Arctron 3 D

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Case Study
3D High-Tech Aerial Prospection
• Airborne Laser Scanning - Applications
in Archaeology and Cultural Heritage
Karabalgassun, Mongolia – “The Orchon Expedition” 2007.
Airborne laser scan documentation of an entire city complex from the 8th/9th century CE (approx. 43 km2 ).
Commissioned by the German Archaeological Institute (DAI Bonn, Prof. Hüttel).
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Airborne Laser Scanning - Applications in
Archaeology and Cultural Heritage
During the last 10 years, airborne laser scanning has become well established
as an indispensable method of generating digital elevation models. Data models
such as these form the foundation for planning projects in a diversity of fields
such as the mining sector, water management, road construction and urban development.
In archaeology, this prospection method has only come into systematic use
within the last few years and has revolutionised the task of source data retrieval.
Terrain scanning enables detailed images of relatively large areas of landscape
to be generated in very little time. Eroded features which survive only as
low protrusions, such as settlements, ditch and rampart systems, tumuli, ancient
pathways or field borders, can be identified in the scan data. The high penetration
depth of laser scanning enables vegetation, in particular trees, to be filtered
out of the scans allowing historical sites hidden in wooded areas to be accurately
pinpointed.
The emergence of this technology has signified a revolution in the recording of
prehistoric and historic feature landscapes in forests. This huge advancement
can perhaps be likened to that occasioned by archaeological aerial photography
in the 70s.
The ArcTron 3D archaeologists, engineers and data analysts use their expert
knowledge of heritage issues and specialist software solutions to generate highprecision
surface models. During this process, previously-known and potential
or uncertain archaeological features are identified in the laser data and mapped
accordingly.
Vianden Castle (Luxembourg).
Left: 3D visualisation of
helicopter fly-over.
Right: 3D terrain model in
various views: With and
without vegetation and
buildings and with texture
data from RGB aerial photogrammetry
cameras.

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Titelberg (Luxembourg). Celtic Oppidum (city complex) with enclosing rampart system.
Top: This scaled, rectified orthophoto of the significant Celtic settlement was created using high-resolution aerial photogrammetry.
Bottom: Vegetation data is filtered out for the DTM. The enclosing ramparts – i.e. the collapsed hillfort walls – usually hidden under the trees, can be very plainly
seen in this model. Of further note are the many deep grooves in the ground. These proved to be collapsed mining galleries caused by iron ore extraction
(19th /20th century). Alterations in the mountain’s structure can damage preserved archaeological material. Re-scanning the area at intervals of several years
allows such changes to be reliably monitored.
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Technical Principle
In airborne laser scanning, scanning systems are integrated into aeroplanes, helicopters
or – as in our newly-developed cost-effective and flexible system for
archaeology and heritage – into a microlite paraglider.
Objects and terrain are recorded by emitting a fan of pulsed laser beams at specific
intervals. Current instruments send and receive up to 160 000 laser
measurements per second. The pulsed laser installed in the aircraft uses a rapidly
rotating mirror to scan strips of terrain which lie perpendicular to the direction
of flight and which can be up to several hundred meters wide. The laser beams
are reflected by the Earth’s surface, vegetation or man-made structures.
In this way, hundreds of millions of points can be surveyed in a very short time.
Measurements are made at the speed of light, namely, the time-of-flight difference
between the emitted and reflected laser beam.
Determining the exact position and angle of each individual laser impulse is a
complex task. It is achieved using a combination of two very different technologies;
inertial navigation (autonomous, spatial angle definition) and satellite navigation
(GPS – Global Positioning System, localisation using a network of satellites).
During post-processing, angle and time-of-flight calculations are used to separate
captured laser data into individual points with 3D world coordinates.
ALS technology can be integrated
into a variety of aircraft.
For archaeology and heritage
tasks, however, only helicopter
and microlite aerial surveys are
really suitable, due to the higher
point density they achieve.

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Seamless Data Models
In the next stage of processing, a seamless model is generated from the separate
scan strips.
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Relative calibration (alignment of elevation differences): Neighbouring flight
strips must correspond exactly in position and elevation. Depending on the
times at which the recordings were made, differences in GPS satellite constellations
can lead to considerable errors of position and elevation. Neighbouring
strips must, therefore, be inspected and adjusted where necessary,
particularly if they have been recorded at significant intervals.
Vianden Castle (Luxembourg).
Left: This 3D terrain model, generated from airborne laser scan data, has been blended with additional terrestrial laser scan data
Right: High-resolution 3D orthophotos of surveyed monuments can also be generated as a final result.
Conversion of individual flight strips into a regular grid and determination
of the final grid width: To do this, a tight, rectangular grid with variable grid
width (0.25m–100m) is created from the laser measurement values. The
high penetration of vegetation-covered areas allows two separate, surfaceconsistent,
high-precision digital elevation models to be produced:
• DSM (Digital Surface Model): Surface model with vegetation and
buildings
• DTM (Digital Terrain Model): Ground model without vegetation or
buildings

