Some questions about planning a TLS survey

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caskar
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Some questions about planning a TLS survey

Post by caskar »

Hello all,
I am a new member of the forum and a newborn in TLS. Therefore, sorry if my questions are not in the correct form.
I am working on a research project, the product of which is an indoor navigation app. My task is scanning the pilot building and creating a 3D model for BIM. LOD for BIM is 200. The main idea is having walls, columns, stairs, doors etc. I will scan only open spaces, not personal offices. The building has long narrow hallways ( e.g. W:2-3m, L: 50 m, H:3m). On one side of the hallways, there are offices with glass doors and windows. Sometimes both sides of the hallway are glass.
I planned to set up my instrument approximately every 7 m. I will use targets besides the C2C registration. Also, I will measure a few control targets around the stairs to connect the floors and for georeferencing.
The scanning instrument I have is ZF Imager 5016.
Would using only B/W paper targets in these narrow, long hallways be a problem? Should I put more targets on the glass surfaces? Would this help in cleaning the artefacts in the office? I read some people use some materials to cover the glass during surveying, but I don't have this chance.
Is setting a range limit during scans a good idea or not? The max range of the instrument is around 300 m. In indoor surveys with short intervals of scan positions, I thought, I might set a range limit to maybe 50 m. Do I miscalculate it?
Lastly, do you think choosing the high resolution&balanced quality setting with Imager 5016 be enough in this survey?
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Re: Some questions about planning a TLS survey

Post by MichaelKassler »

Hello Caskar

Using C2C and 2D checkerboard targets for scanning long and empty hallways may not be the optimal solution.
Never having used C2C registration I can only assume, that there may be more than one possible solution to fit a scan into a repetetiv door/ window pattern.
2D targets, such as checkerboards, shall only be scanned in a steep angle (±30°/ ±45°) making them improper to be put on halway walls.
Unless you attach them to a swivel joint; but I would rather suggest spherical targets.

About your glass problem...
Having had plenty of windows and sometimes mirrors in my scans I can only tell you this:
The quality setting, that eliminates reflection points from glas/ mirror surfaces has yet to be invented.
While there are post processing filters for stray points in your pointcloud, there is no such (direct) option for reflected points.
So the best solution is to cover up any glass in your scan (maybe by using scan spray) or to remove these points manually after registration.
Fitting more targets to a glass surface doenst chang this.

Trading scan range for scan positions is not a good idea when scanning indoors. Because with less scan positions you get more blind spots. If I interpret your project correctly you would want to aim for a 100% point covereage for your navigation app. The best way to achiev this is to trade scan resolution for scan positions. When I scan industrial facilities I sometimes end up with multiple scan positions as close as less than half a meter apart. But keep in mind that this will increase the size of your point cloud and may not be necessary for scanning empty halways.

I hope this was helpfull.

Sincerely Michael
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Re: Some questions about planning a TLS survey

Post by landmeterbeuckx »

My take on c2c scanning as i only work this way :
- in a hallway : scan in the middle of the hallway in front of a door
- scan in the door opening -> always do this
- scan in the room with the amount needed, in rooms with 2 windows i always take at least 2 scans so everything is on the scans.

- hallways : change the height of the tripod from time to time. If a stairwase is very repetitive you're bound to get scans from level 3 in level 2 end so on. Also note on a piece of paper which scan number is on wich floor.

- always start from the outside and try a loop before entering inside.
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Re: Some questions about planning a TLS survey

Post by Scott.Warren »

paper targets can totally work in a hallway, you need to be mindful of their orientation, and the angle of incidence the laser will make when it hits the target.

We often tape targets to the equipment boxes for places like hallways, or use stands to hold the targets, where having a paper target perpendicular to a long hallway is worth several poorly captured ones parallel to the long hallway.
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Re: Some questions about planning a TLS survey

Post by MajorDomo »

For your targets, generate unique numbers using forum user MrK creation here:
https://www.tlstargets.com/

Print them out and just tape the to the walls.
Vary the height of those targets. Uniformity is a big no-no.

