P oceedings o he 8 h
In e na ional Cong ess
on A chaeology,
Compu e G aphics,
Cul u al He i age and
Inno a ion
‘ARQUEOLÓGICA 2.0’
in Valencia (Spain),
Sep . 5 – 7, 2016
DOI: h p://dx.doi.o g/10.4995/a queologica8.2016.2582
Recei ed: 26/01/2016
Accep ed: 18/05/2016
* Co esponding Au ho : En ique Vale o, e. ale [email protected]
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EDITORIAL UNIVERSITAT POLITÈCNICA DE VALÈNCIA
COMPARISON OF 3D REALITY CAPTURE TECHNOLOGIES FOR THE
SURVEY OF STONE WALLS
COMPARATIVA DE TECNOLOGÍAS 3D PARA LA INSPECCIÓN DE MUROS DE PIEDRA
En ique Vale oa,*, Alan Fo s e a, F édé ic Boschéa, Lyn Wilsonb, Alick Leslieb
a Royal Academy o Enginee ing Cen e o Excellence in Sus ainable Building Design, He io -Wa Uni e si y, EH14 4AS, Edinbu gh,
UK. [email p o ec ed]; a.m. o s[email p o ec ed]; [email protected]
b Sou h Gyle Conse a ion Cen e, His o ic En i onmen Sco land, EH12 9EB, Edinbu gh, UK. [email p o ec ed];
[email p o ec ed]
Abs ac :
The main enance o he ex e nal ab ic o his o ic buildings cons i u es a la ge po ion o o e all building li e cycle cos s.
Ad anced eali y cap u e and da a p ocessing echnologies ha e he po en ial o ans o m exis ing su ey p ac ice,
p o iding su eyo s wi h objec i e da a pe aining o building ab ic, in a mo e apid ( equen ), sa e and cos -e ec i e
manne . In his pape , we p esen a unique e alua ion o se e al Te es ial Lase Scanning (TLS) and pho og amme ic
(PG) sys ems ha assess hei ela i e s eng hs o he su ey o s one walls. The assessmen is conduc ed using an
his o ic building selec ed o i s ep esen a i eness o o m, ab ic and condi ion. The wo k conside s pe o mance in
e ms o da a accu acy and p ecision, da a comple eness, and p ocess e iciency. The esul s show ha , while TLS
p o ides good geome ic da a o gene a e accu a e and aluable 3D models, he quali y o PG econs uc ions can be
also be su icien in such con ex s. And conside ing he ela i ely low-cos and po abili y o mode n digi al came as
compa ed o lase scanne s, pho og amme y can cons i u e a ealis ic al e na i e o TLS. In addi ion, moun ing a came a
on a UAV could u he sol e access issues, p e en ing he need o any addi ional in as uc u e (e.g. sca olding), which
would be equi ed when employing TLS. Howe e , a lesson d awn om his wo k is ha e ec i e acquisi ion o
pho og amme ic da a equi es ca e ul planning o selec he app op ia e came a se ings and pic u e densi y (and
loca ions) o ensu e accu a e and eliable pho og amme ic econs uc ion. This p ocess may be e e ed o as: Planning
o Pho og amme y (P4P).
Key wo ds: i ual a chaeology, his o ic buildings, cul u al he i age, documen a ion, su ey, 3D econs uc ion, lase
scanning, pho og amme y
Resumen:
Las labo es de man enimien o de la es uc u a ex e na de edi icios his ó icos cons i uyen una g an pa e de los cos es
asociados al ciclo de ida de es os. Las ecnologías empleadas pa a la adquisición de da os es uc u ales y su
p ocesamien o ienen el po encial de ans o ma , en g an medida, las a eas de inspección adicionales,
p opo cionando in o mación ela i a al edi icio de una mane a más ápida, segu a y económica. En es e abajo, se
e alúan escáne es láse y sis emas basados en o og ame ía, demos ando su solidez pa a la inspección de pa edes
de pied a. Es a e aluación se ha lle ado a cabo en un edi icio his ó ico ep esen a i o y en un buen es ado de
conse ación. En el a ículo, se compa a la ac uación de los di e en es disposi i os en é minos de p ecisión,
epe ibilidad, densidad de da os y e iciencia del p oceso. Los esul ados mues an que, mien as que los escáne es
p opo cionan buena in o mación geomé ica y pe mi en la gene ación de modelos idimensionales p ecisos, la calidad
de las econs ucciones a pa i de o og ame ía ambién pueden se su icien es en de e minados con ex os.
Conside ando, además, el ela i o bajo cos e y la po abilidad de las cáma as digi ales mode nas en compa ación con
los escáne es láse , la o og ame ía puede cons i ui una al e na i a eal a es os úl imos. Además, la ins alación de
cáma as en ehículos aé eos no ipulados (d ones) puede esol e p oblemas de accesibilidad y minimiza el uso de
in aes uc u as adicionales (como andamiajes), las cuales son empleadas en la digi alización median e escáne es. Sin
emba go, una lección que se desp ende de es os expe imen os es que pa a consegui una e ec i a adquisición de da os
o og amé icos se equie e una cuidada plani icación y una selección ap opiada de los ajus es de la cáma a, así como
su co ec o posicionamien o en la escena, con el obje i o de consegui una adecuada densidad de imágenes y ob ene
una econs ucción 3D p ecisa. Es e p oceso se denomina Plani icación pa a la Fo og ame ía.
14
Vale o, Fo s e , Bosche, Wilson, Leslie, 2016.
