266
May 2020
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
R
outescene
’
s
T
echnology
D
ata
C
ollection
Flythru used Routescene’s integrated UAV lidar system
for this task. Designed in 2014 for use on drones,
Routescene’s
comprises an array of sensors
including the Velodyne HDL-32 Lidar scanner. With a
pulse rate of up to 1.4 million points per second from
32 different lasers angled in a 40-degree field of view.
The HDL-32 has a maximum range of 110m, which is
benchmarked at 10% reflectivity on a white surface.
Given that natural features have a lower reflectivity,
the “usable” range is around 80m. This sensor
enables high resolution vegetation penetration
and helps users uncover ground features which
are not easily visible from the ground or which are
obstructed by thick vegetation.
The number of laser hits achieved on the ground
is up to 400pts/m2 through thick vegetation. Lidar
is an ideal method when surveying sites which are
overgrown or woods and forests that are inaccessible
using conventional survey methods. Without UAV
lidar it would have taken considerable time to
survey the Alderney sites. Due to the thick cover of
vegetation it would not have been possible to visually
detect the structures found, and the area was
surveyed by the UAV lidar team with just a couple of
15-minute flights.
S
peedy
D
ata
P
rocessing
Michael Mays’ team from Flythru,
,
processed the data on-site immediately after the UAV
survey using
, Routescene’s 3D processing
and visualisation software and presented it to Caroline
Sturdy Colls the same evening.
The filters used in an automated sequence were sector
reduction, laser ID reduction, coordinate conversion, grid
creation, the purpose-built “Bare Earth tool”, a skim grid
and finally a LAS export filter. Using the Bare Earth Tool
within LidarViewer the team quickly and easily removed
all the non-ground points to reveal the surface below and
produce a Digital Terrain Model (DTM).
The DTM exposed structural remnants of the camp,
and previously undiscovered features that had been
completely obscured by the vegetation covering the site.
The
Tool is newly available in the Routescene
LidarViewer software package. Designed specifically for
heritage applications like this, it considerably reduces the
amount of time it takes to digitally remove all features
above ground level.
O
ther
survey
techniques
In addition to using UAV lidar, Professor Sturdy Colls
also used basic photogrammetry using hundreds of old
Royal Air Force (RAF) photographs stitched together to
create an overall three-dimensional model.
Ground Penetrating Radar (GPR), which sends radio
waves into the ground and allows artefacts under the
ground to be detected, was also used. Overlaying all of
the results from the three different techniques provided
an accurate, highly detailed 3D digital model of the camp,
highlighting features of interest both above and below
ground for further investigation.
In the Smithsonian documentary, Adolf Island,
the results of the study helped recreate a digital
representation of the prison camp and identify the
location of possible grave sites.
Alderney ground cover.
Routescene LidarPod.
