Feet Per Minute

I recently have found a new hobby: Geocaching. I've been wondering just how precise a GPS receiver can get (assuming perfect reception). Latitude and Longitude are measured in degrees, latitude measured North or South from 0 to 90 degrees, longitude measured East or West from 0 to 180 degrees. Each degree is divided into 60 "minutes". Each minute is divided into 60 "seconds", though most displays of coordinates just break the minutes into three decimal places, like "N 38 degrees, 47.213 minutes". Assuming the Earth is a sphere with a circumference of 25000 miles, how many feet are in each minute? And thus, how precise is a thousandth of a minute?

The answer for latitude is constant, but the answer for longitude will vary with the latitude. (At the poles, no distance is traveled for a change in longitude.) Can you find a formula for both? Feel free to send URLs of web references, too.

Source: Original.

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