Drawing accurate ground plans using opti

Drawing Accurate Ground Plans Using Optical Triangulation Data Kevin Cain INSIGHT kevin@insightdigital.org Philippe Martinez École normale supérieure / INSIGHT pmartine@ens.fr Abstract

Drawing Accurate Ground Plans Using Optical Triangulation Data

Kevin Cain

INSIGHT kevin@insightdigital.org

Philippe Martinez

École normale supérieure / INSIGHT

pmartine@ens.fr

Abstract

Here we consider optical triangulation scanning as a

means of creating permanent architectural archives in

the form of accurate ground plans and other

orthographic views We present plan drawings created

with laser scan data and use these documents to make

comparisons with existing documents Finally, we

present a new technique for decreasing the laser

scanning field time required to create plans and other

views

1 Preface

This brief paper introduces an effort to create

accurate ground plans for a Fatimid-era monument in

Cairo The Egyptian Antiquities Project of the American

Research Center in Egypt (EAP, ARCE), having

undertaken conservation of the Zawiya and Sabil of

Sultan Farag ibn Barquq (Al-Dehisha) in Cairo, Egypt,

requested digital documentation as a basis for their

physical restoration This monument, shown below, is

listed as Supreme Council of Antiquities Islamic and

Coptic Antiquities Monument No 203

Fig 1—Digital photograph of north façade, Farag Ibn Barquq

2 A Simple Review of Optical

Triangulation Scanning Practice

Laser scanning can be used to create a representation

of an object in space, but the construction of this image

comes by a very different method than is used in

conventional photography When a conventional photo

is taken, the image is captured through a lens The

specifications of the lens determine the observed

perspective in the final image Also, the lens introduces distortions that make it difficult to extract accurate drawings or measurements The orthographic drawings commonly used by architects and engineers are drawn without perspective in order that the relationships between any given points on the drawing can be measured at a constant scale

During optical triangulation scanning, a sensor measures the distance from the scanner to a specific point on a given object By making these measurements, the relationship between the gathered points can be expressed in the constructed 3D space of the computer

By taking a many measurements, a “cloud of points” emerges that accurately describes the subject being scanned Finally, when rendered from the point of view

of a synthetic camera in the 3D world space of the computer, digital drawings are generated

3 Our Work at Farag ibn Barquq

The scanning and registration process at Barquq involved multiple viewpoints, which were later correlated

Fig 2—Digital photograph of the monument’s west façade

Four main techniques were used to document

selected parts of the mosque: photography, acetate drawings, acoustic measurement, and laser scanning

Conventional ink drawings on acetate were used to record the epigraphy set in limestone near the mosque’s entry Freehand drawings were made of all areas in the

1063-6919/03 $17.00 © 2003 IEEE

interior and notated with measurements as they were

taken A handheld digital measure was used to verify

the accuracy of the laser scan data The device produced

results accurate to (+/- 1 cm); these objective

point-to-point measurements were then compared to the

measurements drawn from scanned data

3.1 Scanning Methodology

The viewpoints needed for comprehensive

documentation of the monument as a whole were first

worked out on paper The main interior viewpoints

(1-12) and exterior viewpoints (A-G) are shown below (Fig

3), in plan view

Fig 3—Primary scan viewpoints for the interior and exterior

The goal of creating an integrated interior and

exterior model presented unique problems because of the

number of common reference objects required to survey

the entire monument For the method we used, three

distinct entities (spheres or otherwise) are needed to

accurately reconcile two viewpoints Care was used to

place reference objects where they could be seen in as

many viewpoints as possible In the case of scan #3, for

example, the spheres in Area A had to be visible through

the narrow passage to Area C At the same time, the scanner itself had to be positioned so that it could see the spheres in Area A and C, which further limited the placement of spheres Finally, it was crucial that all spheres be left in place during scans, and that all spheres were named in the computer for registration purposes Viewpoints and their corresponding objects were planned on paper before scanning

Fig 4—Scanning viewpoint #1, with a red reference sphere

In Fig 4, it is possible to see one red reference sphere on a tripod, and two spheres placed on the floor All viewpoints and their corresponding reference spheres were planned on paper before scanning

Fig 5—View from the roof of Barquq towards the scanner and team below; note reference sphere on ledge

