Introduction to Map Use

study outline – navigation

You should be able to:

1. given an azimuth angle from true north or grid north, set a compass heading to travel in that direction

2. determine your position on a map by measuring angle to features you can find on a map

3. describe how a the GPS system operates

A. direction determination – direction is described as an angle relative to some reference direction.

1. an azimuth is the angle from 0 to 360 deg clockwise from north

2. a bearing gives the direct as the direction east or west of the north or south line

B. reference directions

1. true north – the direction to Earth’s northern rotational axis

2. magnetic north – the direction that a compass needle points

3. grid north – the direction of the north south lines in a grid system

C. magnetic declination

1. a compass lines up with the local magnetic field

2. there are only a few places on Earth where the magnetic field lines up with true north

3. declination is the angle between magnetic north and true north and is described by the direction (east or west) and size of the angle from true north to magnetic north

4. Earth’s magnetic field is complex and constantly changing if the declination is printed on a map, it should be rechecked if it is more than 10 years old.

5. corrections

i. when using a compass to determine the direction from true north, east declinations most be added to the compass reading and west declinations must be subtracted

ii. when using a compass to determine the direction to travel given a heading angle from true north, east declinations must be subtracted and west declinations must be added

iii. drawing a diagram can be very helpful in remembering the above

D. map orientation

1. means holding a map so that the features on the map line up with the features on the ground

2. can be done by inspection

3. or by aligning a compass with magnetic north on the map and then rotating the map until the compass needle lines up with the north arrow in the compass housing

E. position determination

1. by resection or triangulation using measurement of angles

i. find the azimuth angle to features that you see in the real world

ii. draw a line at that angle through that feature on the map. you should be somewhere on that line.

iii. lines for sightings on two to four features should all cross at your location

2. by global positioning system (GPS) using measurement of distances

i. GPS receivers can determine your distance from satellites orbiting the Earth

ii. if you know your distance from four or more satellites and you know the position of the satellites, you can determine your exact position in space

iii. problems

(1) GPS signals are intentionally degraded by the government to prevent their use by enemies in armed conflict

(2) GPS signal are degraded by weather, tree cover and buildings

(3) the accuracy of handheld units is typically 10s to 100s of feet

(4) a map and compass should always be carried as backup to GPS when travelling in unknown territory.

3. the special needs of ship and airplane navigation require additional specialized maps and electronic navigational aids.