Alternative to Sphere / Checkerboard Targets

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PipeMan
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Alternative to Sphere / Checkerboard Targets

Post by PipeMan » Fri Jan 05, 2018 5:50 pm

Hi all - I would like to know if an algorithm has / could be developed to automatically recognize cones as targets for laser scanning, being stackable they would be so much easier to handle in the field, without the bulk of spheres or attention required for tilt and turn types of targets.
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Re: Alternative to Sphere / Checkerboard Targets

Post by brian715 » Mon Jan 15, 2018 7:36 pm

We use cones...but we put spheres on top of them.

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Re: Alternative to Sphere / Checkerboard Targets

Post by LPaulCook » Mon Jan 15, 2018 10:32 pm

I use cones to hold my 200mm sphears..

Where would the radius point of a cone be?

An accurate target needs to be a point or the intersection of 3+ planes or 2 lines. Cones don't provide any of those elements.
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Re: Alternative to Sphere / Checkerboard Targets

Post by mikajahkola » Wed Jan 31, 2018 9:55 pm

In theory a cone would make a very good target. It does have a well-defined tip point, and besides easy stacking it has the advantage that you could mix cones of any length as long as they all have the same opening angle. Measurement data could be matched to a cone just as easily as to a sphere.

The general equation of a sphere centered at x=y=z=0 is:
(x^2/r^2) + (y^2/r^2) + (z^2/r^2) = 1

The equation of a cone with the tip on z=y=z=0 and symmetry axis = z looks somewhat similar:
(x^2/a^2) + (y^2/a^2) - (z^2/b^2) = 0

I don't know that any commercial point cloud software would recognize conical targets, but some do recognize conical surfaces for modelling purposes.

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Re: Alternative to Sphere / Checkerboard Targets

Post by tadol » Thu Feb 01, 2018 1:03 am

I think you would be much more limited in mounting options with a cone - While it might look like a cone from quite a few angles, there are going to be many viewpoints that it will resemble an uneven sphere, or worse, and could throw off registration by more than a bit - Also, from greater distances, you would have a harder time identifying and manually selecting a cone, and getting an accurate registration point.

But if they write algorithms to make it all work easily and accurately, I'll make the best d--n scanning cones out there -

:D


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Re: Alternative to Sphere / Checkerboard Targets

Post by sreed » Wed Feb 07, 2018 1:23 pm

mikajahkola, you stated mathematically what i've been thinking for some time - and this thread raised the issue. A cone is a shape that has a discrete point (coordinate) that is recognizable from any perspective, similar to a sphere but unlike a Checkerboard target (unless rotatable). The advantage of a coneis it could be any size, provided it is well machined/manufactured (#KoppaTargets). The "strongest" shape cone would be a 45 degree angle, but anything approximating that (say 30 degrees) would be sufficient for determining the tip with relative precision. An algorithm could be developed that would identify and fit a cone for a particular slope, then the target cones could be large (think traffic cones or larger), medium (think party hat), small or even micro - all with the same target settings. As has been stated here, cones stack well and could be manufactured within a set tolerance for less investment than spheres. They are also inherently more stable on desktops, bedposts, asphalt, etc.

This could be a very good development and i'd be happy to work on/fund it with anyone more capable than me at SDK/C++/programming. I've developed 2 other apps with consultants, one of which is an alternative target device for registration. How about it YanKoch /Taglabs?

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Re: Alternative to Sphere / Checkerboard Targets

Post by 3DForensics » Wed Feb 07, 2018 2:49 pm

Interesting thread gang...I think the cones have some merit, especially for roadways or open areas. It is true they may not be as good for different mounting options but in terms of stability on the ground, you will likely not have them move or tip over easily.

What do you think the optimal size would be for the base/height? Should it end at an approximate point or do you leave a large flat on the top for other mounting options?

Eugene

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Re: Alternative to Sphere / Checkerboard Targets

Post by sreed » Wed Feb 07, 2018 2:54 pm

In terms of calculating the target point, wouldn't matter if pointed or flat top. (shape would be best fit, and tip calculated from shape). In fact, any edge artifacts on scan returns may be more pronounced with a pointed tip, so a cropped top may be even better for the fit algorithm.

For practical reasons, i'd think NOT having a point would be better. Approximately the shape of a traffic cone would be my suggestion.

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Re: Alternative to Sphere / Checkerboard Targets

Post by 3DForensics » Wed Feb 07, 2018 2:57 pm

Someone mentioned a 45 degree angle to the cone. I don't know what the actual angle is on a regular traffic cone but I would guess it is roughly 20 degrees or so.

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Re: Alternative to Sphere / Checkerboard Targets

Post by Oxbow » Thu Feb 08, 2018 11:15 am

Hi,

a cone is less suitable than a sphere. When detecting a cone, the fitting accuracy in XY and Z direction are not the same. Due to the approaching geometry in the Z direction upwards, larger errors can occur here. There are also totally different types of cone. These would probably also be difficult to compensate for. Spheres are probably the best way to adjust targets that are viewed from different angles.

We have also very accurate way to fix your sphere on top of a cone and to measure it by totalstation.
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