Scan first survey after.
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Re: Some questions about planning a TLS survey

Post by pmalatzky »

Hello caskar,

Your project should be a little easier given you are using the Z+F IMAGER 5016.

Of course the IMAGER 5016 is optimised for in-field registration and I would encourage that you use this functionality. If you intend to use targets, you should activate the "Find Targets" functionality which will automatically use targets, if available and of higher enough quality, in conjunction with the C2C for registration.

You will find a template of the paper targets in the Help folder in the software install folder on your PC. These flat B&W targets may struggle in the narrow hallways as they are only effective between approx. +/- 45deg of perpendicular. Do also note, targets must be common between scan positions, you need at least 3, but 5+ is better.

The glass walls may prove problematic as you are likely to experience two scenarios. Reflection, especially at very acute angles and penetration in less acute angles and sometimes both. This can mess with your registration, the in-field manual alignment tools can help, you may need to use objects on the other side of the glass.

In the hallways, try to zig zag your way down them to improve the geometry and overlap. I would avoid putting targets on objects that move, doors for example, too easy for these to change in between scans and then become unusable.

Definitely use targets when you are going between levels in stairwells, again use the "find targets" function to assist the C2C. In a perfect world you would loop back down another stairwell to close the level, and the targets should probably be "surveyed" but this might not be possible for you.

You cannot range gate data acquisition on the IMAGER 5016, filter by range in your post-processing if you wish.

Finally, High/Balanced may be overkill for you project (walls, columns at LOD200), you should consider Middle/Balance, it will in fact produce a lower range noise dataset and will take half the time (~90secs) to complete the scan. Data density will be fine given the short distances between scans.

Good luck and enjoy!! :D
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Re: Some questions about planning a TLS survey

Post by sim.herrod »

I seem to remember there is a post on here somewhere showing why you shouldn't place paper targets on glass.

In corridors we use targets and/or spheres at different heights, and try to make sure they are visible in 3 scans, 2 as an absolute minimum.

If windows open, we will always try and shoot at least one station on each floors, and then shoot in targets from that station. If you can't do this, your stair scans become very important.

A good site plan with scan locations/names is a big help back in the office.

Noise from glass is an occupational hazard, and sometimes there is no alternative to manual cleaning of the data.
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Re: Some questions about planning a TLS survey

Post by caskar »

Thank you all for sharing your valuable opinion! It will help me a lot.

May I ask the benefit of changing the instrument height in the hallways?

Another question is about georeferencing. My thought is establishing a traverse from reference stations outside the building to the upper floors along the stairways. I won't extend the traverse down to the hallways. Then I will measure 4 - 5 targets on each floor close to the stairways. Would this plan work, or would I have a problem in georeferencing? Do I have to measure more targets with a total station throughout the whole floor?

Thank you so much!
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Re: Some questions about planning a TLS survey

Post by landmeterbeuckx »

caskar wrote: Fri Jan 08, 2021 9:42 am Thank you all for sharing your valuable opinion! It will help me a lot.

May I ask the benefit of changing the instrument height in the hallways?

If a staircase is very repetitive, you're bound to get some troubles sometimes because where you position your scanner is always approx in the same place.

I've had floor 4 scans be inserted at floor 3 with floor 5 the same then floor 4. It is difficult to recognize. it doesn't happen all the time.

It's also my own experience.
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Re: Some questions about planning a TLS survey

Post by max72 »

I would definitely go with spheres. I have some 145, 200 and 300 mm, and while it takes longer to set up everything but in this case it's worth the time in my opinion.
Distance between stations depends on the detail, but for registration rely on spheres, at least when you are talking of narrow hallways with glass walls.
Other parts can be done with C2C too.
Stairs are very tricky in C2C, being so regular and similar. In Scene I divide the scans in many clusters. I recently started using Scantra, and its workflow is perfect for registering stairs (and not only).
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