Outdoor Scanning Advice

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amoignes
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Outdoor Scanning Advice

Post by amoignes » Fri Oct 04, 2019 7:09 pm

We normally work on indoor scans and exterior facade scans and have been asked by a client to scan a large outdoor area.

We normally have no issues registering scans - indoor scans require so much scan density to deal with shadows that there is always well over 30% overlap. This won't be true for this upcoming scan - there is very little obstruction on the site. This also means I have almost nowhere to post checkerboard targets to help with registration.

In our experience spherical targets need to be within 25m of the scanner or they fail to help with registration. Given the size of the site (100,00+sqare meters) this will mean a LOT of unnecessary scans just to register them together. Does anyone have any advice on how to set up targets or plan this scan in order to minimize the wasted scan time just for registration's sake?
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Re: Outdoor Scanning Advice

Post by TByers » Fri Oct 04, 2019 7:37 pm

Hi there!

We do quite a bit of exterior scans. We've done some in the past for job site grading, and overall as-built condition.

One of the biggest tips I can give you for using only spheres is to set them up from scan to scan in a different pattern than the last. For example, if you have five spheres in between scan A and B that are in a straight line, the next spheres between scan B and C should be in any other pattern than a straight line. We've had luck with this in the past, and automatic registration doesn't get confused as each pattern is different in terms of spheres, so it likely won't get "too stressed" and try to tie in scan K with scan A due to similar patterning/thinking it's the correct link.

Another way to tighten it up would be to keep three or so spheres in the original scan position (Scan A). Try and position your scans in a way that your final scan (Scan Z) can see the spheres left at your first scan (Scan A). This is really for the simple surveying principle of "closing the loop". This will tighten up your registration immensely, as cloud to cloud will have some trouble with exterior scans.

Don't get discouraged if Auto registration doesn't work..we've had problems in the past with auto registration on exterior jobs due to the constantly moving trees, blades of grass, surroundings etc.....

If you can, do a test scan to determine the best setting for your lighting (even weighted, horizon weighted etc...)


Anyways, I hope this gives you some sort of idea how to tackle it! This might all be redundant info to you as you've scanner before so take ti with a grain of salt :)

Feel free to contact me directly if you have any questions on exterior scanning!

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Re: Outdoor Scanning Advice

Post by Felix_the_Cat » Fri Oct 04, 2019 7:40 pm

Need to get your scanner up high

Really high

Or you are going to need to make thousands of scans

Get the biggest spheres you can find

I have seen 18”

Put em on elevating tripods

That is a really big job


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Re: Outdoor Scanning Advice

Post by TByers » Fri Oct 04, 2019 7:43 pm

I may have misunderstood your inquiry and given you very general scanning info...

What is the client hoping to get out of the scan? What kind of tolerance are they looking for?

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Re: Outdoor Scanning Advice

Post by amoignes » Fri Oct 04, 2019 8:23 pm

All advice is helpful!

The client is looking for a 3D model of the topography of the site, which is a mostly open field. This is my reason for trying to cut down on scans - I don't need the point density that I would get from scans as close as I usually do. If I could set my scans a full 50-70m apart and still manage to register them together that would be plenty of data for what the client needs.(We're using a Faro Focus 70)

How high should I be shooting for? 7' off the ground? More?
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Re: Outdoor Scanning Advice

Post by jaredmatthews » Fri Oct 04, 2019 8:40 pm

That's a tricky one.

There's honestly no shortcut around it. You either have to place a large amount of checkerboard targets/spheres throughout the site so that the scans have something to register off of, or you have to hire a SLAM based scanner and then not have to worry about registration.

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Re: Outdoor Scanning Advice

Post by Felix_the_Cat » Fri Oct 04, 2019 8:50 pm

You need to go find a field and do some test scans.

If it is bare ground you have a better shot.

You will be dismayed at how small the return circle is. Vegetation moves. Grass moves. With that scanner on the standard tripod you will likely get 20’ diameter circles of usable scan MAX. Think about how many scans it will take using this process.

An open field is second only to a featureless elevator shaft for registration nightmares.

You will lose your shirt scanning that unless you have unlimited resources

Hundreds and hundreds of scans

Terabytes of data

Weeks of processing time

Hire a drone

Use photogrammetry

Keep your shirt


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Re: Outdoor Scanning Advice

Post by MajorDomo » Fri Oct 04, 2019 9:13 pm

Outside? Clear sky? Topography?
GPS.

Or if you want more precision, robotic Total-station.

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Re: Outdoor Scanning Advice

Post by topogeo » Fri Oct 04, 2019 9:38 pm

I use a Leica RTC360 on a 2.76 m high SECO tripod
I make pairs of scans 20 meters apart. To the right and left of each I put two targets on a tripod that I detect with GPS. The scans are recorded through the 4 goals that I also use to georeference the group. I put the next couple at 50-70 mt. Each pair covers an area of ​​around 75 x 50 mt .. or even 100 x 70. So to cover 100,000 square meters you should do about 20 -30 pairs. The project will consist of 20-30 separate but georeferenced groups (Leica Register 360 allows it).
The precision is about 2-5 cm but it is a little significant value because the blades of grass distort the DTM ... It is more reasonable to think of a 5-10 cm precsione (..but it depends on the height of the grass ..)
With GPS you are more precise (the pole always rests on the ground) and you put the same time and you have much much lighter data.
In my opinion ..in open land .. it makes sense to use the laser scanner only if the area is a part of a larger project where the scans are really useful ..

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Re: Outdoor Scanning Advice

Post by SAttaya » Sat Oct 05, 2019 12:27 am

amoignes wrote:
Fri Oct 04, 2019 7:09 pm
...Given the size of the site (100,00+sqare meters) this will mean a LOT...
is that 10,000 (10K) sq meters (2.47 acres)
OR
100,000 (100K) sq meters (24.7 Acres)
???

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