Measuring distances in point cloud

[awakefield]

Hi there, love your software. I'm still learning how to use it, but I'm very impressed.

I want to measure distances within an image derived point cloud and I can think of two ways to do it.

1. Scale an image derived point cloud to a (georefrenced and accurately scaled) lidar point cloud and measure distances within that. (I've tried this method and it works pretty well!)

2. Put a object of known size (piece of paper with 4 targets drawn on) in the scene when the images are collected so that it's a part of the model. Then use that to scale the whole point cloud without the need for lidar data. This is shown here: https://www.youtube.com/watch?v=PjiJBqckv90

I just don't understand why we need to load a second point cloud. What info is contained in it? Can I do this with only one point cloud? What are your thoughts about the best way to georeference an image derived point cloud?

Thanks in advance for your help.

[daniel]

Where do you see a "second" point cloud? Anyway you are right there's no need for one (you can use the 'Align with point pairs' tool with only one cloud and input the equivalent (virtual) scaled points.

You could also do this by measuring the distances between two or three points (with the "Point picking" tool) and then use the 'Edit > Scale / Multiply' tool (but it's generally less accurate).

[awakefield]

Thanks for your quick response. I was referring to mark :20 in the video, but I think I've got it now. Thanks for your help! I achieve an accuracy within ~1% using that method compared with the lidar method. There is a downside, though.

If my scale marker was leaning up against the building at an angle, the points I input to scale it will tell the program to align the axes with the scale, not the building. This makes it difficult to measure some properties that I need. Is there a way to align the building to the axes?

Almost the same problem as this person: http://www.danielgm.net/cc/forum/viewtopic.php?t=180

[daniel]

Well you could just use this method first to get the scale (it appears in the final report as well as in the Console). And then apply this scale on the original cloud (without modifying its axes - see 'Edit > Multiply/Scale').

Otherwise you could use the 'Level' tool that lets you pick 3 points to define a horizontal plane. The cloud will then be transformed so as to make this plane parallel to the XY plane. But it's a manual process so it's sometimes a bit tricky to get accurate results.
http://www.cloudcompare.org/doc/wiki/in ... itle=Level