The requested final output for the project was orthographic drawings of the monument (plans, centerline sections, front elevations, and details) as well

as digital reconstructions of the mosque at different phases of its history Accordingly, different

methodologies were used for the interior and exterior

scans For scans #1-12, the goal was to capture an

accurate cross-section of all interior walls; these scans

were used to construct the plan view and did not require

the highest resolution Viewpoints A-G, the exterior

scans, required scanning over larger distances For the

north and east facades, it was also important to capture

the entire surface ARCE-identified areas, both interior

and exterior, were scanned in greater detail as a

reference for future study of the monument The specific

rationale for these decisions is addressed in Section 6

Considering the scans of Farag Ibn Barquq as a study

case, the following generalizations can be abstracted:

1 Laser scanning can be an accurate technique for

full 3D documentation While it is still a challenge to

deal with the large amount of data generated from 3D

scans, the scanning process remains the best digital

method to quickly gain comprehensive 3D

documentation of a site This is especially crucial for

imperiled sites, where details risk being lost before they

can be documented In such “crisis” cases, 3D scan data

could potentially be the only resource available to future

researchers

2 An integrated approach is helpful, balancing laser

scanning with traditional techniques While 3D

scanning can be accurate and rapid when compared to

traditional techniques, it is important to test the data

provided via traditional techniques, where possible

There are also clear advantages to traditional techniques

in terms of cost and time, depending of the project

Technique Advantages Limitations

Photography Low cost, Fast Limited accuracy

Traditional

Illustration

Relatively low cost Slow, Limited

accuracy QTVR / digital

panoramas

Allows user to navigate a 3D scene

Fixed viewpoints Traditional

surveying

Accurate, Established technology

Limited number of 3D points 3D Laser

scanning

Accurate, High number of 3D points

High cost, Technically demanding

Figure 6 Comparison of 2D / 3D documentation types

3 Advantages of 3D Scanning for Cultural Heritage

Projects 3D scanning enables a site to be accurately

measured in a relatively short amount of time 3D

scanning remains the only viable way of documenting

the precise measurements of a complex subject such as

the Sabil’s deteriorated stalactite ceiling

Importantly, scan models can be transferred to

popular formats (i.e., AutoCAD) for use by architects

and engineers These same files can be used as the basis

for reconstructions, physical models, or object movies

Provided that the files are continually migrated, 3D

scanning is a permanent, durable record of the site

4 Limitations of 3D Scanning for Cultural Heritage Projects During fieldwork, our team has found that 3D

scanning hardware is inevitably delicate While scanners differ in the robustness of their performance, all require special handling and careful operation Also, the costs required to complete large-scale scans and the heaviness

of the resulting data files are currently a significant problem

Also, while laser scanning can achieve sub-millimeter accuracy, it is difficult to accurately capture epigraphy with this technique Large-scale scanners are not designed to record detailed inscriptions while small-scale scanners are not equipped to deal with the small-scale of

a building Since epigraphy must be scanned at high resolution to capture crucial detail, heavy files are again

an issue

3.2 Interior Data

As shown in Fig 3, 12 viewpoints were taken of the interior walls and floor regions for the purpose of generating an accurate ground plan While it would be possible to proceed with fewer points, sampling a significant section of every wall reveals implicit angles

in these vertical surfaces Once the 12 viewpoints were registered using 20 distinct entities (including registration spheres as shown in Section III), a 3D model for the whole interior plan was generated Below, two orthographic views of this point cloud model are shown:

a ¾ view of the interior from above, and a traditional plan view of the point clouds

Fig 7—An orthographic view of the interior walls as seen from above

Fig 8—An orthographic plan view of the interior point clouds

As described previously, the 12 interior scan

viewpoints were merged into an integrated model by

using reference spheres common to two or more views

The proper correlation of these separate views is crucial,

since the registration process can introduce errors The

integrity of the data taken from each viewpoint is

initially secure, resulting in a high level of confidence in

the measurements made from the data When two

viewpoints are registered, however, the accuracy of the

union is limited by the three or more common reference

entities designated To improve the accuracy of these

unions, entities were created to supplement the reference

spheres For the plan shown at left, the average possible

error was computed as 0 - 0.6mm

The resulting ground plan, shown in Fig 11-13, also

incorporates all exterior scans

Fig 9—Detail of Area A Entry

Fig 10—Orthographic ¾ view of Area A

3.3 Correlation with ARCE / SCA Working Drawings, Undated

The following drawing compares the SCA Architectural Working Drawings for Barquq (black, dashed line) to the 2000 ground plan (shaded)