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Palab as cla e: a queología i ual, edi icios his ó icos, pa imonio, documen ación, inspección, econs ucción 3D,
escáne es láse , o og ame ía
1. In oduc ion
The main enance o he ex e nal ab ic o his o ic
buildings cons i u es a la ge po ion o o e all building
li e cycle cos s. In Sco land alone, i is es ima ed ha
he e a e app oxima ely 400,000 adi ional (p e 1919)
buildings ep esen ing 20% o building s ock and
cons i u ing an impo an po ion o cons uc ion
spendings (Muelle e al. 2014). “Repai , main enance
and imp o emen accoun s o app oxima ely 46% o he
o al cons uc ion indus y ou pu , o which one hi d
ela es di ec ly o p e 1919 buildings (accoun ing o
app oxima ely £1.2 billion)” (His o ic Sco land 2011).
En i onmen al clima ic condi ions and associa ed
wea he ing ac as ‘agencies o ma e ials change’, ha
lead o he de e io a ion o building ab ic. A es ing his
de e io a ion equi es egula , app op ia e main enance.
This ope a ion is becoming mo e one ous due o clima e
change and mo e speci ically inc eased incidence o
ex eme ain all in Sco land. P ac ically, highe le els o
mois u e can be co ela ed wi h accele a ed de e io a ion
in po ous building ma e ials. P e alen decay
mechanism in mason y include inc eased se e i y and
equency in eeze- haw cycling and spalling e en s,
and biological g ow h associa ed wi h long e m
sa u a ion o ab ic. Logically, inc eased agg essi i y o
en i onmen al condi ions equi es highe equency
e alua ion o condi ion o he ex e nal e elope o
buildings.
Main enance decisions a e made ollowing de ailed
su eys o ab ic. Ad anced eali y cap u e and da a
p ocessing echnologies ha e he po en ial o ans o m
exis ing su ey p ac ice, p o iding su eyo s wi h
objec i e da a pe aining o building ab ic, in a mo e
apid ( equen ), sa e and cos -e ec i e manne . In
pa icula , g ea e olume and be e accu acy o
in o ma ion can suppo su eyo s in ensu ing objec i i y
and consis ency when assessing condi ion. This is
pa amoun as hese su eys a e u ilised o de elop
epai s a egies.
Su eying building ab ic encompasses many dispa a e
and in e connec ed componen s ha equi e he
acquisi ion o a ious ypes o da a. P incipally, amongs
in o ma ion is cons uc ion o m and ma e ials. Bo h o
hese i s o de su ey objec i es ely upon speci ic da a
including he dimensions/geome y and ex u e o he
ab ic componen s. Ad anced h ee-dimensional and
isual eali y cap u e echnologies ha e been de eloped
in he las ew decades o acqui e such da a. These
echnologies mainly include: pho og amme y and lase
scanning. Bo h can deli e dense 3D poin clouds o
scenes, possibly augmen ed wi h colou in o ma ion.
Pho og amme ic sys ems a e based on he use o
came as and he p inciple o s e eo ision, while lase
scanning is a mo e ecen echnology ha is based on
he con olled command o lase beams and ime-o -
ligh p inciples o de e mine he dis ance o an objec .
Despi e he quali y o he da a deli e ed by lase
scanne s, hei use is limi ed by he need o se up he
scanne on he g ound o a s able pla o m a mul iple
loca ions ha p oduce a comple e 3D model when
indi idual scans a e egis e ed oge he .
Pho og amme ic sys ems ha e shown g ea po en ial o
esol e access- ela ed issues bu hei pe o mance can
be signi ican ly limi ed by he ex u e and/o ype o
ma e ial being scanned. Ano he limi a ion o
pho og amme ic sys ems is ha he 3D da a hey
p o ide has unknown scale; con ol ne wo ks o known
dimensions in he da a a e necessa y o con e he da a
o me ic scale, which ine i ably in oduces addi ional
e o .
The di e en s eng hs and weaknesses o bo h sys ems
ha e led esea che s o de e mine which solu ion is mos
app op ia e in a ious con ex s. In his pape , we p esen
a unique e alua ion o se e al Te es ial Lase
Scanning (TLS) and pho og amme ic (PG) sys ems ha
a e compa ed o assess hei ela i e s eng hs o he
su ey o s one walls. The assessmen is conduc ed
using an his o ic building selec ed o i s
ep esen a i eness o o m, ab ic and condi ion. The
wo k conside s pe o mance in e ms o da a accu acy
and p ecision, da a comple eness, and p ocess
e iciency.
This pape is s uc u ed as ollows: Sec ion 2 con ains a
e iew o p io wo ks on he use o lase scanning and
pho og amme y o he su ey o his o ic buildings,
leading o he iden i ica ion o he knowledge gap ha
his esea ch con ibu es o ill. Sec ion 3 desc ibes he
echnologies assessed and he me hodology ollowed o
compa e hem. Sec ion 4 p esen s he on-si e da a
acquisi ion and econs uc ion s eps. Sec ion 5 epo s
and analyses he esul s on he compa ison o he da a
ob ained o he di e en sys ems. Finally, Sec ion 6
p esen s he conclusions and u u e wo ks.
2. Backg ound
Lase scanning is a ela i ely ecen echnology ha
cons i u es a e olu ion in land and building su eying.
Wi hin he con ex o his o ic monumen su ey,
no ewo hy examples o he use o TLS include he wo k
o Wilson e al. (2013) who illus a e he ad an ages o
TLS o he su ey o la ge and complex his o ic
monumen s ia case s udies o UNESCO Wo ld
He i age Si es. Ca daci e al. (2011) show ha TLS
p o ides signi ican alue compa ed o adi ional manual
su ey. They also show how CAD models gene a ed
om he da a can be success ully used o s uc u al
analysis using he Fini e Elemen Me hod (FEM). Ne ley
e al. (2013) use TLS and ligh De ec ion and Ranging
(LiDAR) – i.e. ae ial lase scanning – o ob ain a
pho o ealis ic geospa ial model o he his o ic quayside
a Co ehele Quay in eg a ed in an accu a e Digi al
Ele a ion Map (DEM) in o de o assess he po en ial
impac o ising sea le els esul ing om clima e change.
