Sometimes you need to do a scan project and you need to register without targets, either because you don’t want to lug them around or because you can’t. However targetless registration both automatic and manual can be difficult when there are not many vertical, physical features to match to.
There can be many reasons for scanning where there are not many physical features. The example I will use in this tip is scanning a boat launch ramp during low water for the purpose of checking the slope and comparing to differing water levels. I would usually use targets for large open spaces like this but as luck would have it they got lost in shipping. Sooo…. scanning without targets it was.
In the image you can see that there isn’t much to register to.
The strategy I employed was to use my vehicle as a vertical reference. A giant target if you will. I started by placing it in a central location for my scan project. As a note you want to make sure the vehicle doesn’t move so no jumping into it to keep warm or leaning on it when you need a rest.
For the scans close to the truck the automatic registration did a great job. However, as I mentioned, the area I needed to scan was pretty large, so I needed to get farther and farther away from the vehicle with each scan. As I got farther away the automatic registration started failing and I needed to start manually registering the scans. This was done quickly and easily as I could use the split screen view in Trimble Perspective to quickly pick common points on the truck to match to.
Now I will admit that using a single vehicle in the middle of the project isn’t ideal so I had to look around for any other objects I could identify in the scan. Sometimes I use the scanner case itself as an identifiable object especially when in small tunnels or parking garages where everything looks the same and there are lots of repeating patterns. However, on this project I was able to use a few objects in the scan scene. These concrete parking stops worked great.
I mentioned earlier that the site was large, and I couldn’t see my truck from all the scans, so I had to get creative. I set the scanner so that it would capture the truck early in the scan and then using Perspective I paused the scan just long enough for me to move my truck and park it in a new location closer to where my next scans would be. Then I un-paused the scan and the truck was then captured in two positions but in the same scan. Now I could use it as a visual reference to register my next group of scans.
To complete the project, I brought the data into Trimble RealWorks where I tightened up and verified the registration. To tell the truth, as the day got colder, I pushed the spacing between scans a bit farther than I probably should have but with the arsenal of registration tools in TRW it was no problem getting a great fit. Then I ran the automatic classification to separate ground from vegetation and remove vehicles and people from the scans. I was able to create textured surface meshes, structural models, and perform analysis to measure the slope of the launch ramp and compare against variable water levels.
So next time you are in a bind and need to perform a targetless registration in a large, open space get creative.
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