Page 1
LiDAR in Lahar Mapping
David Napier,
Dr Vernon Manville
NZ Aerial Mapping Limited
Hastings, New Zealand
The accuracy and repeatability of LiDAR surveys is now entering the legendary arena; can this help us in the world of volcanic hazards?
At Mt Ruapehu, in the central North
Island of New Zealand, a series of eruptions
in 1995 and 1996 expelled the
summit Crater Lake, generating a
sequence of eruption and later raintriggered
lahars. Early in 1996, the
potential for a significant Crater Lake
break-out lahar was identified as the
Crater Lake began to refill behind a
fragile barrier of volcanic material, or
tephra, deposited on the hard rock rim.
Lahar is a fast flowing mixture of rock
debris, sand, silt and water (other than
normal stream flow) originating from a
volcano, along a river valley. It has the
consistency of concrete: fluid when
moving, then solid when stopped and
can be extremely dangerous.
The threat of such a lahar was taken
very seriously as the last time this situation
occurred, in December 1953, a
major lahar destroyed the Tangiwai
Rail Bridge crossing the Whangaehu
River at the very moment a train was
crossing it. Since then, lahar monitoring
equipment has been installed 14km
upstream of the bridge to ensure a
repeat does not happen.
The Whangaehu valley is one of the
most active lahar channels in the
world. In addition, eruption-triggered
lahars have also passed through New
Zealand's largest and biggest ski area
on the northern slopes of the volcano.
As the lake rose above the base of the
tephra dam in early 2007 it became a
matter of when, not if, the lahar would
occur. The worst possible scenario in
these circumstances was sudden collapse
of the new tephra dam causing a
lahar as large as or bigger than the 1953
Tangiwai lahar.
Nowadays, Mt Ruapehu and the sister
peaks of Ngauruhoe and Tongariro are
constantly monitored by GNS Science,
based at the Wairakei Research Centre,
just north of Taupo in an area of abundant
geothermal activity. The GeoNet
team has installed a comprehensive
suite of sensors around the mountains
to monitor volcanic activity.
One component of installing a dedicated
a lahar-monitoring system was
the need to produce a highly accurate
3D map of the predicted lahar path
along the upper Whangaehu River both
before the lahar occurred, and most
critically immediately afterwards. Specific
requirements of the data needed
led the researchers to consider the use
of LiDAR equipment. The pre lahar data
was flown on 18th February 2006 as a
joint operation between Fugro Spatial
and NZ Aerial Mapping Limited using
Fugros Leica ALS50 LiDAR unit mounted
in NZ Aerial Mappings Cessna 402b.
Data from this survey was processed by
Fugro and supplied to GNS.
Just over one year later, the tephra
dam failed on the morning of 18th
March 2007, releasing 1.3 million cubic
metres of warm acidic lake water. The
lahar passed Tangiwai, 40 km downstream
within 2 hours and reached the
coast 155 km away in the early hours of
the following morning. Thanks to a
detailed inter-agency response plan
and the ERLAWS lahar warning system
installed by the Department of Conservation
no lives were lost and infrastructural
damage was minimal.
Within less than three weeks of the
lahar the post-lahar survey was performed,
on 6th April 2007. The post
lahar data was captured with an
Optech ALTM 3100EA again mounted
in Cessna 402b, both owned and operated
by NZ Aerial Mapping Limited.
Operationally, the project was rather
difficult from two quite diverse angles.
Firstly, the area of survey spanned
more than 2,000m of vertical relief
requiring very accurate flight planning
and flying to ensure correct overlaps
and point density were achieved.
Additionally, as the area is situated
within an Military Restricted zone, formal
clearances were required to fly
within it. To complicate matters further,
at this time a major international
exercise was taking place which
required the highest levels of cooperation
and coordination between the Military,
Weather Office and Aircrew.
In addition to the LiDAR data, digital
imagery was also captured by the cosited
medium format camera. This
imagery provided both checking
imagery for the LiDAR classification
and also a set of orthophoto images for
eventual supply. For the processing, the
LiDAR sensor positioning and orientation
(POS) was determined using the
collected GPS/IMU datasets and
Applanix POSPac software. This work
was all undertaken using NZGD2000
coordinate system.
The POS data was combined with the
LiDAR range files and used to generate
LiDAR point clouds in NZTM map
projection but NZGD2000 ellipsoidal
heights. This process was undertaken
using Optech REALM LiDAR processing
software. The data was checked for
completeness of coverage then the relative
fit of data in the overlap between
strips was checked. The point cloud
data was then classified into ground,
first and intermediate returns using
automated routines tailored to the project
landcover and terrain. This and subsequent
steps were undertaken using
TerraSolid LiDAR processing software
modules TerraScan, TerraPhoto and
TerraModeler. The data was converted
from NZGD2000 ellipsoidal heights
into Moturiki 1953 vertical datum using
the LINZ NZGeiod05 separation and offset
model. Comprehensive manual
editing of the LiDAR point cloud data
was undertaken to increase the quality
of the automatically classified ground
point dataset. Independent of the aerial
acquisition work, Opus International
Consultants field surveyed a series of
check sites in open ground, to be later
used to verify the accuracy of the
processed datasets. Sites were chosen
outside of the lahar flow path. The
height accuracy of the processed data
was checked using the provided check
site data. This was done by calculating
height difference statistics between a
TIN of the LiDAR ground points and the
checkpoints. The positional accuracy of
the processed data was checked visually
by overlaying the check site data
over the LiDAR dataset displayed with
its intensity values. The data was found
to fit well in position.
LiDAR data was processed for delivery
into two main sets of data: thinned and
unthinned. The thinned dataset contained
ground classified points only
of the entire area and was made up
of approx. 45 million points. The
unthinned dataset was divided into:
First of Many, Intermediate, Only & Last
of Many and Water Point Cloud and
consisted of approx. 87 million points.
All the LiDAR data was supplied to
GNS as ASCII XYZ files whilst orthophotography
tiles at 0.21m GSD were supplied
as TIFF/ESRI TFW files. Analysis of
the pre and post lahar data sets has provided
a never before insight into the
behaviour and outcome of this lahar.
Most significantly for this particular
event was the realisation that the lahar
was approximately 25% larger than the
1953 Tangiwai lahar.
For further information please visit
http://www.gns.cri.nz/what/earthact/volcanoes/nzvolcanoes/ruabookprint.htm