A quick survey

Crew scans 24 T-REX bridges in 40 days

Bridges Article April 18, 2002
Printer-friendly version

Using Cyrax scanning technology, David Evans and Associates Inc. (DEA), Denver, surveyed 24 bridges in 40 days without disrupting traffic on the heavily traveled I-25 through the Denver metropolitan area. T-REX, the Transportation Expansion Project launched by the Colorado Department of Transportation (CDOT), includes highway expansions, improvements and the addition of light rail along 18 miles of I-25 and I-225. The $1.67 billion design-build project includes reconstruction of interchanges and bridges, a new drainage system and improved pedestrian and bicycle access. One of the
goals of the project is to accommodate the addition of a light rail along
sections of the interstate.

CDOT selected Southeast Corridor Constructors (SECC),
Denver, for the project, based in part on SECC’s aggressive work plan for
finishing T-REX 22 months ahead of CDOT’s schedule. SECC is a joint
venture be-tween Kiewit Construction Co. and Parsons Transportation Group
(PTG). Turner Colley Braden (TCB), Sverdrup and DMJM are assisting PTG in the
project design. Kiewit is responsible for gathering the necessary design survey
information as well as construction staking and final right-of-way surveys. DEA
was hired as the design contractor of the project.

Time was a critical element, as was the need to avoid lane
closures during surveying. A total of 22 structures (out of a total of 67)
needed to be surveyed with a combination of Cyrax and other terrestrial survey
methods in 40 days.

Traffic, crew create a buzz

After reviewing the tight schedule and design requirements,
Jim Bodi, survey manager for SECC, decided to combine conventional terrestrial
surveys with aerial mapping and to use a Cyrax scanner to gather the bridge and
mainline freeway as-built data. He hired my company, DEA, to provide
terrestrial surveys and Cyrax scanning services.

My task as the survey manager was not made easier by the
fact that DEA crews had 40 days to complete field surveys for base maps of 24
bridge sites over or under I-25. Right at the start I decided we needed two
Cyrax crews for the Cyrax scanning and three field crews. DEA performed mapping
under and over many structures while six to eight lanes of traffic buzzed by
uninterrupted. In the office, DEA employed six survey technicians to manage the
Cyrax data: three to process the data, one to process the field survey data and
two drafters to produce the base maps.

We learned a lot. One of the more amazing facts related to
this project is that even with five field crews working 12- to 16-hour days we
did not require traffic control or lane closures for any of our work. We worked
through morning and evening rush hour while listening to the helicopter traffic
reports on the radio talk about the work we were doing, but at no time did we
impede the flow of traffic.

The Cyrax scanner has been used for the past three years;
however, it has never been put to work on so many structures in such a short
time. DEA had extensive experience using the Cyrax equipment and processing the
data for bridges, intersections, roadways, parking lots and buildings. Our
survey technicians are trained and experienced in merging the scan data with
the field crew’s topographical mapping to prepare maps for design.

For those who are not familiar with the 3-D laser scanning,
the Cyrax system is a self-contained unit that consists of a scanner the size
and weight of a small computer monitor, a power supply, a Dell laptop computer
and the Cyclone software installed on the laptop. The scanner sweeps a green
eye-safe laser over the area it surveys and with each sweep gathers 1,000
points per second. It has a practical range of 180-200 meters (594-660 ft), and
it is calibrated to achieve survey grade (6 mm/0.24 in.) precision of each
individual point for up to 50 meters (165 ft).

In a few minutes the scanner captures up to 1 million points
and displays these as a dense “point cloud” on the laptop’s
screen. Looking at the point clouds is almost like looking at a photo, except
that this “photo” is 3-D and can be rotated and viewed from every angle.
In addition, Cyclone allows for the registration (stitching together) of many
individual scans, which could come in handy when surveying a large bridge or
building or when scans are taken from different angles to capture all sides of
an object. Further, Cyclone provides a powerful set of tools to
semiautomatically convert point clouds into 3-D CAD objects, which in turn can
be exported to AutoCAD or Microstation. This last feature was important to us,
as our deliverable to the clients was 2-D CAD drawings either in AutoCAD or in
Microstation.

The bottom line is that we knew it would work for T-REX and
believed we had the right combination of field and office procedures to
complete this project in the time allotted. The further we got into the work,
the more we found our normal survey procedures also applied to the new
technology. Even though the scanner collects data differently (quicker and more
of it), the same type of control is necessary. Each scan has to have at least
four control points or registration points in it. If the control is poor, the
scan data are useless. Proper planning and attention to detail is essential for
a successful project.

The way it was handled

We used one scanner mounted on a tripod and the other
scanner mounted on a boom truck, dubbed the Scan Van. The tripod scanner
performed the under-bridge scanning and tight-area work, while the Scan Van
provided proximity for the railroad scanning and mainline work. The Scan Van
has a boom arm that raises the scanner and increases the range and visibility,
thus increasing the amount of data that is captured. We could park off the
shoulder with the van and scan the freeway while the cars drove by. The cars
would show up in the scan as a single line. The scanning technicians could then
delete the cars from the scan before sending the data to the mapping
technicians. If the traffic was going very slowly or was stopped we did not
attempt to scan.

We also learned that by setting up a conventional
instrument, such as total stations with each scanner, we were able to increase
our productivity. We could tie in the control points while the scanner was
operating and pick up the obscure area topo the scanner could not observe. The
scanner can only pick up what it can see, therefore the top of a curb may be
mapped but the flow line would not be measured. Likewise, we found that areas
around parked cars, guardrails or brush could be mapped using the conventional
instruments.

This is so far our largest deployment of Cyrax technology
that we used in conjunction with other methods. In fact, the main challenge was
to coordinate the use of a number of different hardware and software platforms.
These included the Cyrax systems (used mainly on bridges and overpasses) and
total stations, aerial survey and GPS equipment for the main roadway and
control. The Scan Van-mounted Cyrax achieved about 240 ft distance and the
tripod-mounted Cyrax about 180 ft.

DEA used the following work flow:

               Collect
control survey data and Cyrax scan data—two-person field crew;

               Process
survey data in Listcad (create control point coordinates);

               Load
survey control data into Cyclone and register scan data (in-cludes quality
control check of registration);

               Prepare
3-D polylines of all key break lines and bridge features. Line types are
assigned colors and are put on appropriate layers for later use in AutoCAD;

               Launch
AutoCAD from Cyclone and save file as AutoCAD;

               Review
line work in AutoCAD and correct any layer errors; and

               Translate
from AutoCAD to Microstation (InRoads) for final file/ drawing preparation.

Final deliverables to the client were Microstation files for
2-D planimetric drawing files and divisional terrain models (DTM) for contours.
The client can use planimetric drawings for their horizontal designs and the
DTM for all of their vertical calculations. DEA delivered the survey on time
and on budget.

All in all we spent 40 days scanning in the field using both
Cyrax 2500 (Circle 935) scanners with a crew of two for each. During this
period we took about 400 scans. In the office it took the office crew 50 days
to model all data, but since they worked parallel to the field crews they were
able to finish modeling 10 days after the last scan day.

About the author: 
Service is vice president/survey manager, David Evans and Associates Inc., Bellevue, Wash.
Overlay Init