Scanning Using Survey Control

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EricPullins
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Scanning Using Survey Control

Post by EricPullins » Mon Mar 11, 2019 6:56 pm

Hi Guys,

So I've been perusing this forum a lot over the past few days... and one thing that keeps coming up is scanning using Survey Control. But more what I'm finding is this statement, but nothing about workflow or how I would even start to scan using such a thing. So I suppose my question is, can someone give me a rundown of how using a survey control works? Are the targets the things that are surveyed? What about scanner location?

Any info anyone can give would be very much appreciated!

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Re: Scanning Using Survey Control

Post by Scott.Warren » Mon Mar 11, 2019 7:15 pm

The basic idea is that targets you use in the field have coordinate values which accurately related to some coordinate system. This can be done by setting up targets on top of surveyed points using Bipod/tripod and measuring up, or by locating the center of chevron targets using conventional surveying methods (i'm sure there are other ways as well). The main take-away is that when registering the data, you have some way to tell the software that the targets relate to a coordinate system, and that it should attempt to fit the cloud to this system. There a several advantages to this, with the main ones being is you have a quantitative way to check your cloud registration's accuracy, and that you can place it in space in a meaningful way. You know for sure that the position of an object within the scanning data can be consistently and accurately related to real world coordinates/locations.

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Re: Scanning Using Survey Control

Post by EricPullins » Mon Mar 11, 2019 7:33 pm

Thank you for your response. Much appreciated.

I get the idea behind it, i guess my hangup is understanding how one incorporates this into a workflow. Like, do you have a surveyor survey the area wanting to be scanned first and make sure that they mark where they run their survey?

I'm really trying to understand how this all stacks up, as we may be venturing into this territory with some projects we have in the forefront. The more I have understood, the better off I'll be.

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Re: Scanning Using Survey Control

Post by Scott.Warren » Mon Mar 11, 2019 9:22 pm

We are a survey/engineering company, so our field crews are surveyors, and 95% of our scanning projects use survey control. If it's a larger site, we spend a day running control through the site, if a smaller project, we'll set control as we go. Surveying is a profession in and of itself, and not something I'd want someone to do without adequate training or oversight, too easy to make mistakes that can be very costly. Met lots of folks with scanners that think their uncontrolled scanning surveys are more accurate than a well observed, controlled, and adjusted conventional survey traverse, boggles the mind.

If you're not trying to tie into a defined coordinate system, and just want some QA/QC, using spheres might be the way to go.

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Re: Scanning Using Survey Control

Post by LPaulCook » Tue Mar 12, 2019 5:46 am

Im a surveyor and do georeferenced scans often. Besides translateing the scans to fit the boundary lines and NAVD88 elevation datum it also checks the calibration of the scans.
Here's two photos of a site i started a boundary topographic survey on last Saturday. I screwed 3 12x18" checkered targets into 3 trees which you can see 2 of if you zoom in.
I had not started and boundary or elevation BM yet but i will be back there Wednesday and shoot these targets in reflectorlessly then and tie it all into my boundary and elevation datum.
The only other way to do this is fake it wich is not acceptable.
Surveying is another thing from scanning. Much more to surveying than scanning. Hire one, don't try to fake it.
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Re: Scanning Using Survey Control

Post by landmeterbeuckx » Tue Mar 12, 2019 7:54 am

As stated before always use some kind of control. Surveyors like to check everything double! I always measure in the Belgian Lambert system.

Maybe it would be handy for all those non-surveyors to look into a simple total station. Nothing fancy, no robotic but something reflectorless. RTK is also overkill. I would suggest al the non-surveyors call a surveyor if it needs to be tied to a certain coordinatesystem ;)

If that is out of range a simple automatic level with a rod can save you a ton of headache. Measure the height difference between some points which are on the ground (manholes,...) and check these differences with the registered scandata.
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Re: Scanning Using Survey Control

Post by smacl » Tue Mar 12, 2019 8:28 am

What I see with most of my clients on larger jobs is a control network put down with a total station prior to the scanning taken place. Where the scanner used is one which you can set a back sight, e.g. P40, this might be a network of scanner positions. It can also often include sphere positions, where the position may have been shot from multiple total station positions and may also form part of a resection network. Where control is vital is on linear jobs such as tunnels, roads, rail etc... where scan to scan registration will accumulate error over the length of the job without additional external control. The same can happen on large buildings. As per previous posts, any survey is only as good as its control.

The mechanism I see used most frequently on large jobs is least squares adjustment of total station measurements, including a number of fixed and/or constrained GPS coordinates to tie into a known grid or pre-existing control. This is typically more flexible that a conventional traverse and better at accommodating short legs where there is sufficient cross-bracing.

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Re: Scanning Using Survey Control

Post by Daniel Wujanz » Tue Mar 12, 2019 10:17 am

Dear colleagues,

I agree with the vast majority of statements yet I'd like to add some thoughts.

a) You CANNOT check the calibration (I assume axes errors were meant) with some targets that you use for registration. The redundancy and accuracy is way too low!

b) EVERY sensor is subject to noise / errors and thus has stochastic characteristics! Hence, errors accumulate in every network regardless if it's based on levelling, tacheometry or laser scanning. It is defo a good idea to tie in some control based on tacheometry but you should consider the stochastic properties of every single point (that you get via error propagation). If you treat these points as God-given you may introduce tensions into the network. The reason why observations captured by total stations are more accurate is because you usually need less setups to conquer larger distances.

If you don't believe me here's an idea: one could do an error propagation race! Set up a total station and a scanner side to side and measure a traverse and a scanning network with a distance of 2 metres between the viewpoints. It'll be a close one ; )

c) How do you know that your scanning network is error free? Most people use control points to check and / or stabilize their network (again that's not a bad thing) BUT you wouldn't check every single station by tacheometric measurements since that wouldn't be too smart from an economical point of view. In addition you can use redundant registrations (more connections between scans than needed to initially register the lot), inclinometer observations and SOME control points. What you want is the lowest possible amount of control points that help you to satisfy the specified accuracy that the customer wanted. The magic of a block adjustment then helps you to identify discrepancies and tensions in your network even if there are no control points around.

The attached figure shows a beautiful example of newtork design:
- 587 scanning stations (one level of a building)
- 1013 registrations
- 24 control points (green circles)
- Absolute 3D accuracy: 5.2 mm

Note that the area highlighted in red was hardly accessible.
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Re: Scanning Using Survey Control

Post by EricPullins » Tue Mar 12, 2019 12:18 pm

Thanks everyone for your responses.

Maybe I'm missing something, but I'm asking about workflow and what people's process is with using survey control. From what I've read, there really isn't much explanation about what you do to actually work with a survey control. Is is literally as easy as loading in survey data and just letting it do its thing? What kind of work do I need to do to get these survey controls to work their magic?

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Re: Scanning Using Survey Control

Post by Tookie » Tue Mar 12, 2019 12:28 pm

Eric,

In its simplest form..

Survey the known points. Scan the known points.

Import surveyed point data and scan data and combine.

I don't know the exact workflow in scene, but I would start by looking at the user manual and or getting some training of your delivering this to a client you want to be sure it's right!

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