2.2 Finding North on the planet Earth PDF

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Lecture notes and what was taught. this is the complete information about what was done and what was noted....

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Finding North Another important item that all good maps will have is some indication of how to properly position the map when viewing it. This is the orientation.

Orientation Traditionally you orient a map with north pointing towards the top and parallel to one edge of the paper, however, there are exceptions. Look back at the campus map of SMU. Notice that the map is actually on an angle and the north arrow is pointing to the left. As we will see, it is better to use bearings when referring to maps, rather than directions like " right" or "down". These terms will be relative to how the person is holding the map, while bearings are referenced to the north direction. So, no matter if you are holding the map on its side, or even upside, down a bearing will always give the right direction.

The Rotational Pole When we use the term "north" we are generally referring to the north rotational pole. This is the axis about which the planet revolves - this is sometimes referred to as "true north". Most maps will have some kind of symbol indicating the orientation. Most commonly this is an arrow indicating the direction of the north pole. But how do we find north?

Magnetic North

Earth’s magnetic field can be illustrated as a bar magnet coinciding with the core

The traditional method for centuries was the compass with a magnetized north-seeking needle. A more recent method is to use a GPS (global positioning system), which we will discuss later. The north arrow of a compass points to the north pole correct? Well actually it points to the magnetic north pole. The Earth's magnetic field is generated within the iron - nickle outer core as the planet rotates. Often this magnetic field is illustrated as if it was a large bar magnet inside the Earth - do realize that this is just an analogy to help you visualize this field, there is not really a large magnet inside the planet. The magnetic pole is not perfectly aligned with the rotational axis. The two poles are separated by almost 1300km, however, this is changing constantly because the magnetic pole tends to wander. So where is the magnetic North Pole? The North Magnetic Pole is slowly drifting across the Canadian Arctic. The Geological Survey of Canada keeps track of this motion by periodically carrying out magnetic surveys to determine the Pole's location.

On this figure you can see how the location of the magnetic north pole has moved over time

The location of the pole was first measured by James Clark Ross in 1831, and for the most part since then it has slowly wandered around the Canadian arctic region. But in the mid-1990s the rate of this random movement increased from 15 to 55 kilometres per year. In 2001 the North Magnetic Pole lay near Ellesmere Island in northern Canada at 81.3° N 110.8° W.

In 2009 it was still in the Canadian Arctic at 84.9o N 131.0o W In 2018 it crossed the International Date Line and moved into the Eastern Hemisphere and is currently moving towards Siberia. You can find more information about the Magnetic Pole at Natural Resources Canada (NRC). Declination

The declination is the angle between magnetic north (towards which your compass points) and true north. This value is dependent upon where you are located relative to the pole. You can see on the isogonic chart that the East Coast is east of the 0 degree declination line (the agonic line) and so a compass will point to the west of True North and so the value given will always be less than True North. At present the declination at Halifax is 18 degrees 22 minutes west. If you were in Vancouver, on the West Coast, the compass will point to the east and values will be higher. How to remember this? There are a number of rhymes such as East is Least, but I find it just as easy to picture the chart. At this point you may be asking how much of a difference can being out a few degrees make. Pretend you are in the field and were to walk to another location on your map, a distance of 10 km.

If your compass was off by 1 degree you would miss your target by about 170 metres; not too bad. If your compass was off by 5 degrees you would miss by about 870 metres. If you are in Halifax, more than 18 degrees off, you would end up over 3 km west of your target! To further complicate things, because the magnetic pole is not fixed in one location the declination is changing each year. To determine the amount of declination for a given location you need only check a topographic map for that area (other scientific maps will also give the declination). There will be a value, the year in which it was measured and the rate at which it is changing. Multiple the rate by the number of years and add this to the given declination. Most compasses that a geologist would use will allow the declination to be set so that the needle will be pointing to true north. Thanks to Natural Resources Canada we don't even have to go through this now. You can just go to their web site and enter the coordinates for any point and get the current declination.

Three Norths

Magnetic declination from the Wolfville topo map sheet

So, in fact most topographic maps will display three north directions. True North: the direction of the rotational north pole. Magnetic North: the direction of the magnetic north pole for that map sheet. Grid North: the direction of the vertical reference lines shown on the map as part of the UTM grid. It is important to know to which north you are referring. Magnetic declination is measured between the magnetic north and the true north, but on some maps the declination is shown in two parts- magnetic north to grid north and grid north to true north. The declination for Wolfville NTS map sheet. For the year 2000 the declination between magnetic north and grid north was 19 degrees 5 minutes. Grid north to True north was 0.53 minutes.

You should make sure you can identify the the declination information on the Wolfville topographic map (look in the Media Library).

Measuring Map Direction There are basically two methods for measuring direction (bearing): quadrant or azimuth.

Quadrant bearing:

When you hear someone speak of northwest or southeast they are using quadrant bearings. This is an ancient system which divides the compass (or circle) into four quadrants of 90 degrees each: northeast (NE), northwest (NW), southeast (SE), and southwest

(SW). It can start getting confusing with terms such as south southeast (SSE). This is OK if you are speaking to someone but the directions are pretty vague. Compass quadrant bearings include a number in degrees measured from north (zero) along with the quadrant. The bearing is expressed by stating the pole (north or south) from which it is measured, the magnitude (0 to 90 degrees) and the direction (east or west). For example, N30W (which means 30 degrees west of north). Azimuth bearings:

Geologists tend to use a slightly different system. You probably have already had some experience with the azimuth system - most of you will remember the protractors found in geometry sets. This system divides the circle into 360 degrees in a clockwise direction. North is 0, east is at 90, south at 180, and west at 270. So to give a direction you would simply write 045 degrees or 200 degrees. Notice that it is common practice to always use 3 digits. Geologists often also add the quadrant at the end - 265SE. This is just a quick method to ensure that you haven't accidentally read your compass backwards and are 180 degrees off. Both systems are commonly used in Canada and we will be using both throughout the course.

Important Terms or Concepts

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Magnetic north Declination Bearing Quadrant Azimuth...