Careful Planning Yields Accurate Sewer Scan Results in Romania
Trimble Geospatial distribution partner in Romania, Giscad, is an expert in topographic equipment and measuring instruments. The experienced staff is always ready to problem solve and share their knowledge, particularly to address unique surveying situations. We interviewed Sergiu Ceausu to learn more about a recent challenging sewer surveying job.
Question: What is your name, your profession, and your company?
My name is Sergiu Ceausu, Head of the 3D Scanning Department for Giscad, the Trimble Geospatial distribution partner in Romania.
Q: How long have you been in the geospatial industry?
As of September 2022, I’ve been working in this industry for a decade. At the beginning I didn’t know what it was all about, but once I started to study and gain experience with the technology, I knew this was important for the future and I wanted to be a part of it here in Romania.
Q: Tell us about a recent project.
The first one that comes to mind is an especially challenging project involving a sewer. The customer told me there was a constant flow of water in the sewer between 30 cm and 50 cm high, so I had doubts about how we would be able to collect the necessary data. In the end, we decided to test the Trimble® X7 3D Laser Scanning System in these difficult conditions.
Point cloud view of the scanned surface and the sewer, registered together and georeferenced. Courtesy: Giscad
Q: What was the most important objective of this project?
The main goal was to measure the path of the sewer in relation to a building on the surface that had apparently been built above the sewer. There was concern that if something went wrong with the sewer, responders could not intervene in a timely manner. They wanted to confirm the exact location to be better prepared.
Q: What was the biggest challenge?
There were three big challenges, all equally important. One, confirming the correct correlation of the sewer with the surface at both ends, two, registering the stations in the repetitive environment of a sewer, and third, keeping the scanner out of the flowing water.
Q: What made you decide on the hardware and software solutions utilized for the project and what advantage did that give you?
I knew from the beginning that we were the last ones invited to the “show” so I must provide something that no one before us managed to put on the table — a feasible and efficient workflow with quality results. After a short analysis, I concluded that scanning with the Trimble X7 using Trimble Perspective and Trimble RealWorks software was the ideal combination. The Trimble X7 is easy to move between stations, features auto-calibration and self-leveling, and is IP55-rated against water. In the sewer there was a constant danger of sudden bursts of water. Trimble Perspective ensured in-field control and registration of all scans and images that I collected. Trimble RealWorks helped me georeference the surface and verify with precision the correlation between the surface and the sewer.
Q: How long did this project take your team to complete?
In the field, X7 with Trimble Perspective did its job very well. The data collection took around three hours, plus the set-up, so a total of 5-6 hours in the field. Afterwards, I checked and georeferenced the data in Trimble RealWorks in about two hours. The data was ready for delivery by the end of the day.
Q: What was your preparation for the project before and on site?
The tricky part was to go down in the sewer, which is 6-7 meters deep, and find points to connect with the surface through a hole about 60 cm in diameter. To accomplish that, I used two pieces of metal that were very well fixed and could be seen from the sewer.
View from the surface at the first point of entrance, and view from the sewer at the first point of entrance. Courtesy: Giscad
Then, we went down in the sewer individually equipped with waterproof suits! (Not my real smile 😊)
In the sewer, I was accompanied by three of the client’s workers, who helped me paint crosses on the sewer walls to use as common points for easy registration.
Crosses painted on sewer wall to help with registration. Courtesy: Giscad
Q: Walk us through the scanning process and describe the data you collected.
The scanning process consisted of placing the scanner on a wooden board, to keep it out of the water, at an interval between 5 and 7 meters to have a better overall overlap percentage from station to station. I was able to check the data in real time to make sure everything was working correctly.
Trimble X7 balanced on a board in sewer. Courtesy: Giscad
I began with a 4-minute resolution setting with color ON to see the density of the points at certain distances. Going forward, all the stations were set at two minutes with color, just enough to have a very good quality of point cloud. After I reached the other end of the 20-m long sewer, I went back up to street level and started measuring the surface to close the loop. That resulted in 49 stations, most of them done at minimum resolution, but with enough level of detail due to above 80% overlap percentage overall.
This was another opportunity for everybody to see how good and versatile Trimble X7 and Trimble Perspective really are. The workflow managed to produce a point cloud with impressive resolution and quality in a short time.
— Sergiu Ceausu, Head of the 3D Scanning Department for Giscad
Q: How was the data processed and analyzed?
For me to provide quality data, I analyzed the data in Trimble RealWorks. First, I needed to be sure that the ends of the sewer fit precisely with the ends from the surface. For that, I changed the colorization to Station Color and with the help of Limit Box I isolated the common surfaces. The result showed a perfect match.
With the help of the Cutting Plane Tool, I generated a horizontal section at the lateral extremities of the sewer to make sure there weren’t any doubled walls. Then, using the 2D Easy line tool, the automated vectorization was done with a 90% fidelity. Only a few artifacts needed to be corrected.
Next step was to georeference the project and import CAD drawings to observe the fit.
Q: How did the hardware and software solutions work together in your workflow?
Trimble X7 and Trimble Perspective worked like a charm. The fact that I could see how much of the sewer is captured from every position offered me the certainty that the next position would have around 80% overlap.
Q: What deliverables were part of this project?
We delivered a georeferenced point cloud for the customer to compare with other measurements from our National Agency of Cadaster. Our registration error was less than 5 mm and the georeference error was less than 1 cm. Also, I vectorized a horizontal section to compare with the same measurements.
Vectorized horizontal section of sewer. Courtesy: Giscad
Q: What was the outcome of the project?
This was another opportunity for everybody to see how good and versatile Trimble X7 and Trimble Perspective really are. The workflow managed to produce a point cloud with impressive resolution and quality in a short time. Any other scanner would have had difficulties. We were able to confirm the location of the sewer in relation to the building with high accuracy and confidence.
Q: What would you do differently the next time?
For me to do a quick workflow, I sprayed pairs of crosses to be used as common points when I did the registration. Unfortunately, the spray wasn’t visible on grayscale intensity and I had to rely on the colored pictures. So, next time a better spray.
Also, I would position the scanner at a better interval to ensure that all crosses are seen. This goes hand-in-hand with the first point. And I would use three pieces of metal at the surface of the hole to be seen from below and position much easier in the sewer.
If I had more time, I also could have used the X7 laser pointer to measure the control points, then refine the scans and georeference the project in the field with Perspective.
Q: What advice do you have for people trying to start a career in laser scanning?
If one has the opportunity, my advice is to give laser scanning a try. It is an awesome and fun way to use the latest technology and see how challenges can be met in innovative ways.