The Apple iPhone includes an integrated Global Positioning System satellite navigation device that tracks the exact position of the phone. One of the features on all iPhones is a compass application that uses the GPS data to find and display the phone's longitude and latitude position. After you have the correct latitude and longitude for your location, convert these coordinates to an eight-digit National Grid Reference with an online tool that you can access on the iPhone's Safari Web browser application.
Tap the Compass application button on the iPhone's display screen.
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Read the latitude and longitude coordinates at the bottom of the Compass display screen. The longitude coordinate ends with "W" and the latitude coordinate ends with "N." Write both figures on a slip of paper so that you can read them easily when you need to input them into the online converter.
Open the iPhone's Safari Web browser. Go to the United States National Geodetic Survey's online conversion tool at Ngs.noaa.gov.
Type the latitude into the first input box. Place the "W" before the number coordinates rather than after them. Enter the longitude into the second box. Again, place the "N" before the number coordinates.
Click the "Submit" button. This displays the full U.S. National Grid spatial address for your location; for example, 18SUJ23480647. In this example, "18S" is the Grid Zone Designation and "UJ" is the Square Identification. The numbers that follow this comprise the eight-digit U.S. National Grid coordinates. In this example, the grid reference is for the Washington Monument, Washington D.C..