The Bridge - Issue 1, 2023 - 16

Feature
Ground-based Lidar for Historic Preservation, Increased Accessibility, and Virtual Tourism
second, dramatically decreasing scan times. In 2017, the
P50 increased the P40's maximum distance of 270 m out
Figure 3 shows the resultant point cloud of Trim Castle in Trim, Ireland
merged into a spherical panorama using a program such as
PTGui and then exported as Cubed-faced images for import
into the Leica processing program, Cyclone. Common
points are manually selected in the point cloud and
photographs to give the software enough control points to
orient the Cubed-faced photographs with the point cloud.
Once oriented, the software assigns RGB values to the XYZ
points in a fashion similar to the onboard camera method.
Using the onboard camera takes longer in the field, with
less control over the image quality. However, it saves time
post-acquisition with the Leica software auto-registering
the images with the scan data. Taking photographs with
an external camera allows one to use a high-resolution
DSLR, a high-quality lens, and control all camera settings.
The resulting images may produce a noticeably better
colored point cloud, but the real advantage is the
photographs themselves. Along with the point cloud data,
the photographs are part of the documentation and can be
used to create 3D views displayed on websites for virtual
tourism. Note that digital photographs are currently not
acceptable for submission to the Library of Congress as part
of HABS documentation. Conversely, color digital images of
the documented resource are accepted and included in the
field notes accompanying documentation project materials.
V. A Scanner for Every Task
To understand the timeline of improvements to groundbased
Lidar instruments, Leica's 2006 release of their
HDS6000 is a good starting point [7]. With a range of
79 m at 90% reflectivity, a field of view of 360 x 310,
and an integrated computer with a control panel, the
HDS6000 phase-shift scanner looks similar to today's
scanners. The real breakthrough was the incorporation of tilt
compensators, which allowed for survey-grade data similar
to data captured with a total station. The C10's release
in 2010 improved upon the HDS6000 by moving to a
pulsed, time-of-flight system that achieved distances of 300
m at 90% reflectivity but came with a decrease to 50,000
pts per second. The P40 was released in 2015 with an
increase in operating temperature range, a distance of 270
m at 34% reflectivity, and achieving 1,000,000 points per
THE BRIDGE
to 1 km at 80% reflectivity.
In 2016, Leica released the BLK 360, and in 2018, the
RTC 360, to complement their P-series scanners. These
scanners are designed to be fast, lightweight, small, have
a scan distance of only a few tens of meters, and improve
onboard image quality. Scans are acquired through a onetouch
onboard screen or via a wirelessly connected tablet.
The RTC360 uses a Visual Inertial System where internal
sensors monitor the movement of the scanner from one
location to the next, with the software auto-locating the
scanner in a local grid system with no need to set out and
backsight points. The RTC360 has tilt compensation for one
axis and is limited to a 10-meter range.
A scan at Detailed Scan settings takes about two minutes,
and HDR images take an additional two minutes. Once the
first scan is complete, the scanner is moved to the next
location and set up without the need to level, measure
instrument height, or locate over a distinct point within the
local grid. The movement of the scanner can be monitored
on the tablet. The tablet displays the scans after automatic
transfer at the end of each scan and can be used to
further refine linking two scans together. This process is
accomplished by visually rotating and moving the scans.
The software then runs a registration algorithm to link
the two scans. The RTC360 brings improvement to the
hardware along with improved software.
Each scanner has its application, with a combination of
scanners ideal for historic preservation. While documenting
an entire city block in downtown Denver in the summer of
2022, the team used the P40 for exterior scans where the
precision of the two axes tilt compensation was needed out
to a distance of 130 m to document a 50 m tall apartment
building. Over seven days, the two scanners captured 660
scans for a total of 0.63 terabytes of data.
A recently acquired DJI Matrice 300 RTK with a Zenmuse
L1 Lidar scanner payload would have been ideal to
operate during the summer of 2022 to document a site
with buildings spread out in a larger landscape. The team
did not have the drone-based Lidar at the time, and it
took four days and 155 scan locations to document the
buildings and landscape. The first test flight of the Matrice
300 covered an area approximately an eighth the size of
the summer project. The total flight time was about eight
minutes, with three minutes of laser scanning time. In
the future, what took us four days on the ground will be
replaced by 30-45 minutes of drone flight time.
We also have a suite of structured light scanners that range
in resolution from 0.5 mm to 0.05 mm. These are used
for documenting fine architectural details, such as the
papier mache decorative ceiling at the Philipse Manor Hall
historic site in Yonkers, NY. These scanners are designed to
document small to medium-sized objects such as a desk.
This scanner and an RTC360, and a DJI Matrice drone are
shown in Figure 4.

The Bridge - Issue 1, 2023

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