Latitude and Longitude 101
(Another no credit course of course!)

Lately, I have been confused about what GPS coordinates mean so here's what I have discovered:

GPS coordinates
are in Latitude ( [0 to 90] Horizontal bands around the earth) and Longitude ( [0 to -180] Vertical bands North to South).  Latitudes progress from 0 at the equator to 90 at the North Pole or -90 at the South Pole. Longitude progress's from 0 to 180 in a Westerly direction or 0 to -180 in an Easterly direction. Longitude 0 (aka the Prime Meridian) is arbitrarily selected to be the one longitude passing through Greenwich, England. So every coordinate is stated to be East (-) or West of Greenwich, England.

Horizontal lines = Latitude
Vertical lines = Longitude

A Latitude of 37.000000 is further North from the Equator then a Latitude of 34.000000

e.g.  If you are at 37 and you want to get to 34, you have to travel due South.  If you are at 34 and you want to get to 37, you have to travel due North.

A Longitude of -122.000000 is farther West then -121.000000

e.g.  If you are at -121.000000 and you want to get to -122.000000, you have to travel due West.  If you want to get from -122.000000 to -121.000000, you have to travel due East.

An Increasing Longitude means you are traveling in a Northerly direction.  If decreasing, then you are traveling in a Southerly direction.

A larger negative Latitude means you are going in a Westerly direction, less negative, in an Easterly direction

Another Example:
To get from 37.388877,-122.071224 to 37.219374,-121.936388  You would set a course towards the South and East (SE)

or vice versa
To get from 37.219374,-121.936388 to 37.388877,-122.071224  You would set a course towards the North and West (NW)

If the latitude you want to go to is larger then the one you are at, you have to go North

If the longitude is a smaller number then where you are at (you are at -122 and you want to get to -121) you need to go in an Easterly direction.

e.g. Priest Rock Trail head is at 37.196478,-121.984002  and Alum Rock Trail head is at 37.395255,-121.825486.  From this, you can observe that Priest Rock is further South then Alum Rock and further West. Looking at a map verifies what the coordinates tell you.

General Notes:
1. A Statute Mile is 5,280 feet, a Nautical Mile is 6,076 feet (1.15 miles)
2. The circumference of the earth at the equator is 24,901.55 miles, at the poles it is 24,859.82 miles (smaller).
3. The distance from one Longitude to the next at the Equator, is 69.170972 miles
4. The distance from one Latitude to the next is 69.055055 miles
5. The distance from one Longitude to the next, gets smaller as you go away from the equator.
6. My GPS coordinates are to 1/1,000,000 resolution between any two longitudes or latitudes.  One millionth of the distance between two longitudes at the Equator would be 69.170972/1,000,000 or 0.000069170972 miles, so, in theory, a six place GPS coordinate is accurate to 0.365 feet (4.38").  In practice, +/- a yard is generally the rule.


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December 13, 2013

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