Best scanning technique for iPhone LiDAR in architectural/crawl space captures?

[Coldaine]

First off, I want to say I really appreciate this community and the maintainers. Thank you for all the work you do.

I apologize if this is a basic question, but I've had trouble finding a clear, practical answer. Every time I search YouTube for scanning technique, I just get influencer-style content rather than someone actually walking through the mechanics.

My use case: I'm scanning architectural spaces with an iPhone LiDAR, specifically houses and crawl spaces. (Crawl spaces here are essentially half-height areas under the entire house where all the plumbing runs.)

My questions:

1. What is the optimal angle to hold the phone? Should it be perpendicular to the wall, or angled?

2. How do you handle capturing both the ceiling and floor from about 2 meters away? At that range, I can't seem to get both in a single pass. What's the recommended approach?

   • Do I do one full pass at a consistent height (capturing the wall and part of the floor), then complete a second loop angled upward to catch where the ceiling meets the wall?
   • Or do I shuffle sideways parallel to the wall one step at a time, panning up and down at each position?

3. What is the general body/camera movement technique? I'm a bit neuroatypical and tend to think in very concrete, mechanical terms. I'd really benefit from a step-by-step description of how to physically move through a space during a scan.

Thanks again to anyone who can help. I'd very much appreciate it.

[matlabbe]

Hi!

1. The LiDAR would be more accurate when looking perpendicularly to surfaces, thus better geometry. That also helps to get better texturing too.
2. (and 3.) We made this video a while ago with a spin-off app from RTAB-Map. This shows the "painting-style" scanning. I generally do a scan from floor to ceiling motion (up and down) while moving slowly sideways. One thing that is not clearly explained in that video is that when we come back to where we started the scan, we should remap a little bit to find a loop closure. Doing only floor first of the full room and doing ceiling afterwards gives generally not good results, unless you pan down or up regularly to find loop closures (that's it, connecting the floor trajectory with the ceiling trajectory). RTAB-Map should let you know when a loop closure is detected, then you can safely stop the scanning if you are done. There is another video that shows some loop closure detections. For single rooms, they can be done in one pass like this. For multiple rooms or one big large space in one session, when you are moving back close to an area you already scanned, re-scan it a little bit to find a loop closure. The loop closures will help to have a more globally consistent map at the end. For example when mapping multiple rooms, I'll make sure I find a loop closure in each room, then when I exit the room, I make sure I find a loop closure in the corridor with any of the previous rooms mapped.

The don'ts:

• Avoid scanning straight to a texture-less surface. Scan textureless walls from little farther so that the tracking camera can track more reliable features.
• Avoid looking straight in empty space or the sky, try to keep some close surfaces with texture in field of view of the camera