Fig 11—SCA working drawings v 2000 ground plan

3.4 Correlation with Saleh Mostafa Ground

Plan, 1972

The following drawing compares the Dr Mostafa

ground plan for Barquq (black, dashed line) to the 2000

ground plan (shaded)

Fig 12—Dr Mostafa ground plan v 2000 ground plan

3.5 Correlated Ground Plan Before

Reconstruction, 1917

The following drawing compares a ground plan from

1917 (black, dashed line) to the 2000 ground plan

(shaded) Note: The 1917 plan shows few exterior lines

Fig 13—1917 drawings v 2000 ground plan—note the substantial

reworking of Area D

3.6 Ground Plan Comparisons

The following general notes were culled from the

2000 ground plan:

1 For the range sampled, interior walls were

observed to be vertical within apprx 5-10mm

variation The interior wall of room C leans

east

2 Since the building was moved during 1922-23,

there are obviously many changes between the

1917 plans and all others; it is therefore left out

of direct comparisons of dimension below

3 The exterior dimensions of the plans vary, and are being examined against the 2000 plan and each other On the southern façade, the 1974 plan understates the thickness of the southeast exterior wall, while the ARCE plan overstates this dimension The same is true of the eastern façade, where the southeast corner is likewise distorted

Unless otherwise stated, the following comparisons are made relative to the 2000 ground plan A reference

is given for each note on Fig 14 below

Fig 14—Key to specific notes

ENTRY (NORTHERN) FAÇADE

1 The 1972 plan simplifies the entry contours

on the entrance niche, omitting the vertical grooves at the perimeter of the entrance façade

threshold and entry steps is common to all four plans; for placement, the ARCE plan matches the 2000 plan most closely

3 The extreme right edge of the northern

façade on the 2000 plan is drawn at less

than a 90-degree angle This area is not

drawn in the 1917 plan; the 1972 plan

draws this angle at apprx 90 degrees and

the ARCE plan show the angle at greater

than 90 degrees

AREA A

4 The ARCE plan closely matches the 2000

plan dimensions; when rotated clockwise to

correct for angle, the 1972 plan also fits the

2000 plan

5 The 1972 and ARCE plans shows a second

header in the western passage to area B

AREA B

6 The walls of the eastern passage between

areas A and B are shown as flared in the

2000 plan This is a possible artifact of the

passageway doors

plans in showing no pitch to the south wall

niche The 2000 plan agrees with the pitch

drawn in the 1917 plan; the 1974 version

shows the pitch running in the opposite

direction

AREA C & AREA D

8 The ARCE plan omits the beveled corner

present in the northern wall of area D On

this point, the 2000 and 1972 plans closely

match if rotated for alignment

9 The south walls of areas D and E establish

a parallel line The ARCE plan presents

the south walls of areas D and G as

parallel; the 1917, 1972, and 2000 plans

show otherwise

AREA E

relationship between areas E and G; the

1972 plan does not

AREA F

11 The niches along the east wall are not

shown in the ARCE plan, and were not

scanned for the 2000 plan They are shown

on the 2000 plan according to their position

on the 1972/1917 plans

12 A boundary return in the north wall divides

the room into a rihab and Mihrab section

The location of this return on the 2000 plan

is east of the same feature in the 1972 and ARCE plans If verified, the implication is that the rihab roof members are not perfectly parallel

13 The bookcase in Iwan was closed during

scans, accounting for lack of depth information at this point

14 The 2000 plan indicates that the west wall

is not perfectly true over its course All other plans show this wall as true

niche are more inclined in the 2000 plan than the others

AREA G

16 On the ARCE plan, the north wall of Area

G is not parallel with the south niche wall

of Area B The other plans agree on this point On the 2000 plan, the line of Area G’s north wall is also shown as parallel with the north wall of Area E

4 Section Views

The construction of sectional views follows the methodology described for generating the 2000 ground plan Again, using the scan data, dimensions can be extracted as accurate, orthographic drawings

