How do I achieve the best accuracy?
With proper technique, you should typically get results within 2% of the actual distance.  There are three factors of prime importance:

• Motion at the end points.  The algorithm used to compute distance relies on the fact that the device is completely stationary when you press the button at the end points.

• Changes in tilt.  Gravity affects the acceleration measurements, and the relative direction of gravity changes when the device is tilted.  Keep the device as level as possible - typically flat on a table or other surface.  If you are using an iPhone 3GS or an iPod Touch with a rounded back, try measuring along the long axis of the device, so that the angle reading is near 90°.

• Speed/acceleration.  Faster is usually better.  An accelerometer loves to accelerate -- use it!

Other tips

• Flat-backed iOS devices, such as the original iPhone, iPhone 4, iPad, and the iPod Touch work great with AccelaRuler.  Owners of the iPhone 3GS and other rounded-back devices need to be bit more careful while measuring since the phone’s rounded back can allow the device to wobble, upsetting the measurements -- try measuring along the long axis of the device, so that the angle reading is near 90°.

• Sharp accelerations resulting from bumping into objects will cause measurement errors.  Try to keep all movements smooth.

• Make sure that your device is completely motionless at the end points.

• If you have a grippy case on your device that sticks to the surface when you try to slide it, try placing it on a napkin or other piece of paper to slide more smoothly.

• Manufacturing tolerances.  Accuracy testing was done on multiple devices without issues, but it’s possible that there are some accelerometers out there that don’t behave the way that AccelaRuler expects.  Let us know at
Still doesn’t work for me.  Now what?
Take the device to a countertop or table where you have a clear, smooth path of about 2 feet (0.6 meters).  Often it’s easiest to learn to use AccelaRuler at this distance before proceeding to measure shorter or longer distances. 

Try to measure the distance, taking about one second to slide the device one way and another second to slide the device back to the starting point.  Follow the on-screen tips, if any, and try this test several times until you get measurements in the green on the quality meter.  At that point, the measurements displayed should be close to the true distance, typically within the plus or minus amount shown.
Can I measure distances while walking around?
AccelaRuler is really only meant to be used on flat surfaces.  Of course, feel free to experiment with this by carrying the device as smoothly as possible.
Why does AccelaRuler show a measurement if I don’t move the device?
Remember, AccelaRuler measures to the outside of the device.  The measurement you see will be one of three things -- the width of your device, the height of your device, or the diagonal length of your device.  The accelerometer will register even the smallest tap of your fingers on the screen, and depending on which direction that primary force is in, you will get one of the measurements listed above.
What can I do with the Recent Measurements screen?
Measurement memory:  It’s an easy way to keep track of distances you have measured without copying and pasting or writing them down.

Accuracy enhancement:  One technique for increasing accuracy is to measure a distance several times.  Selecting several measurements that were in the green area on the quality meter will average them together, providing an increased level of accuracy.

Area of a rectangle:  If you measure two sides of a rectangular surface, you can select both to see the square area of the surface computed for you.

Adding up multiple distances:  Selecting two or more measurements provides a sum of the selected measurements.
How does AccelaRuler work?
Warning:  nerd speak ahead.  AccelaRuler samples the accelerometer data stream about one hundred times per second.  AccelaRuler then applies a simplified form of a Kalman Filter known as an alpha-beta-gamma filter to estimate and correct out errors in the raw accelerometer data and produce position information.    

I’m a contractor building a house.  Should I throw away my tape measure and just use AccelaRuler?  But you could certainly use AccelaRuler as a convenient way to get length and measurement estimates of any number of things around the job site.
I read the FAQ and I still have questions.  Who do I contact?
AccelaRuler Frequent Questions

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