| ??? 01/17/08 12:54 Modified: 01/17/08 13:09 Read: times |
#149640 - Adjusting for latitude Responding to: ???'s previous message |
Ap said:
Can you please explain me why Adjust east-west distance for latitude is required. Sure. At the equator, one degree of longitude corresponds to an east-west distance of about 69 miles. (Circumference of the earth / 360 degrees = 25,000 miles / 360 ~= 69 miles.) However, as you move north or south away from the equator, each degree of longitude corresponds to a smaller and smaller distance as the longitude lines get closer and closer together. Right at the north and south poles, where all the longitude lines come together, there is no space between them. That's why the adjustment is necessary. Ap said:
Also is the formula equally well
d = acos(sin(Lat1) × sin(Lat2) + cos(Lat1) × cos(Lat2) × cos(Lon1 – Lon2)) where d= distance and result is in radians. That formula gives the exact distance, and takes into account the curvature of the earth. In other words, it does not make the approximation that a small chunk of the earth's surface is essentially flat. There are two problems with that formula. One is that it is time consuming to calculate. (I count six trig functions!) The other is that for small distances, you're going to be bothered by roundoff errors in the calculations. Look here for more discussion. For what you're doing, stick with something simple. -- Russ |