Temize e al. (2013) show he alue o TLS o su ey
unde g ound s uc u es like he Byzan ine cis e n
si ua ed benea h he cou o he Sa nicli Han building.
They also in es iga e he impac o a ious le els o
poin il e ing on he accu acy o he mesh p oduced
om he da a.
15
COMPARISON OF 3D REALITY CAPTURE TECHNOLOGIES FOR THE SURVEY OF STONE WALLS
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Pho og amme y is ano he well-es ablished me hod o
ob aining 3D eco ds o his o ic monumen s as he
e iew o Ogleby (1995) o example shows. I s use
ou da es ha o TLS by decades, almos a cen u y
(Sche me ho n 1940). Howe e , signi ican p og ess has
been made in he las wo decades, bo h in e ms o
ha dwa e and so wa e, ha has enabled signi ican
au oma ion o PG da a acquisi ion and p ocessing o 3D
model econs uc ion. The de elopmen o obus
au oma ed ea u e de ec ion and ma ching in digi al
images, (e.g. SIFT (Lowe 1999) o SURF (Bay e al.
2006) ea u es), as well as dense ma ching app oaches
(S echa e al. 2003) ha e d ama ically imp o ed, e en
au oma ed he image p ocessing s age. Fu he mo e,
high- esolu ion and po able digi al came as a e now
widely a ailable a a ela i ely low cos , meaning ha
pho og amme y can be easily deployed o p oduce
dense ex u ed 3D poin clouds and meshes. Thanks o
he ligh weigh o mode n came as, single-came a
pho og amme y has also shown g ea po en ial o
esol e access- ela ed cons ain s.
Rega ding he ield o his o ic monumen su ey,
ema kable examples o he use o pho og amme y
include he wo k o Cappellini e al. (CAPELLINI, V.,
STEFANI, C., NONY, N. and DE LUCA, L., 2012) who
apply i o p oduce 3D models o monumen s ha a e
used o gene a e 2.5D o hopho os o walls. Using he
example o Roman walls, hese o hopho os a e
employed o conduc he seman ic anno a ion o he
opus o di e en sec ions o he wall. Le ma and Mui
(2014) p esen a compa ison o lase scanning wi h
pho og amme y, econs uc ing a 3D model by means
o Visual S uc u e o Mo ion (VS M) and Pa ch o
Clus e based Mul i View S e eo So wa e
(PMVS/CMVS), and concluded ha o accu a e
documen a ion o ca ed de ailing on s one,
pho og amme ic echniques p o ided he g ea es le el
o lexibili y and eliabili y, al hough he wo echniques
used in combina ion may p o ide he bes esul s.
Howe e , PG sys ems also ha e limi a ions. Fo
example, hey a e no obus o a ying ligh ing
condi ions and ex u e-poo o e lec i e ma e ials
(Tho nbush and Viles 2008). Fu he mo e, hei p ecision
quickly d ops wi h he dis ance o he came a o he
a ge scene when compa ed o TLS (Rasz o i s and
Do ninge 2013). Also, single-came a PG sys ems
p o ide 3D econs uc ions only up o scale, and equi e
he use o use su ey ne wo ks o ex ac known
dimensions in he images o adequa ely scale he
econs uc ions (Ska la os and Kipa issi 2012).
Wi h ega d o access p o ision, PG and TLS sys ems
can bo h be moun ed on poles o ex endable ipods ha
enable posi ioning hem a heigh s up o app oxima ely
5m. Fo g ea e heigh s, ae ial solu ions can hen be
conside ed. In ac , ae ial pho og amme y and LiDAR
(i.e. ae ial lase scanning) sys ems ha e long been
conduc ed using planes o helicop e s wi h he senso s
moun ed unde hem. Howe e , hese solu ions do no
allow he acquisi ion o images a close dis ances, which
limi s he accu acy and densi y o he esul ing 3D
econs uc ions. Recen and apid de elopmen s o
Unmanned Ae ial Vehicles (UAVs), in pa icula
helicop e - ype UAVs, o e new pla o ms ha a e
pa icula ly sui ed o ligh -weigh PG sys ems. PG
came as moun ed on helicop e UAV enable he
acquisi ion o images a close anges a any heigh ,
p o iding a se ious ad an age o e lase scanning
sys ems in e ms o access p o ision. I should be no ed
ha he e a e a emp s o moun lase scanning sys ems
on helicop e UAVs bu he weigh o he scanne s
equi es e y la ge UAVs and ensu ing ha he acqui ed
da a has high p ecision and accu acy emains a g ea
challenge.
UAVs ha e al eady been conside ed o use in he
applica ion o pho og amme y in he con ex o his o ic
monumen s. Fo example, Püschel e al. (2008) p opose
he use o e es ial and UAV pic u es o c ea e an
accu a e 3D model o Cas le Landenbe g. Remondino e
al. (2011) e iew he di e en s ages o da a acquisi ion
and p ocessing, such as: planning, came a calib a ion,
3D econs uc ion and applica ions; and Remondino
(2011) p o ides an ex ensi e e iew o de ices and
so wa e o eco ding and modelling o ien ed owa ds
cul u al he i age. La ely, Kou soudis e al. (2014)
p oposed a PG sys em combining UAV and e es ial
pic u es and compa ed he esul ing econs uc ion wi h
ha ob ained wi h TLS, epo ing p omising esul s. In
cons as , Xu e al. (2014) ac ually combined 3D da a
om TLS and a UAV-moun ed came a o he
econs uc ion o a his o ical monumen in Fujian, China.
