Map North/True North

[Coral Rhedd]

What is the degree variation between Map North and True North and how can one tell if a map is accurate?

 

If this is not a good place for this question, where should I post it instead?

[ecoli]

Did you mean True North and Magnetic north?

 

Magnetic North is constantly moving, it's documented location in 2003 was 78°18' North, 104° West. (wikipeida)

[Coral Rhedd]

Did you mean True North and Magnetic north?

 

Magnetic North is constantly moving' date=' it's documented location in 2003 was 78°18' North, 104° West. (wikipeida)[/quote']

 

No. I know about magnetic declination. What I am trying to find out is if most maps (I believe they should be using something called Grid North.) match True North. For instance, if cities or a park service publish a map, are they usually accurate and, if there is a possibility that they may not be, how do you find out?

[Spyman]

how do you find out?

Do You know how to use a compass ?

[Coral Rhedd]

Do You know how to use a compass ?

 

Yes, but this is not about magnetic north.

 

I am trying to resolve an argument that arose at a dinner party. I said my house faces due west and a guest (more an interloper ) said I could not be sure if I was going by the sun. I told him I was not going by the sun, I was using the city map which should be pretty equivalent to True North and he said "not necessarily, blah, blah . . ."

 

So is Map North and True North always the same? How to I find out?

[coquina]

Yes' date=' but this is not about magnetic north.

 

I am trying to resolve an argument that arose at a dinner party. I said my house faces due west and a guest (more an interloper ) said I could not be sure if I was going by the sun. I told him I was not going by the sun, I was using the city map which should be pretty equivalent to True North and he said "not necessarily, blah, blah . . ."

 

So is Map North and True North always the same? How to I find out?[/quote']

 

"Map north" is the same as "true north", because a map is set up by parallels of latitude and meridians of longitude.

 

If you look at a nautical chart, it is set up the same way. There will be a compass rose on the chart, and the inner circle will indicate magnetic north at the time the chart was printed. There will also be a notation indicating the annual change, so that if one is using a chart that is several years old, one can determine the difference betweeen true and magnetic for the given time.

 

In navigation, the difference between true and magnetic is known as "variation". If one navigating by a magnetic compass, one also has to contend with "deviation" - the amount of compass error caused by magnetic objects on the boat. One of the first things you learn in a navigation class is "TVMDC". (true +/- variation = magnetic +/- deviation = compass) The standard ditty used for memorization is "True Virgins Make Dull Company".

[Skye]

It depends what you mean by map north. If you mean grid north, then no.

 

The vertical lines that we normally use to divide up the earth are lines of longitude, also called meridians. All the meridians pass through the north and south poles. But on a flat map only one of these can by a straight line, the rest have to be curved, and the further they are from the one straight meridian the more curved they become. So if you have a map that is divided into squares by straight horizontal and vertical lines, as in a street map, the vertical lines WON'T pass through the north pole. They are, on a flat piece of paper, parallel to that single meridian that is straight. Also, except at one point on the map, they won't be perpendicular to a true east-west axis.

 

The angular difference between grid north and true north is called the convergence, and this can be used to calculate the angular difference between the grid north and the true north at a point on the map. You also need to know which meridian the convergence is relative to. These things will be given on maps that are to be used for navigation, but not usually on things like street maps.

 

Sorry if any of this is wrong, it's been a while since I've done nav.

[Spyman]

Yes' date=' but this is not about magnetic north.

 

I am trying to resolve an argument that arose at a dinner party. I said my house faces due west and a guest (more an interloper ) said I could not be sure if I was going by the sun. I told him I was not going by the sun, I was using the city map which should be pretty equivalent to True North and he said "not necessarily, blah, blah . . ."

 

So is Map North and True North always the same? How to I find out?[/quote']From Your house I guess the difference of True North and Magnetic North would be very, very small...

(Assuming that Your house is not an igloo.)

 

So I would verify the city map with a compass, if I don't trusted the map.

(Maps is made by humans and can have errors.)

 

Map North and True North is almost the same and for most practically reasons it's good enough for me.

 

BTW Maybe Your guest was thinking of the Magnetic North ?

 

EDIT: Another way to check the direction is by in fact using the Sun.

Put a "stick" or something in the ground so You get a long shadow, when the shadow is shortest, around 12:00, it points towards North.