have also reported conversion of

existing physical models of a city or other entities into 3D CAD models by reverse

engineering technique.

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2.6 Objectives of the Thesis Work

GIS data of a terrain is the data of a surface. It is not a closed volume in itself and thus,

not a solid model or a faceted model. Conversion of DEM data into faceted model has

been done by previous researchers in several steps and using multiple commercially

available softwares. There is a data loss associated with this procedure and the user has

less control over the conversion process. A critical review of the literature in the related

area revealed that there is a scope to develop a methodology which can convert GIS

surface data formats directly into faceted models for additive manufacturing and eliminate

any data loss associated with translation of data through intermediate file formats.

Obtaining a 3D solid from a surface data is quite an involved process. The reason is that

the surface of a terrain is not a regular surface. To make a physical model of the terrain,

vertical walls on the surface have to be built and a base has to be made.

Following objectives have been set for this thesis work to achieve the above mentioned

goal:

To develop a methodology and a software program in the C language to obtain 3D

STL part from DEM ASCII XYZ surface data directly. The STL part can be used in

an AM machine for fabrication of a terrain model.

To develop a methodology to obtain 3D STL part from USGS DEM data directly

and use the STL part for fabricating a physical model of a terrain.

To obtain 3D STL part from Surfer Grid file format.

To develop a methodology and a software program in the C language to obtain 3D

PLY part from DEM ASCII XYZ, USGS DEM and Surfer Grid DEM data formats.

The above mentioned objectives are dealt with in detail in the following chapters.