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    <p>Remember a short history of position, going back a few seconds. 5 seconds should give you a reasonably accurate result that updates fairly quickly...</p> <pre><code>// delay is the time difference between the 2 samples you have delay = 5; // 5 second delay // figure out how far along x and y we have moved since last time dx = newx - oldx; dy = newy - oldy; // distance travelled distance = sqrt(dx*dx + dy*dy); // find the speed. if the positions were measured in metres and the time in seconds // this will be the average speed in metres per second, over the last 5 seconds speed = distance / delay; </code></pre> <p>The longer you can wait between samples (eg, if you keep the last 30 position samples and use a 30 second delay), the more stable your answer will be (ie, the less noisy it will be), but the slower it will be to react to any changes in speed.</p> <p>Why do you need to add this delay stuff? well, the GPS unit in your phone probably isn't very accurate. If you are standing still, the position it returns each second might wobble about a fair bit. This wobble noise will make it look like you are sprinting randomly around the room and might cause you to report a moderately high speed, even though you're not moving at all. The solution I have listed will not really help when you are standing still, as the result from 30 seconds ago will be just as wrong as the position from 1 second ago. What you really need to do is average the position over a while, then compare that to the average position from a slightly earlier time. eg...</p> <p>Take 10 samples of position and average them. This is position 1.<br> Take another 10 samples and average them. This is position 2.<br> Use these 2 positions with the code above to get the speed.</p> <p>Again, the more samples you can take, the more accurate and stable your positions will become, but this will make your speed measurement less responsive.</p>
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    1. POI'm getting a callback with latitude and longitude, how can I best calculate "current speed", and other derived values?
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    1. COMy question is really just about the 5 seconds. Did you just make that up, or does it come from actual field experience with GPS imprecision? How do you know that 4 isn't a better answer, or 6?
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    2. COI've updated a bit to try and make it clearer. Basically, the more samples you can do, the more stable your answer gets, but the slower it is at reacting to changes in speed. How much accuracy and how responsive you need your application to be is up to you. It also depends on the speeds you are travelling at the how much position noise is coming from the device. If your device is accurate to +-2 metres, then that is a massive amount of noise at walking speeds, but probably no problem at aircraft speeds. There are too many "it depends" to give you a good guess without knowing more.
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    3. COYou added a lot more to text to your answer, but now there is another magical number, "10". How do you know that "9" isn't a better number?
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