Tape MeasureWhile the hands on approach of using a trundle wheel will always provide learning value in a classroom/lecture setting, GPS built into the iPhone 3G is providing news ways to measure distance. TapeMeasureTape Measure by Limekiln for the iPhone gives you the ability to take measurements from a previous position to your current position. It displays the latitude, longitude, altitude, and accuracy of your current position, as well as shows you the current measurement being taken. Measurement information includes the distance, height, and direction (heading) of the current position from the previous position. This information can be displayed in either feet/miles or meters.

Once you find a position with enough desired accuracy, you can start a measurement, exit the application, and continue the measurement later. This feature is useful for saving power when GPS is enabled. You can pause measurement updates and then either resume measuring from the same start point, or reset your measurement and start again whenever you desire.

The iPhone 3G will provide greatest accuracy for TapeMeasure because of its powerful GPS feature, but TapeMeasure will still work well for long distances with the original iPhone. Note that only the iPhone 3G can provide altitude information, and only with a good GPS signal. TapeMeasure will also work with the iPod touch, provided that different WiFi connections are active when starting and ending a measurement. Note that not all WiFi access points can produce location information.

AppStoreLink App Store URL: Tape Measure


2 Comments to “Tape Measure - the iPhone’s trundle wheel?”  

  1. 1 Graeme Payne

    This looks like a very useful learning tool. There are at least two areas of improvement that can be made to improve the learning, though. The application can probably be used as-is (the developer would have to make the suggested improvements) and the educator can discuss the two following points.

    First, the result of the measurement (distance between two locations measured using the phone’s GPS receiver) appears to be displayed to seven significant figures. I believe, given the level of accuracy inherent in the GPS system, is would be more appropriate to display measurements with a resolution of no less than 1 meter (m). (Resolution of 1 foot (ft) could be used in the USA with the understanding that 1 m is a little more than 3 ft.)

    The second improvement could be proper use of measurement uncertainty. With GPS Selective Availability turned off (as it is now) the inherent accuracy of a typical GPS receiver system is approximately /- 10 m. (If SA is turned on the inherent accuracy degrades to approximately /- 100 m.) IF the GPS receiver is capable of using enhanced or differential measurements, the position accuracy can be improved to approximately /- 3 m. (These methods include WAAS or Differential GPS - commonly used in surveying or precision navigation instruments - and comparison to locations of known fixed points such as cell phone towers.) For the purposes of discussion I am assuming that the error figures given are 95% confidence values, each standard measurement is made with at least three satellites in the sky more than 20 degrees above the horizon, and for enhanced measurements at least three cell phone towers can be “seen” by the phone, and all measurements are made using the same device. Using standard measurement uncertainty methods, the approximate 95% values for the error of a single distance measurement between two locations are listed below:
    Standard measurement, SA on: error = /- 141 m
    Standard measurement, SA off: error = /- 14 m
    Enhanced measurement, SA off: error = /- 5 m

    As you can see, if the estimated distance error is /- 14 m, it is not useful to show a value with resolution of 0.0001 foot (0.0012 inch, 0.03 millimeter)! (It also shows that it’s not necessarily so when the mapping application on my smartphone (not an Apple) tells me that the blue dot indicates my position within /- 3 m.)

    If the learning module includes (or the students are already familiar with) statistical methods and spherical geometry, the educator can demonstrate a simple way to improve the measurement uncertainty. At each location, the students could take a series of measurements (at least 5, up to about 20) several minutes apart, find the mean position and its standard deviation, and correct the sample standard deviation using the appropriate value of Student’s t. Then they would use spherical geometry to find the distance between the two positions, and combine the standard deviations to find the uncertainty of the distance.

    Error contributors include the orbital mechanics of the satellites, atmospheric distortion and absorption of the signals, the geodesic model used by the device (there are several different ones), the accuracy of mapping corrections to the geodesic model, the amount of sky obscured by buildings or vegetation, the position of the antenna, the time of day (which affects reception), solar weather conditions (which also affects reception) and probably more that don’t come to mind right now.

  2. 2 Paul Reid

    Fantastic comment and ideas Graeme! I will be sure to share these ideas with teachers I work with.

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