TLS poin clouds a e used o model he açades and
pho og amme ic in o ma ion is used o comple e he
oo a ea (no comple ely isible by he TLS sys em
loca ed on he g ound).
Despi e he g ea e e o in es ed in o in es iga ing he
po en ial o TLS and PG o he su ey o his o ic
monumen s, we ound ha u he s udies we e s ill
necessa y o compa e he quali y o he da a deli e ed
by hese echnologies in he speci ic bu common case
o s one walls eco ding. In pa icula , he eam wished
o assess whe he UAV-based pho og amme y could be
used as a eliable means o ob ain su icien ly accu a e
ex u ed 3D models o mason y walls o hei su ey
and main enance, as such solu ion would conside ably
educe he need o and he e o e he cos and sa e y
isks associa ed wi h adi ional solu ions o access
p o ision (e.g. sca olding, abseiling).
3. Me hodology
The compa ison o 3D eali y cap u e echnology was
conduc ed expe imen ally in he ield using an ac ual
his o ic monumen as es si e. The s udy conside ed:
wo e es ial lase scanne s employing di e en
measu emen p inciples; a hand-held/pole-moun ed
single-came a PG sys em; and a UAV-moun ed single-
came a PG sys em.
This sec ion i s p esen s he cha ac e is ics and
se ings o he es ed sys ems (Sec ion 3.1). The es si e
and he easons o i s selec ion a e desc ibed in Sec ion
3.2. Finally, he c i e ia de ined o compa e he
pe o mances o he di e en sys ems a e p esen ed in
Sec ion 3.3.
3.1. Sys ems
The wo TLS de ices conside ed a e he Leica
ScanS a ion P40 and he Fa o Focus 3D. The Leica
ScanS a ion P40 (he ea e TLS-1) is he la es ime-o -
ligh (TOF) scanne p oduced by Leica. I can scan
objec s up o 270m away, wi h an accu acy o 3mm a
16
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50m and up o 1 million poin s a e eco ded pe second
(Leica Geosys ems 2015). In con as , he Fa o Focus
3D (TLS-2) is a phase-based scanne . I s ad e ised
pe o mance (Fa o 2011) cha ac e is ics show ha his
scanne can measu e dis ances up o 120m, wi h sligh ly
lowe bu s ill good single poin p ecision (2mm a 25m)
and can scan up o 1 million poin s pe second. An
addi ional impo an ad an age o he Fa o Focus 3D
scanne is ha i is signi ican ly smalle and ligh e
weigh han scanne s such as he Leica ScanS a ion
P40, which makes i mo e po able.
The i s single-came a PG sys em conside ed (he ea e
PG-1) is a digi al single-lens e lex (DSLR) Nikon D810
came a equipped wi h a 14mm lens ha is ope a ed
manually hand-held o moun ed on an ex endable pole.
This came a can acqui e 36 megapixel (MPx) pic u es
and he sys em allows he acquisi ion o high-quali y
da a wi h he came a aised a heigh s up o 5m ( he
pole heigh ). The second single-came a
pho og amme ic sys em conside ed (he ea e PG-2) is
a DSLR Sony -7R came a equipped wi h a 35mm lens
ha is moun ed on a UAV and ope a ed emo ely. This
came a can also ob ain 36MPx pic u es. The UAV on
which he came a is moun ed is an Asc ec Falcon 8
Mul i- o o ha can ly a signi ican dis ances and
heigh s – al hough he pilo should con inuously main ain
eye con ac , and Ci il A ia ion Au ho i y egula ions
ac ually p e en lying such UAVs a heigh s abo e 120
me es.
3.2. Tes Si e
The Eas ga den o he medie al C aigmilla Cas le, in
Edinbu gh, Sco land, was selec ed as es si e. This
cas le was buil in he 14 h cen u y and is cu en ly
managed and main ained by His o ic En i onmen
Sco land. Wi hin he Eas ga den, he ampa wall
( acing Eas ; see Figu e 1) was selec ed o he ollowing
easons:
1. Complexi y: The a ea o wall was selec ed due o i s
ela i e complexi y and mo e speci ically: he andom
na u e o he s onewo k; a ia ion in wid h and dep h
o mo a join ; plana and cu ed su aces; s one
soiling and associa e colou and ex u e a ia ion.
2. Su icien heigh : I is impo an o s udy he
pe o mance o he chosen echnologies a
inc easing heigh s; he ampa wall is app oxima ely
10m high, which seemed su icien o challenge
ce ain echnologies, in pa icula he g ound-based
TLS de ices.
3. Wind p o ec ion: The UAV can only be ope a ed in
condi ions wi h low wind ( o p e en collision wi h he
monumen and ensu e good quali y pic u es). To
enhance he chances o being able o ope a ed he
UAV on he selec ed day o da a acquisi ion, he
Eas - acing wall was ound bes due o he
p edominan ly wes e n winds in Edinbu gh;
4. Accessibili y: The e should no be ees o o he
occlusions p e en ing he sa e use o he UAV and
occluding he su eyed scene. The Eas -ga de n
does no include any ees.
3.3. Pe o mance Assessmen C i e ia
The pe o mances o he ou 3D eali y cap u e sys ems
o he eco ding o he 3D geome y o s one walls we e
assessed and compa ed using se e al quan i a i e and
quali a i e c i e ia.
Da a Comple eness: his is measu ed by he densi y
and uni o mi y o da a ac oss he acqui ed su ace.
