Leave the tape measure at home. iPhotoMeasure is piece of software that will let you measure anything just by taking a photo!
All you do is place a small square (called the DigiTarget) into the shot when you take the photo. The program will use this target to calibrate your photo so it can give you accurate measurements.
I think it's most useful for builders, architects, or anybody who has to quote on work that needs measurements. You can get your client to take the shot for you so you don't need to visit on-site for the quote!
There are a few things to watch for when taking the photos. The DigiTarget needs to be on the same plane as the object to measure (meaning the same distance from the camera), and you need to take the shot as square-on as possible. You can't take a photo of a high window from ground level and be able to work out the height.
But other than that, I think it's a great idea, and well worth a look if you need to measure lots of things.
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