The Bridge - Issue 1, 2023 - 14

Feature
Ground-based Lidar for Historic Preservation, Increased Accessibility, and Virtual Tourism
at 50 m; 6 mm at 100 m. The Leica P40 is set up similarly
to how one sets up a total station by placing the device
on a tripod and leveling the instrument using a tribrach.
Depending upon the selected data-gathering mode, the
instrument may be set up over a known point on the
ground. Ground-based Lidar technology, used for historic
preservation and many other applications such as forensic
investigations, bridge repair, and interior design to name a
few, continues to improve in speed capture and accuracy.
We will discuss some of the newest instruments in Section
V, A Scanner for Every Task.
II. Common Field Methods for
Historic Preservation
Free, Traverse, and Resection are three standard techniques
for gathering data with the Leica P-series scanners. When
scanning in Free Mode with no known points, orientation,
or scanner height, the scanner is positioned in the desired
location to gather the maximum data with minimal hidden
features. This point is also selected with the next survey
location in mind to ensure at least 30% overlap of the
two scans to facilitate registering the scans to each other
post-acquisition. The scanner is leveled, the scan and
photographs acquired, and then the scanner is moved to
another unmarked location with no azimuthal orientation.
The following scan is initiated after leveling is completed.
This mode minimizes field time but requires additional time
post-acquisition to register all scans to make a single
point cloud.
Traverse mode uses marked points on the ground (or
on the floor) and targets to link all points together while
acquiring scans. While using this mode, the first scan
location is identified as a primary site datum, a location
unlikely to move. Surveyor pins or markers are ideal, but
metal access covers, edges of sidewalks, or other fixed
features will work. Along with leveling the scanner, it is
aligned over the marked point using a laser plummet,
and the scanner height from the known point is entered
into the system. The primary site datum may have a
known coordinate such as latitude, longitude, or Universal
Transverse Mercator (UTM) used for the project. A local
survey grid system is established by assigning 1000.000
m x 5000.000 m x 500.000 m for Easting, Northing, and
Elevation. This local grid system can be converted to a
universal system post-acquisition if GPS data are collected
for these points. A second point is then chosen to define
the azimuth of the local survey grid. A target is placed over
the second point and leveled. The P40 acquires the target
similar to how one would use a total station and after
the acquisition, the instrument sets the azimuthal angle
for point one and records the location of point two in the
local or universal grid system. After acquiring the scan and
photographs at the first location, the scanner is moved to
the second location and the target to the first. Once leveled
THE BRIDGE
Figure 1: The Leica P40 in traverse mode backsighting to a prior point
using a target outside of the curtain wall at Trim Castle, Ireland [6].
and over the second point, a backsight to the first location
is conducted to align the scanner back into the survey
grid. Scan, photograph, set out the next location, move,
backsight, and repeat. This field work is shown in Figure 1
at Trim Castle, Ireland.
In Resection Mode, multiple targets are set up over known
points. The scanner is placed over an unmarked point,
leveled, scanner height is measured, and the various targets
are selected and scanned to determine the scanner's
location in the local grid system. Recording targets while
in the field allows the scanner operator to know the level
of precision of the scanner's locale. The P40 has options
for four parameters, six parameters, or auto resection.
Alternatively, a scan can be conducted without resection,
and the multiple targets captured during scanning are used
post-acquisition as part of the registration process.
The two main workflows involve taking the time in the
field to register each scan location to the local survey grid
with very little processing post-acquisition or spending less
time in the area and registering each scan location back in
the laboratory/office. With recent improvements in Leica's
registration software (Leica Registration 360), the time to

The Bridge - Issue 1, 2023

Table of Contents for the Digital Edition of The Bridge - Issue 1, 2023