Scanning Manhole Inverts at Scale
We recently had a request come through our door for a massive DOT project that is going to be flown with aerial lidar & they want the manhole inverts scanned as well.
I'm trying to figure out if there is a feasible way to provide this in a cost effective manner. We have numerous scanners in our arsenal but I'm leaning towards using our BLK2GO. Here is what I'm thinking so far and if anyone has any suggestions please chime in...
1. Two man crew, one person cones off the area & pulls the lid then sets 3 small black and white targets with a portable table top number plate beside each target that is set to the point number that will be stored when they he shoots them in via RTK. As for positioning of the targets I was thinking maybe 3-5 meters from the opening.
2. The second man has the BLK2GO, starts the scan a few meters back from the opening, once initialized, he takes it off the stand and screws it onto an extendable pole. He scans the opening, the targets & number plates then plunges the BLK2GO through the opening of the manhole where they scan barrel & inverts. He pulls the BLK out, unscrews it and sets it back on the base then shuts it down.
The "walk" would be exported from register360 and brought into CloudCompare for alignment to the RTK points. I most likely would only shift in XYZ and rotate in Z letting the scanner set the scale and "flatness" of the dataset.
I imagine they should be able to do a manhole in 5-10 minutes +/- and the backend shouldn't take more than 10 minutes each if a bunch of scans are done at a time. Obviously this methodology isn't provided centimeter level accuracy but I think 5-10 cm wouldn't be hard to achieve.
Engineers and their affinity for lidar data...
It's likely you won't "have numerous scanners in our arsenal" when you're done.
Do you have a wearable SLAM scanner? We've used our VLX3 on shallow <8' Manholes with success.
Does a scanner go beyond the surface of the water. How about floaters, what does the scanner see?
They will penetrate an inch or two of water in most cases. If the water isn't too murky. Nevertheless, the scanner will get enough of the dry inside surface of the pipe that true invert can be readily deduced.
We have a VLX as well but the extra time and cost that it would take to use it over the BLK makes it not feasible at scale. Plus no one is lowering a VLX into a manhole.
Are there any particular techniques to getting a useable point cloud for deducing inverts and pipes sizes? I tried scanning a few manholes a couple months ago with a demo FARO Orbis. The cloud was awfully fuzzy at the bottom of the manhole and I wasn't able to determine any useful information from any of the three scans. I haven't pursued purchasing a SLAM scanner since that test as a result.
What's the required precision? Dot3d with Apriltags and a disto as a check is pretty economical. If it is precise enough depends on the project specs.
If you can determine the diameter of the pipe bore, it's easy but the invert is only as good as the vertical.
Before I spent any time figuring out the best way to scan the inverts I'd ask the client what they actually want and need. If the answer comes back "inverts," I'd put the scanners away and put the Pipe Mic and a 25' fiberglass leveling rod in the truck instead.