E iciency: his is measused by he ime equi ed o
acqui ing and p e-p ocessing he da a o ob ain a uni ied
geo- e e enced dense 3D poin cloud.
Da a Accu acy and P ecision: aking he mos
accu a e and p ecise TLS da a as ‘g ound u h’, da a
accu acy and p ecision is measu ed by he ‘dis ance’ o
he o he da ase s o his g ound- u h da ase . I is
p oposed o measu e he ‘dis ance’ be ween wo poin
clouds using he Hausdo dis ance me ic (Hu enloche
e al. 1993). Fo his, he olume con aining he wall da a
is di ided in oxels (i.e cuboids); we use oxels o size
1cm x 1cm x 10cm. The la ge oxel side co esponds o
he dep h (pe pendicula o he wall plane); i is se la ge
o ensu e ma ching poin s a e ound despi e possible
global misalignmen s. No e ha he Hausdo dis ance
di e s om he s anda d poin - o-poin dis ance ha
simply conside s he closes poin s in he wo poin
clouds. In cons as , he Hausdo dis ance is based on
he s udy o minimal and maximal dis ances be ween
poin clouds wi hin each oxel, which makes i mo e
sensi i e o noisy da a. In e es ingly, i also makes i
sensi i e o a ia ions in he uni o mi y o he cloud poin
densi y.
4. Da a Acquisi ion and Recons uc ion
S ages
A su ey ne wo k, composed o h ee geo- e e enced ie
poin s se wi h 150mm ci cula a ge s, was es ablished
wi hin he Eas ga den si e o geo- e e encing he TLS
da a. Un o una ely, such su ey ne wo k canno be
used o he geo- e e encing o PG da a. This is due o
he inabili y o he su eyo s (and he echnologies
employed) o simul aneously ocus he came a on he
wall and he ie poin a ge s loca ed in he ga den.
The e o e, a second su ey ne wo k based on a di e en
se o geo- e e enced a ge s had o be es ablished wi h
17cm checke boa d pa e ns ixed o he walls. Bo h
su ey ne wo ks a e shown in Figu e 1. All
measu emen s o he es ablishmen o he ne wo ks
we e conduc ed using a Leica MS50 mul is a ion.
To ensu e a ull co e age o he ga den, h ee lase
scans we e acqui ed wi h TLS-1, wi h a esolu ion o
1.6mm a a ange o 10m. The h ee scans we e hen
aligned using he TLS su ey ne wo k, esul ing in a
uni ied clean poin cloud con aining 374 million poin s,
including 180 million poin s om he ampa wall.
The same p ocedu e was applied o he da a acquisi ion
using TLS-2, wi h h ee scans acqui ed a simila
loca ions and wi h a esolu ion o 3mm a 10m. The inal
uni ied poin cloud con ained 87 million poin s o e all,
and 51 million poin s o he ampa wall sec ion. In his
case, colou in o ma ion has also been acqui ed using
he Fa o Focus 3D in e nal came a.
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Figu e 1: Poin cloud o he Eas ga den o C aigmilla Cas le.
The ampa wall is ma ked in ed. The TLS su ey ne wo k is
illus a ed in ed, and he PG ne wo k in yellow.
Using he PG-1 sys em, 260 pic u es we e aken om
many loca ions a ound he ga den and wi h di e en
o ien a ion. The dis ance o he came a o he walls was
kep o app oxima ely 5m, wi h a ew addi ional pic u es
aken a u he dis ances. While some o he images
we e acqui ed wi h he came a hand-held, o he images
(speci ically o he ampa wall) we e acqui ed wi h he
came a moun ed on he pole and igge ed using a
mobile phone app. An o e lap o 40-50% be ween
adjacen pho os was aimed o . The 3D econs uc ion
was conduc ed using Agiso Pho oScan .1.1.6,
deli e ing a dense poin cloud con aining 79 million
poin s o e all, including 51 million poin s o he ampa
wall sec ion. The PG-1 poin cloud was scaled and geo-
e e encing using he PG su ey ne wo k.
Using he PG-2 sys em, 460 pic u es we e acqui ed a
simila dis ances o he walls. The pic u es we e hen
p ocessed using he exac same p ocedu es as o he
PG-1 sys em, deli e ing a geo- e e enced and scaled
dense poin cloud con aining 34 million poin s o e all,
including 20 million poin s o he ampa wall sec ion.
The econs uc ion p ocess used o bo h
pho og amme ic econs uc ions (PG-1 and PG-2)
in eg a es he op imisa ion o he came a in e nal
calib a ion, and his p ocess is ca ied ou using as ini ial
alues hose p o ided by he manu ac u e s (EXIF
in o ma ion).
The poin clouds co esponding o he ampa wall o all
hese econs uc ions a e shown in Figu e 2. No e ha
he colou in o ma ion was acqui ed wi h he TLS-2
sys em bu no TLS-1 sys em. None heless, colou
in o ma ion appea s o signi ican ly be e quali y o he
PG sys ems.
5. Resul s
5.1. Da a P ecision and Accu acy
Fo he epo ing and analysis o he esul s on he
accu acy and p ecision o he sys ems, we ocus on
h ee a eas o he ampa wall ha we ound we e
ep esen a i e in highligh ing he s eng hs and
limi a ions o he di e en su ey echnologies ega ding
accu acy and p ecision. These a eas a e shown in
Figu e 3. A ea 1 is an uppe pa o he wall, a 5 me e s
heigh and i s dimensions a e 4m x 2m. A ea 2 is loca ed
jus below a human heigh and has simila dimensions.
Finally, A ea 3 co e s a e ical egion, wi h dimensions
2m x 6m.
a)
b)
c)
d)
Figu e 2: Wall econs uc ion o he employed sys ems.
