What can I buy to perform accurate no-contact architectural measuring?
August 11, 2017 2:25 PM   Subscribe

The dream is to put a device in a secure fixed position, let it take images or whatever, and output accurate measurements of what it's seeing.

My interest is in window openings. Basically, the dream would be a "scanner" of sort that uses photo imaging with rectification, and/or laser, or other things I've never heard about, to produce highly accurate, architectural-quality measurements of existing structures.

It looks like such a thing was done for this project.

The linked description uses the phrase "a variety of state-of-the-art digital measuring techniques."

I want to know what those techniques are, and what equipment is on the open market.
posted by anonymous to Technology (5 answers total) 4 users marked this as a favorite
I do this for a living actually. Im on mobile now, but some good terms to look up are BIM modeling and laser scanning.

Laser scanners are made by a variety of companies, the largest two being Leica and FARO probably. The cost can be small ($16k) to large ($150k) depending on things like accuracy and range.

I'll check back and fill in more later when I have an opportunity.
posted by sanka at 2:34 PM on August 11, 2017 [3 favorites]

Another term of art is "point cloud", which is the data captured by the laser scanners sanka mentions. Google image search
posted by misterbrandt at 4:28 PM on August 11, 2017 [1 favorite]

OK, so I have a bit of time now.

So I laser scan and model stuff like this all the time. I actually do a lot of different laser scanning, from small intricate parts for reverse engineering to 10 story 500,000 sq ft buildings. I have done several churches like this one (though not this exact one). We do building refit stuff like this all the time for historical preservation and such. If you can think of a weird place, I have probably scanned something like it in the last 10 years. I have done a fair amount of work for a local window manufacturer using a FARO articulating arm and laser head that is typically accurate to about .003".

There are several different scanning options depending on what you want to scan, and what you want to do with the data. As for laser scanners, there are two different types, phase based scanners and Time Of Flight (TOF) scanners. In the past, phase based laser scanners offered a much faster data collection rate (1M points/sec) at the cost of some accuracy (typically 3-6mm at range). Phase based scanners measure the difference in the wavelength of the laser burst put out and coming back to measure distance. TOF laser scanners work much like total station reflectorless surveying equipment that's been around for decades. They use one single pulse of light to and back from the target to measure distance. This results in a more accurate measurement (typically under 2mm), but slower capture speed. In the past (showing my age here) a TOF laser scanner might capture 5000 points per second with an attached laptop and huge battery pack. Now, with computing power being so cheap and compact, TOF lasers will also scan at 1 million points a second, with the advantage of accuracy.

For something like this church project I would use a Leica P30-P40. This instrument is a TOF laser scanner. The specs say the accuracy is within 2mm. In practice, since we have so many kinds of scanners, it is usually much more accurate. We recently scanned a feature with a Leica P30 scanner and compared it to the data from a laser tracker. A laser tracker is like a robotic total station, but much, much more accurate. A typical laser tracker these days will be accurate well under .001". When we did some tests we found the Leica P30-40 results were within .030" of those tracker results. So about 3/4 of a mm. Keep in mind, for this church window thing, I wouldn't use a single point on a window sill to make measurements. Each sill would have like 100,000 points to make an average surface out of, making things much more accurate.

The advantage of laser scanning it is the tracker took about 6 hours to map the feature, leaving only single points that had to be manually connected in the office, while the Leica laser scanner took about 45 minutes, captured 1.5 billion points of the feature and it's surroundings, and a CAD model was made out of the points in office in minutes. The scanner was used for similar projects afterward, because the client always always decides after the fact they want something else. The ground level, the pipes near it, the curbs or manhole covers, whatever ad infinitum.

So laser scanning a church like this would get all the windows and sills (provided there was a lift or balcony to get a higher vantage point) and all that pretty fast. Depending on the needs, I would probably bid this one out at like 2 days of scanning, since it is in color. Color capture takes far longer than scanning. A single laser scan in a large area like this will take me 3 minutes and 30 seconds, and gather 150ish million points, but the color capture will add 10 minutes on top of that.

If you just want to scan window openings, something like an Artec scanner might be better. They use photogrammetry and a structured light pattern to map things. If you have reachable access to a window opening (like a 6x10 window or something), I could do it with an Artec Eva in literally 2 minutes. It would output a mesh that we have found is dimensionally accurate to under .020". It can be converted to CAD simply enough in the office.

If you really, really need dimensional accuracy and a high capture rate, you can use a Surphaser scanner. These use TOF technology, but typically a smaller laser beam diameter and slower capture rate, though I hear their new ones are much faster (haven't seen one in the wild for testing yet though). We use them to scan when dimensions are very important, like entire aircraft or helicopters. They are very expensive, but are accurate to under 1mm. They typically have much less "noise" in a point cloud because their beam pattern is tighter. It is super clean data that makes point cloud nerds like me go "woah".

So anyway, that's the basics of the scanning. The software is a whole other can of worms. And typically at this point people will die when they hear the prices.

A typical phase based scanner is something on the order of $40-60k and has a range of anywhere from 50-350m. I typically don't trust our phase based scanner farther than 100ft though MAX. A Leica P30-P40 (TOF) is on the order of $100-125k and has a range of up to 300m. A Surphaser is around $150k and has wildly varying ranges and thus accuracy (we typically use a 7m range one for under 1mm accuracy for aircraft). An Artec Eva is around $19k.

This is why I have a job really. It's much cheaper to hire out a job to us and have us carry all that cost than buy one yourself. Though Leica has recently released a shorter range less accurate scanner (BLK 360) that only costs $16k. Originally I was a bit worried about it, but now I realize that will only stop all the calls from people who will die on the phone when they call me. It'd be great for people who have a small company to measure things like window openings, or cabinet installers or things like that. It'd be great for our company to scan things above suspended ceilings. I'd like to get my hands on one.

Sorry for the length. Any questions, please ask. I can talk for days about this, obviously.
posted by sanka at 6:36 PM on August 11, 2017 [12 favorites]

At the risk of everone already knowing this... at the other end of the scale, there are cheap laser tape measures. Many have built-in calculations (such as area when you scan points for height and width). Here's the first example that popped up at Amazon.
posted by daisyace at 4:25 AM on August 12, 2017

From PBS This Old House - 3D Scanning Tool for Tablets
posted by canoehead at 9:07 AM on August 12, 2017

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