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Recording Elevation Data
Modern laser scanners perform up to 160 000 distance measurements per
second. The survey point density is constant, subject to system configuration,
selected altitude and flight speed. Depending on the aircraft in use, approximately
4 laser points (aeroplane) or 10-15 points (helicopter) are measured per
square metre. It is also possible to measure up to 50 points per square metre for
special tasks. This high survey density ensures that even fairly small structures
such as drainage ditches or surface fractures can be reliably recognised.
Moreover, the high density of data guarantees that building contours can be
localised extremely accurately. A standard terraced house with an 80m 2 footprint
would be recorded using around 400-800 measurements and described
by 80-160 elevation values in a 1m grid.
Currently, digital elevation models generated from airborne laser scanning data
are created with grid widths of 1m or better and vertical resolutions approaching
the sub-decimetre range. The models’ horizontal positional accuracy is better
than ±0.2m (for every point) and the absolute vertical accuracy (referenced to
the local geoid) better than ±0.15m (valid for 95.7% of all grid values).
Vianden Castle (Luxembourg). 3D models (DTM/DSM).
The scanner data is processed to produce elevation models which precisely describe
the terrain’s surface (top edge of vegetation, roofs etc.). Elevation
models of this kind are often referred to as DSMs or Digital Surface Models. In a
further stage of processing, taller vegetation and buildings are automatically removed
on computer, producing elevation models of the Earth’s surface, known
as DTMs or Digital Terrain Models.

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Basic Data Processing
Post processing of the scanned data is carried out in a number of stages, some
automatic and some manual. The complexity and duration of data processing
depends on the size of the project and the desired results:
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Synchronisation of laser data and INS/(D)GPS positional data using the
GPS time stamps (UTC) present in both datasets.
Data reduction: When surveying with high scanning frequencies, adjacent
scan strips lie only around 0.1m apart in the direction of flight. Therefore,
the data is now inspected for incorrect measurements and reduced
using statistical procedures and plausibility checks. The type of reduction
employed (first or last pulse) depends on the intended application: If exact
terrain models (DTM) are required, higher distances are selected; for
surface models (DSM) on the other hand, lower distance measurements
are selected. For specialist evaluations (e.g. texture analysis),data reduction
can be omitted from this phase.
Geocoding: The positions of reflecting objects on the Earth’s surface are
calculated from the time-of-flight difference and the position and orientation
of the sensor. The results are transferred into the target coordinate system
with simultaneous geoid correction. All survey data is then available in the
desired coordinate system ( e.g. Gauss Krueger, meridian line 9°, Bessel
Ellipsoid, Potsdam Datum, NAP).
Further coordinate transformations: If no general transformation parameters
are available, local transformation parameters can be calculated using
at least seven known points. These points must be known in both WGS84
and the local coordinate system.
The result of this procedure is an ASCII file (xyz values) containing the
measured elevations of every survey strip as metric target coordinates.
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Aerial Survey
Areas selected for surveying are flown over and scanned in
a pattern of parallel strips, overlapping perpendicular to the
direction of flight. All the data required for subsequent evaluation
is recorded during this process. During aerial surveys,
the data detected by all system sensors (GPS, INS,
laser scanner, RGB camera) is recorded simultaneously.
Differential GPS data is recorded by at least one ground
station for subsequent reconstruction of the exact flight
path.
Karabalgassun, Mongolia “The Orchon Expedition” 2007.
Left: An MI 8 helicopter was used for aerial surveying. Right: FH Karlsruhe (technical college) provided GPS reference values using a ground station.
Karabalgassun, Mongolia. “The Orchon Expedition” 2007. Various views of the city’s central area with the palace and temple complex.
The remaining above-ground structures were badly eroded but could be accurately filtered and extracted from the 3D data.
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Filtering Data to Generate 3D Terrain
Models
The result, at this stage in processing, is a precise surface model (DSM) in the
target coordinate system and with the target grid size (usually a 1m grid).
In densely-wooded areas, however, the DSM will still contain residual vegetation
and buildings which would have to be removed to generate a terrain model
(DTM). If required, the DSM can be refined to produce a DTM.
Vianden Castle (Luxembourg). Technical principle.
During laser scanning, some laser beams will reach the ground allowing a terrain model to be generated.
Aerial surveys of forested areas are mostly carried out in periods of reduced vegetation to guarantee the best
results.
One of several methods can be used to remove residual vegetation and buildings,
depending on local circumstances. It should be mentioned that, as a rule,
low-growing vegetation (lower than 1.5m) is very difficult to differentiate automatically
from natural surface effects. Generally, around 80% of vegetation can
be automatically removed using filter algorithms, the remainder must be re-
moved interactively during post-processing.
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Additional Data Products
Further data products can be derived from digital elevation models generated
using airborne laser scanning, including:
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Scaled, georeferenced cross sections of any length (approx. 200 sections
per hour)
Variable digital grids: 1 m, 2 m, ... 5 m, ... 10 m etc. for rapid data viewing
and editing
Simulations of rising water levels (e.g. in meter intervals)
Calculations of isolines with variable intervals (e.g. 0.1m, 0.5m, 1m …)
Calculations of volume & mass with variable planning horizons
Monitoring and industrial safety surveillance
Efficiency control
Planning data and environmental impact analysis for authorisation
procedures etc.
Clemency (Luxembourg).
Airborne laser scan data from a previously-unknown group of Iron Age tumuli. The badly-eroded tumuli lie in a forested area (left). They are marked with three
white points (in the top right of the picture).Their large diameters (> 20m) and low height (< 0.5m) make them almost unrecognisable, even on site in the forest.
In the coloured, finely-graduated evaluation, however, (bottom right) they are immediately apparent (yellow). After the tumuli group was discovered during air-
borne laser scanning, an archaeological excavation led by Dr. Jeannot Metzler provided evidence of an early Iron Age barrow cemetery.
Bottom right graphic: Dr. Andreas Schäfer, University of Jena, Chair of prehistory and early history.

