Maps or charts? (part 3)
December 28th 2006 19:11
Map of Europe
Maps or charts?
Mercator was the man who decided to lay the round earth out on a flat piece of paper that was easy to read and store on a ship. This is still seen on modern charts and called “Mercator’s Projection” It gives a slight distortion to position but it is so slight it does not matter as the final tool in a navigator’s box of black art tricks is the eyeball. Once he is close enough he can visually con the boat.
To decide if you have a map or a chart look at it. If most of the information is about land based objects and only a little about the sea it is a map. A chart has good accurate (or as close as possible) information on the water. It is possible to become pendadtic here but no real need to. Down the sides you have degrees of latitude and across the top and bottom degrees of longitude. Read all the information given on a chart before you go to use it. All the notes and special warnings, the date the chart was printed, has it been manually updated. On old charts lots of drawings were used of real or imaginary objects found in the sea. Some areas were marked, “beware, here be monsters”. This was a way of saying “we have no bloody idea what is here, stay away.” Today it is simply marked as Inadequately Surveyed.
Once you have determined on a chart where you are, you can decide where you are going and how you are going to get there. Using the degrees of latitude and longitude you would say you were at Latitude 31deg 47 min. south and 151 deg 26 min. east. There are sixty minutes in each degree and each minute is divided into sixty seconds. Degrees of latitude are measures of distance, each being sixty nautical miles. Degrees of longitude measure the time from Greenwich so never use this as a measure of distance.
How can time be placed on to a chart? Remember stone age man holding his hand out at arms length to measure the number of hand spans or fingers the sun was over the horizon? Well each degree measures the span on the sun around the earth. !80 deg. East or west of Greenwich. So if we are 151 deg 26 min. east of Greenwich the local time where we are is 152 deg 26 min divided by 15 (the number of degrees the sun moves in an hour. So the answer is very close to 10.5 hrs east of Greenwich. If in reverse we know the actual time in Greenwich, and we can determine the very actual time at our position on the earth, (by measuring the suns angle from the horizon) we can determine our longitude.
Degrees of latitude are measured parallel from the equator. 90 deg south and 90 deg north. As each degree represents 60 miles we can determine how many miles north or south of the equator we are. So at 31 deg 47 min south we are 1907 miles south of the equator (31 x 60 47 = 1907)
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