(a) TLS-1, (b) TLS-2, (c) PG-1, (d) PG-2.
Figu e 3: The h ee a eas wi hin he Eas ampa wall ha a e
used o he assessmen o pe o mance in e ms o accu acy
and p ecision.
As p e iously men ioned, all he da ase s we e geo-
e e enced by means o su ey ne wo ks. Gi en he
ma ches be ween he ie poin s in he su ey ne wo ks
and he co esponding poin s in he acqui ed poin
clouds, he geo- e e encing e o s a e calcula ed o all
de ices and summa ised in Table 1.
These esul s, oge he wi h he be e speci ica ions o
he TLS-1 de ice in compa ison wi h TLS-2, lead o he
conclusion ha TLS-1 indeed p o ides he da a wi h he
highes quali y o e all and so can be used as g ound
u h agains which all o he da ase s can be compa ed.
18

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Table 1: Mean esidual e o s o he geo- e e encing o he
di e en de ices.
De ice Value (inch)
TLS-1 1.53
TLS-2 3.73
PG-1 19.8
PG-2 18.1
Figu e 4 illus a es he esul s o he Hausdo dis ance
calcula ions o he h ee selec ed a eas o he ampa
wall, and Table 1 summa izes he mean and s anda d
de ia ions o he Hausdo dis ance o each case. As
can be seen, he Hausdo dis ances a e smalle o he
Fa o TLS wi h a mean below 5mm in bo h A eas 1 and
2. The mean dis ance o he PG-1 poin cloud is only
sligh ly highe and is hus ema kably good. In con as ,
he esul s a e disappoin ing o he PG-2 poin cloud,
wi h a mean be ween 10 and 20mm. The esul s a e
pa icula ly poo o A ea 1. Bu , we iden i ied ha his
was due o he insu icien amoun o o e lap be ween
neighbou ing pic u es acqui ed by he PG-2 sys em. The
easons o he poo pe o mance o PG-2 a e discussed
u he in Sec ion 5.4.
A ea 3 was selec ed o assess he a ia ion o he
accu acy and p ecision wi h heigh o he di e en
sys ems. I is in e es ing o no e he ela i e uni o mi y o
he Hausdo dis ances along he heigh o he wall o
he PG-2 da a which highligh s he ad an age o UAV in
e ms o access, he eby ensu ing consis en esul s. Fo
he pole-moun ed PG-1 sys em, he dis ances inc ease
mo e signi ican ly a he bo om, because ewe images
we e aken o he lowe pa s – he ope a o was ocused
on he co e age o he uppe pa o he wall ha is less
accessible and did no ealise ha an insu icien numbe
o images would be aken o he lowe pa (e.g. aking
pic u es while on hei knees). Howe e , he heigh o he
wall (10m) seemed no o ha e been qui e su icien o
show a simila pa e n in he op pa o he wall esul ing
om he limi ed ex ension o he pole. This sugges s ha
e en wi h a 5m pole, good-quali y da a can s ill be
ob ained up o almos wice ha heigh .
O e all, hese esul s con i m ha TLS de ices p o ide
mo e p ecise and accu a e esul s han pho og amme ic
sys ems. Bu , he cheape pole-moun ed PG-1 sys em
demons a ed compa a i ely good esul s, meaning ha
such a sys em can ce ainly be conside ed as a eliable
al e na i e o he su ey o s one walls.
Table 2: Mean (µH) and s anda d de ia ion (σH) o he Hausdo
dis ance o he TLS-2, PG-1 and PG-2 poin clouds agains he
TLS-1 poin cloud, o he h ee selec ed A eas o he ampa
wall.
De ice A ea 1 (mm) A ea 2 (mm) A ea 3 (mm)
µH σH µ
H σH µ
H σH
TLS-2 3.67 1.06 4.29 0.99 5.23 1.66
PG-1 4.36 1.60 5.09 1.95 6.14 2.8
PG-2 17.62 10.18 10.79 5.62 7.76 3.04
5.2. Da a Comple eness
Pe o mance canno be solely judged on da a p ecision
and accu acy. Ano he impo an c i e ion is da a
comple eness, ha assesses da a poin densi y and i s
uni o mi y. Table 3 summa izes he poin densi y in he
poin clouds ob ained o he en i e ampa wall by
means o he ou di e en sys ems. Poin densi y is
calcula ed by p ojec ing he econs uc ed poin s on he
wall plane and hen calcula ing he densi y o poin s
wi hin ha plane. As can be seen, he densi y is highe
o he lase scanne s, bu again we no e ha he
di e ence be ween he TLS-2 and PG-1 sys ems is
eally no ha signi ican , which u he con i ms he
po en ial o PG-1 sys em as an al e na i e 3D eali y
cap u e echnology.
Table 3: The maximum densi y (MaxD), mean densi y (µD) and
s anda d de ia ion (σD) o he poin clouds o he en i e ampa
wall ob ained by he ou eali y cap u e sys ems.
De ice MaxD (p s/cm
2
) µD (p s/cm
2
) σD (p s/cm
2
)
TLS-1 164 75.94 11.67
TLS-2 49 22.99 3.88
PG-1 31 16.09 3.05
PG-2 23 7.54 2.29
5.3. P ocess E iciency
An addi ional c i e ion o compa ing pe o mance
be ween TLS and PG sys ems is e iciency. This is
de ined as he ime equi ed o acqui ing and
p ocessing he da a o ob ain a uni ied geo- e e enced
dense 3D poin cloud. Acquisi ion ime comp ises he
posi ioning o he de ices a ound he ga den and he
acquisi ion o poin s clouds ( o TLS de ices) o pic u es
( o PG sys ems). P ocessing includes da a ans e ,
egis a ion/geo- e e encing, colou isa ion and da a
cleaning ( emo ing spu ious da a).