4.1 Iwan and Prayer Hall Sections

The EAP team expressed interest in documenting the original beams in Area F The beams are deteriorated and irregular, as seen in Fig 15-16 The surface and volume of each beam is unique, a fact that can be appreciated as easily by rotating the digital scan model

as by direct observation at the site Note fragments of the marble encrustation in the scan model, above the closed cupboard and doorway

Fig 15—Iwan point cloud

Below, a scan data section is drawn at the centerline

of Area F (Fig 16) A complete orthographic section

drawn from this data is shown in Fig 18, including

major beam measurements (As with the 2000 ground

plan, dimensions were verified using manual

measurements

Fig 16—Iwan roof section

Since the character of each beam changes over its

run, a single section view will only partly describe the

three-dimensional detail present However, it is possible

to take sectional views at regular points along length of

the beams as a way of sampling the changing dimensions

of the beams As shown in Fig 17, below, the total point

cloud has been divided into three smaller areas for

analysis Using this approach, section views can be

generated at any desired interval

Fig 17—A subdivided view of the Sabil section data

Fig 18—Prayer Hall Centerline Section

4.2 Sabil Section

Using a similar subdivision approach, section views can be generated at any desired interval for other areas of the mosque, such as the Sabil ceiling In Fig 19, below,

an orthographic centerline section view of point cloud data is shown

Fig 19—Ceiling section view of Sabil point cloud data

Fig 20—A contrasting axonometric view of the Sabil ceiling, seen

from below

5 Epigraphic and Façade Documentation

In addition to recording the building exterior with

laser scanning, traditional line drawings were completed

for principal inscriptions on the north façade

5.1 Texts

The epigraphy on the north entry walls is treated

below (In Fig 21, note the restoration on the right third

of the text: the restored block is clearly revealed by the

color and lighting in this photograph.)

Fig 21—Epigraphy on north façade

For comparison, a fragment of the extracted line

drawing on acetate is shown below After the full-scale

sheets of acetate were inked, they were digitally

recorded The entire inscription, with translation, is shown in Fig 23 below

Fig 22 Extracted line drawing with gradient to accent

the restored, rightmost block

Fig 23—Epigraphy and translation from north façade

Epigraphy running along the top of the north and east walls was digitally photographed and assembled

5.2 Façade Reconstruction

An area of inlaid and mosaic work directly above the main entry was selected for a simple digital reconstruction This area is shown in Fig 24

Fig 24 (left)—Entry portal

Below, a photo showing the existing condition is

contrasted with a reconstructed image (Fig 25-26)

Color reference for the reconstruction came through

research, informed by study of mosaic fragments

elsewhere on the exterior of the mosque

Fig 25—A photo of the portal region

Fig 26—A digitally reconstructed view of the same portal

6 Exterior Documentation

As requested by Dr Vincent, director of the ARCE

EAP, sections of the monument exterior were scanned at

high resolutions as a baseline for future study of environmental effects on the structure

6.1 Digital Architectural Archive

Easily quantifiable landmarks were designated on the exterior of the mosque; these regions were then scanned and archived as 3D clouds of points Areas recorded in the way include the upper Muqarnas rows along the eastern façade and the main entry portal area on the north façade By comparing the data recorded in March

2000 with future 3D scans (of any kind) it would be possible to study changes in the structure over time

Fig 27 (left)—Point cloud view of the north façade entry portal,

(right)—Photographic reference

7 A New Technique for Increased Speed and Accuracy in Site Scanning

During viewpoint framing for long-range laser scanning, nearly all current scan control software assumes a uniform bounding box selection (parametric UxV) within an XYZ world Here we suggest a new system of scanner control that does not make this assumption, but instead uses active parsing of incoming points to enable automated, “subdivided” scan viewpoint framing

7.1 Unique Challenges in Large-Scale Scanning

Because access to archaeological sites is often limited, it can be difficult to scan large sites at relatively high resolution This was true for the three-day schedule allotted for our documentation of Farag ibn Barquq Time constraints were also the case of our laser scanning at the Ramesseum in Thebes, Egypt In this case, like Barquq, our goal was to establish detailed architectural plan, section, and elevation views However, in this case we documented a site many hundreds of meters in dimension