The Meaning of +/- 1/4″

I am sure we have all seen this specification, +/- 1/4″. The question is what does it really mean? Let’s take a fairly large office building – multiple floors, 100+ rooms, a couple of acre footprint and the laser scanning spec is – you guessed it, +/-1/4″.

Does this mean absolute accuracy, or relative precision? Is there any survey control/ laser scanner combination on the market today that can meet that requirement, at a reasonable cost, or any cost for that matter?

But should that really be the question, or should we be thinking more in terms of the way in which the dimensional information is intended to be used. Isn’t the use of laser scanning better than manual point by point survey methods?

These are just some of the questions that I think need to be answered. What do you think?

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2 Responses to The Meaning of +/- 1/4″

  1. lidar says:

    I am currently dealing with this topic for a client that has a large facility scan already in process. After a few false starts, I have landed on just two statements of accuracy. 1) How does the distance measurement between any two scan points compare to the same two points in the physical world? 2) How well does any point in the scan compare to the actual plant coordinate?

    The first recognizes the need to know dimension accuracy and prompts the discussion that physical plant dimensions also change. The second allows for measurements for items relative to the plant. I have neither a survey nor a scanner background, so the ice feels very thin where I stand. Some comments or guidance on this subject from such an experienced group would be very welcome. Thanks.

    Scott

  2. I think they are referring to “relative” positional accuracy – but they probably wouldn’t even know how to explain that.

    The ALTA standards are fairly reasonable when they stipulate that the measurements should be within 0.07′ at 95% confidence level. That means the survey MUST have used a least squares and redundancy in his control network to certify it to that standard. It’s going to be hard to get it down to 0.02′ or a 1/4″ due to the technology currently used to do the control. The bubble on most rods is only a 10′ bubble, and at 6 feet that’s a 0.015′!

    The scanners have a much more accurate level bubble (just like the total stations) but I’d say that measuring concrete buildings, asphalt roadways, and other structures subject to expansion or contraction during variations of heat are going to see a 1/4″ or more themselves. Combined with the error in setting up on instrument, you might see a more reasonable relative error being closer to what the ALTA standards are.

    Most clients don’t want to hear that, and actually understand very little about the theory of measurements. I can measure within 1/4″, but that for staking buildings and bridges, not doing a topo. To located things within 1/4″ would take days. Imagine trying to located a fence within 1/4″? If I person slams the gate shut too hard, it might move 1/4″! Exceptions to this are monitoring surveys and joining into an existing building. However, the time it takes is much longer than simply creating a map of existing conditions. It goes contrary to the purpose of the engineering project. The engineers don’t need the dirt and curbs or even the existing building located within 1/4″. The building are getting torn down and the dirt getting raised by a few feet. There are exceptions to this for buildings, but they cost is greater and is a case-by-case basis.

    I think the best way to change this is to educate the client by citing respected survey sources like books and others like the ALTA Standards. With new technology, comes great expectations. I am constantly being asked if I’m using GPS on construction sites. Some of these sites are covered by trees or next to tall building that would cause errors in excess of 2′ or more but they want to see the GPS because it’s new and “Sexy”. I usually have to explain that it’s great for rough grade, on a large and open site, but not so good for elevations on finish grade, next to building or under tree canopy and the robotic instrument I’m using is the best tool for the job.

    You may run into problems if someone is trying to design based on your “survey” of the plant and they donen’t tie into the joins well. It could cause some liability issues. Not having a “survey” background, you’ll probably see that “knowing how wrong or right you are” i.e., the “positional accuracy” of your work is part of being a Land Surveyor. There seems to be a lot of non-licensed people getting into this, partly due to the romantic idea that “anyone” can take a picture, but once you get into it, you relize that the “devil is in the details”.

    I read up on Measurement and Error theory with Engineering applications to get a better grasp on what you have to deal with.

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