Table 5 shows he app oxima e ime eco ded o each
ask o each su ey sys em. No e ha he p ocessing
ope a ions ha e all been unde aken on he same
compu e (i7 3.60GHz p ocesso and 12 GB RAM).
Rega ding he acquisi ion s age, he scanning ime
appea s signi ican ly sho e o he TLS-1 han o he
TLS-2. Bu , his is essen ially due o he ac ha no
colou in o ma ion was acqui ed du ing he TLS-1
scanning. I colou acquisi ion had been conduc ed, he
acquisi ion imes would likely ha e been simila . An
impo an di e ence is no iceable be ween PG-1 and
PG-2. This is due o wo ac o s. Fi s , he longe ocal
leng h selec ed o he PG 2 sys em led o he need o
acqui e many mo e images wi h he PG-2 sys em ( o
ensu e su icien o e lap be ween neighbou ing images).
Then, he UAV could only wo k o 10-minu e pe iods a
a ime, a e which he UAV had o be landed o change
i s ba e y be o e esuming wo ks. Combined, hese wo
ac o s esul ed in a signi ican ly longe acquisi ion ime
han expec ed o he PG-2 sys em. The analysis o he
pe o mance o he PG-2 sys em is u he discussed in
Sec ion 5.4.
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TLS-2 PG-1 PG-2
A ea 1
A ea 2
A ea 3
Figu e 4: Hausdo dis ance be ween Leica ScanS a ion P40 poin cloud and he poin clouds o Fa o (le ), Nikon (middle) and Sony
( igh ) o he h ee selec ed A eas o he ampa wall.
P ocessing imes a e simila o all sys ems, despi e he
ac ha hey equi e di e en p ocessing s eps. The
main obse a ion is ha p ocessing imes a e in his
case s udy o en mo e han double he acquisi ion imes,
despi e he use o a high-pe o mance compu e .
Table 4: Acquisi ion and p e-p ocessing imes o TLS and PG
sys ems.
Acquisi ion P ocessing To al
TLS-1 40’ 3 h 3h 40’
TLS-2 1h 30’ 2 h 30’ 4 h
PG-1 30’ 3 h 15’ 3 h 45’
PG-2 1h 30’ 3 h 15’ 4 h 45’
5.4. Discussion o he Pe o mance o he
UAV-based PG Sys em (PG-2)
Reasonably good quali y esul s we e achie ed wi h he
PG-1 sys em, sugges ing ha in such con ex s
pho og amme ic sys ems can be conside ed as iable
al e na i es o TLS. I was hoped ha simila esul s
would also be achie ed wi h he PG-2 sys em ha would
ha e demons a ed i s addi ional ad an age in e ms o
access p o ision. Ye , he esul s achie ed wi h he PG-
2 sys em we e compa a i ely e y poo and
disappoin ing o e all. I mus howe e be quali ied ha ,
in some a eas o he wall, he esul s ob ained wi h he
PG-2 sys em we e as good (i no be e ) han hose
ob ained wi h he PG-1 sys em. This sec ion aims o
discuss he easons o hese poo esul s obse ed in
ce ain a eas, highligh ing he ac o s ha con ibu ed o
ge ing hose poo esul s, and how hey could in ac
ha e been ob ia ed. This shall demons a e ha UAV-
based PG sys ems do ha e g ea po en ial o he su ey
o s one walls, as long as he iden i ied ac o s a e
adequa ely managed.
The local a iabili y in he quali y o he econs uc ion
ob ained wi h he PG-2 has highligh ed ha (1) came a
loca ions, and (2) came a p ope ies and se ings can
sign ican ly impac he pe o mance o hose sys ems.
PG-1 and PG-2 ac ually used di e en came a se ups
and came a posi ioning me hods, which we belie e
al oge he explain he poo e esul s achie ed by PG-2.
Fi s ly, Table 5 shows ha he came a se ings o PG-1
and PG-2 di e ed in se e al ways. The PG-2 sys em did
no use en i ely manual se ings, which led o images
wi h a ying ISO and F-s op alues. Bu , an impo an
di e ence is no iced in he lens’s ocal leng h. The ocal
leng h o PG-2 is h ee imes la ge han ha o PG-1,
which implies ha he wall a ea co e ed by each pic u e
aken by he PG-2 sys em was signi ican ly smalle . The
g ound sampling dis ance (GSD) o each sys em can be
ob ained om he pa ame e s p esen ed in Table 5. Fo
PG-1, GSD=2mm; o PG-2, GSD=0.5mm. To ensu e a
su icien amoun o o e lap be ween pic u es, a
signi ican ly la ge numbe o images was hus
necessa y. Al hough wice mo e pic u es we e indeed
acqui ed using he PG-2 sys em compa ed o PG-1, his
numbe appea s o s ill ha e been insu icien . Indeed,
he poo quali y o he econs uc ions achie ed in he
local a eas o he ampa wall A ea 1 and A ea 2 isible
in Figu e 4 appea s o di ec ly co ela e wi h a lowe
o e lap be ween images acqui ed wi h he UAV in hese
a eas, as can be seen in Figu e 5.
20
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Table 5: P ope ies and se ings o he came as used in he wo
PG sys ems.
Se ings PG-1 PG-2
Size (px) 7360 x 4912 7360 x 4912
Senso (mm) 35.9 x 24 35.9 x 24
Focal leng h (mm) 14 35
F-s op /8 4- /8
ISO 250 100-400
Shu e (s) 1/125 1/500
Numbe o pic u es 260 460
A ea o wall co e ed by he
came a a 5m (m)
15.5 x 10.3 4 x 2.7
Figu e 5: Came a loca ions (in blue) o he acquisi ion o da a
wi h he PG-2 sys em.The h ee a eas o he wall used o he
pe o mance assessmen a e shown in ed.
