Where is it? Relative Vs. Absolute Coordinate Systems

You’re running late for the big field meeting with a new client to conduct a UAV LiDAR survey. The directions they gave you list the site on Hot Creek Hatchery Rd, 3.4 miles off U.S. 395 in Northern California. The GPS gives it as 37 40 24.24N 118 48 42.84W. Why are they so different? More importantly, you’re in a hurry, which one do you trust? 

The difference is that the first address is relative to the position of U.S. 395. If the highway gets re—routed, detoured, etc., this address may no longer be accurate. The address depends on the relative position of U.S. 395. On the hand, the coordinates are an absolute designation of a location on Earth. The relative address depends on some locally arbitrarily designated landmark – in this case, U.S. 395. The absolute address depends on a globally accepted coordinate system based in Greenwich, U.K., first established in 1851. Every point on Earth can be referenced to this system. The GPS satellites all use this Prime Meridian, 00 00 000E/W and the established equator of the Earth, 000 00 00N/S, as the origin for their coordinate system, known as WGS-84. This is also the reason that Greenwich Mean Time (GMT), a.k.a. Zulu time, is the time zone of the Prime Meridian, and all other time zones are in reference to GMT. 

So, which one do you use? In this case, as long as the highway hasn’t been moved, either one will work just fine. You will find yourself at the Hot Creek Geological Site. Admire the hot springs, but stay out of the water!