Feet per Second/Millisecond a degree of longitude.?

Question

I am trying to determine how many feet there are per a second/millisecond of longitude at a given latitude.

What I'm trying to do is create a simple radar for my gps to track distance from fire hydrants. However, I only want to go out to a 1000 feet. While I have this working for latitude, the longitude formula is proving much more difficult to put together.

Basically what I want to do is :

CurrentPosition +- (1000/Feet per Second of Longitude)

Answer 1

A degree of latitude is approximately 69 miles, and a minute of latitude is approximately 1.15 miles. A second of latitude is approximately 0.02 miles, or just over 100 feet.

A degree of longitude varies in size. At the equator, it is approximately 69 miles, the same size as a degree of latitude. The size gradually decreases to zero as the meridians converge at the poles. At a latitude of 45 degrees, a degree of longitude is approximately 49 miles. Because a degree of longitude varies in size, minutes and seconds of longitude also vary, decreasing in size towards the poles.

Answer 2

1 minute of arc at equator = 1naut.mile = 6080 feet

1 second of arc at equator = 101 1/3 feet.

At latitude 'w', the above quantities have a multiplying factor

cos w.

(101.3333) cos w feet per second of longitude

At a latitude of 60 degrees (=w)

1minute=3040'

1second=50 2/3 '