The PG 2 sys em ope a o s may ha e expec ed ha , by
acqui ing mo e images wi h a lens ha ing a longe ocal
leng h, be e esul s would be p oduced. Bu , hey
unde es ima ed he ull impac o a longe ocal leng h on
he numbe o images needed o be acqui ed. In his
case, i seems ha hey should p obably ha e acqui ed
wice mo e pic u es han hey ac ually ha e (which would
ha e u he nega i ely in luenced he acquisi ion ime).
Na u ally, o he ac o s may ha e in luenced he esul s,
such as he a iable F-s op and ISO alues. Bu , we
ound he PG-2 pic u es o be o quali y simila o ha o
he PG-1 sys em. This sugges s ha , hanks o hei
mode n gimbals, UAVs can p o ide a eliable pla o m
o he acquisi ion o images o pho og amme ic
pu poses.
O e all, we belie e ha signi ican ly be e esul s (a
leas as good as PG-1) would ha e been ob ained by
using a lens wi h sho ocal leng h (14-16mm) on he
UAV-moun ed came a (PG-2). A mo e gene al lesson
d awn om his wo k is ha ca e ul planning should be
conside ed o selec he app op ia e came a se ings and
pic u e densi y (and loca ions) o ensu e accu a e and
eliable pho og amme ic econs uc ion. This p ocess
may be e e ed o as: Planning o Pho og amme y
(P4P).
6. Conclusions
Te es ial lase scanning and pho og amme y a e
inc easingly used o building su eying, p o iding dense
ex u ed 3D poin clouds (and meshes). In e ms o
geome y, da a om TLS was ound pa icula ly
accu a e. While his was expec ed (since i had been
shown by p e ious esea che s), in he case s udy
conside ed he e he Fa o Focus 3D scanne poin clouds
we e close o he Leica ScanS a ion P40 ones. This
simila i y is in e es ing, al hough i mus be emembe ed
ha hese esul s a e ob ained a a he sho anges
(maximum 10m) and he pe o mance o he Fa o
scanne would be expec ed o de e io a e as e a la ge
dis ances.
While TLS clea ly p o ides good geome ic da a o
gene a e accu a e and aluable 3D models, i also has
h ee impo an limi a ions. Fi s , TLS de ices emain
ela i ely expensi e ( ens o housands o pounds pe
uni ). Also, he came as embedded wi hin he scanne s
do no p oduce good quali y colou in o ma ion
compa ed o wha is achie ed wi h common DSLR
came as. And inally, bu s ill e y impo an ly, cu en
TLS de ices ha e o be ope a ed om s able posi ions,
which educes hei mobili y and he ange o con ex s
wi hin which hey can be eliably employed.
In con as , mode n digi al came as a e ela i ely cheap
and e y po able, making hem sui able in a wide ange
o con ex s. Howe e , hei main limi a ion is hei
a iable pe o mance depending on he le el o ex u e
in he scene being econs uc ed. None heless, we ha e
shown ha scenes like s one walls p esen g ea
ex u es o he applica ion o pho og amme y. In such
con ex s, pho og amme y can cons i u e a ealis ic
al e na i e o TLS.
Moun ing a came a on a UAV can u he sol e access
issues. A cop e - ype UAV can ly a ound close o
buildings and ake pic u es om di e en iewpoin s
wi hou he need o any addi ional in as uc u e (e.g.
sca olding). This heo e ically ex ends he applicabili y o
pho og amme y o signi ican ly mo e con ex s (e.g.
wi hou occluded a eas, as was expe ienced wi h TLS).
Un o una ely, his esea ch was no able o ully
demons a e his. Howe e , he analysis o he ac o s
ha led o he disappoin ing esul s ob ained wi h he
UAV-moun ed PG sys em we e ‘simply’ an inapp op ia e
selec ion o lens and a co espondingly insu icien
numbe o images acqui ed. The esul s showed he
need o pho og amme ic su ey eams o ha e a good
unde s anding o he pho og amme ic p ocess o
es ablish e ec i e da a acquisi ion p ocedu es. This
sugges s he need o es ablish obu s me hods o
Planning o Pho og amme y (P4P), which should be
he ocus o u u e in es iga ions. Bu , he esul s
epo ed he e should no de e p o essionals om
conside ing his o he wise aluable echnology ha
could signi ican ly educe he cos and sa e y isks
associa ed wi h access p o ision, in compa ison wi h
cu en solu ions used o manual su ey as well as he
use o TLS sys ems.
Beyond he echnical conclusions p o ided, his wo k
also highligh s ha mixed adop ion o echnologies o
speci ic echnologies u ilised o su ey a e po en ially
equi ed. Thei selec ion will depend upon weigh ed
conside a ions o da a p ecision, acquisi ion and
p ocessing e iciency, and heal h and sa e y g ounds.
The wo k shows ha i is impo an ha su eyo s
a emp o be e unde s and he p os and cons o
echnologies adop ed and ealise ha some will pe o m
be e han o he s in di e en con ex s. I s esses ha
be e dialogue be ween adi ional su eyo s and
a chi ec s and specialis echnologically-o ien ed su ey
ope a i es is equi ed so ha be e unde s anding o
expec a ions can be achie ed by bo h pa ies.
21
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Acknowledgemen s
The w i e s a e g a e ul o His o ic En i onmen Sco land
(HES) o unding his p ojec , and Cybe hawk
Inno a ions L d. o conduc ing he UAV-based su ey.
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