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Petit DF – A Direction Finder

Application Name: Petit DF

Description: Rhumb line and great circle direction finder

Publisher’s website: inda3

Cost: Free

Version/date reviewed: v.2.7  /  4-17-11

Phone/OS: Droid X / Android 2.2


Android Market (mobile app only)
Android Market (browser)

Petit DF (stands for “small direction finder”) shows the direction to a user-selectable destination. And it works, but with quirks.


Start up the app, and it will show your position plotted as a green dot in what appears to be a Bing Maps interface. The green flag is also your destination, and is set at startup at your current position. The blue dot indicates the current direction magnetic north, red dot magnetic south, and they move as you rotate the device to maintain the correct orientation.


The map zoom controls are the +/- in the left corners; the “v” at the upper right toggles you between the default satellite view and a street maps view, while the button at lower right makes the map view rotate to match the direction you’re currently facing in. Latitude/longitude are shown at the bottom, along with the nearest geocoded address/location for your destination. Magnetic declination is also shown, which makes it odd that the app doesn’t use those to correct magnetic directions to true directions.


You can set a destination to get the direction to in several ways. First is to zoom/scroll the map to find your destination, then tap on it. You can pinch to zoom, but that has an annoying tendency to relocate your destination to a spot on the map you touch during the pinch operation, so using the +/- zoom controls is a better choice (scrolling with tap and drag doesn’t have this problem). The direct “rhumb line” direction to the destination is shown in red, and the distance to the destination is shown at top. Surprisingly, the actual compass bearing direction to your destination is not shown; this would be helpful in giving you information you could use with just a simple compass to maintain a heading to that destination.



There’s also a Search function that lets you enter an address, keywords, or latitude/longitude position; once located, you can save that position in database to load in directly later. For example, entering Mecca as the destination and loading it in would put the green destination flag at Mecca, as above. Pity it doesn’t show the magnetic declination value for the destination, as this would make it a handy reference app for this value.


To go back to your current position, tap the button at lower right to get the view above. The red line shows the “rhumb line”, the straight line you’d draw on a Mercator projection between your current location and the destination; following this line would get you to the destination, but it wouldn’t be the shortest route across the Earth. The app instructions say that it will draw a pink line to indicate the “great circle” route, the shortest distance between your current location and the destination when traveling on the Earth’s spherical surface. For short distances, the rhumb line and great circle will generally be very close to the same; for long distances, like the US to Mecca, they should diverge dramatically. At first glance, that pink great circle line seems to be entirely missing here. However, if you look closely at the two screenshots above, you’ll see a short pinkish stubs indicating the start, stop, and general direction of the great circle route, but the rest is missing.


Other issues: The app appears to be a resource hog; other apps, like my screenshot app, slowed down dramatically when it was active.

Final thoughts: Potentially useful app, but hobbled by the lack of a bearing degrees indicator, use of magnetic rather than true north, and the buggy great circle route line. Could still be useful if you need to determine the direction to multiple points saved in the database from your current location. Hopefully the author will fix these issues in future updates.

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