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Specialist Archaeological Analysis
ArcTron 3D specialises not only in archaeological research and services but
also in the analysis of terrain models according to archaeological criteria. Our
historical scientists, IT specialists and engineers work hand in hand to extract,
analyse and structure evidence of archaeological remains from scan data. The
application of full waveform filters, which analyse the entire spectrum of
airborne laser scanners, enables us to produce excellent results, especially in
heavily forested areas.
Aalen-Wasseralfingen (Baden-Wuerttemberg, Germany).
Documentation of a Hallstatt era barrow cemetery. Left: DSM (Digital Surface Model) located in light to heavy deciduous forest. Centre: DTM (Digital Terrain Model)
without vegetation. Right: Plan interpretation. The tumuli threatened by building measures were subsequently excavated by LAD Baden-Wuerttemberg (Dr. Krauße).
During archaeological plan evaluation and mapping, existing information is combined
with new survey data. Areas in which archaeological features are suspected
can be targeted and systematically searched in the plans. Archaeological
features identified and potentially-identified in the data are extracted by trained
engineers and prepared for on-site inspection, if required. The data is then converted
into standardised GIS/CAD maps.
For larger ALS projects, we also provide easy-to-use 3D software. This application
allows terrain model data to be accessed rapidly and easily and provides
further functions such as distance measurement, area calculation, generation of
cross sections through identified objects, printouts of detailed plans and visualisation
of superelevated 3D models.
ArcTron 3D have carried out a broad range of diverse projects for numerous
clients and developed new standards for the archaeological evaluation of ALS
data.
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A Selection of Project References...
Archaeological airborne laser scanning projects. Since 2005, we have carried out circa 50
projects in the field of archaeology and cultural heritage. Please ask us for a complete list of
references.
Vianden Castle (Luxembourg)
Area investigated: approx. 16 km 2
Bernstorf near Freising (Bavaria)
Area investigated: approx. 16 km 2
Ipf (Baden-Wuerttemberg)
Area investigated: approx. 12 km 2
Bartholomaeberg – Silbertal –
Gaflunatal, Montafon (Austria)
Area investigated: approx. 20 km 2
Roman Limes (Bavaria, Baden-Wuerttemberg)
and Odenwaldlimes (Hesse)
Total length: approx. 230 km
Karabalgassun, Mongolia
Area investigated: approx. 43 km 2
ArcTron
ArcTron 3D GmbH
Ringstrasse 8
D-93177 Altenthann
Germany
Tel.: +49 9408 8501 0
Fax: +49 9408 8501 21
Email: info@arctron.de
Web: www.arctron.com
The historical landscape surrounding Vianden Castle, topography of
the castle mountain, ancient path system and numerous further
features. Combined with terrestrial laser scan recordings. EU-funded
research project.
Bronze Age hillfort settlement with medieval staggered fortification and
surrounding archaeological landscape. Commissioned by: Kranzberg
borough and the University of Frankfurt a.M
Baden- Wuerttemberg’s “holy mountain”. A prehistoric hillfort with
complex rampart structures. Evaluation of existing aerial survey data
from the public surveying authority.
Commissioned by: Baden-Wuerttemberg heritage authorities.
Prehistoric hillfort and evidence of mining (galleries, shafts, waste
heaps). Combined with terrestrial laser scan recordings.
Commissioned by: University of Frankfurt a.M. - Montafon Research
Project (Prof. Krause).
Complete helicopter aerial survey and evaluation of the world
heritage site. Documentation of the Roman limes structure, peripheral
watch towers and fort complexes. Project duration 2007-2009.
Commissioned by: Heritage authorities in Bavaria, Hesse and Baden-
Wuerttemberg.
ALS documentation of an 8th/9th century CE city complex in the
Mongolian grass steppe. Commissioned by: The German Archaeological
Institute ( DAI Bonn, Prof. Hüttel).
Our partners for airborne laser
scanning: