SAR Fundamentals/Compass practical and pacing

00:00

3 min

Introduction

Introduce topic title

Introduce Instructor

Present Objectives

00:03

Review - declination

• Magnetic north is not the same as true north
• point out the approximate direction of true north and magnetic north
• The difference between the two is called declination
• In our part of the world magnetic north is to the east of north, so we have EAST declination
• Declination changes each year as the north magnetic pole moves

• Give declination for lesson location to the nearest degree (ignore seconds)
• We should always ask what the declination is for our search area

Setting declination

• The needle points to magnetic north
• But we want the directions on the compass bezel to show directions relative to true north
• We do that by setting declination on the compass
• We use the key on the lanyard to move the bed
• point out how to read the declination a compass is set at
• On most compasses, each tick represents 2 degrees, so odd numbers are half way between ticks
• point out which side is EAST declination and which is WEST
• Have each student set their declination to zero and then back to the correct declination for this location
• Check that each student has correctly set declination

Review - compass failures

• avoid metal
  • don't hold compass close to your belt buckle
  • don't stand near vehicle, metal building
• avoid magnetic fields
  • don't stand underneath high power lines

Holding compass

• about arm's length away from you
  • don't have lanyard around your neck
• hold the compass about at eye level
• hold the compass level
• demonstrate

Aligning needle

• the north pointing end of the needle is painted red
• the "bed" of the needle is on the plastic underneath and points slightly to the east of North on the bezel
  • because we set EAST declination
• we want to put the "red to the bed"
• two ways
  • rotate the compass until they line up
  • rotate the bezel until they line up
• demonstrate both methods
  • looking down on compass, not using mirror
• have each student align their needles using both methods
• when the the needle is aligned, the directions on the bezel are the directions relative to True North

needle parallax

• it's easy to align the needle when looking straight down
• if you look sideways it's harder
  • called parallax
• we need to be able to do this so that we can "sight" at the same time as aligning the needle
• demonstrate
  • set direction of travel on bezel to East (90 degrees)
  • fold mirror flat (so it can be ignored)
  • hold the mirror at eye level so you are looking obliquely at the compass surface
  • demonstrate rotating your body until the needles align
• have students do the same
• have student check their alignment by then looking straight down on needle
• trick is NOT to try to get the tip of the needle in the middle of the bed
• trick is to try to get the edge of the needle parallel with the edge of the bed
• have students retry until they can get proper alignment looking

• have students try again with direction of travel set to North (0 degrees)
• This is much easier because we're looking along the needle length
• direction near north and south are easy to align, near east or west are harder

Sighting

• sighting requires that you have several things working at once.
• close one eye
• fold the mirror so you can see all of the bezel in it
  • when you hold the mirror away from you, you're looking in the mirror; you're not looking down at the bezel directly
• make sure you're holding the compass level
• make sure you see the center line in the mirror slices through the middle of the needle
• use the sighting notch at the top of the mirror center line to spot your target
  • you may have to raise or lower the compass slightly to bring the target into view
  • some compasses also have a sighting notch at the bottom of the mirror center line
• RECHECK you have all things correct at the same time:
  1. one eye closed
  2. compass level
  3. can see all of bezel in mirror
  4. mirror center line through needle center
  5. can see target in notch
• demonstrate
• have each student sight on an object on the far distance; have them verbally confirm each of the sighting requirements
  • Because you can't put your head where the student's head is, you won't be able to check all of their sighting requirements, but can check whether they are holding compass level, have one eye closed. You can approximate the others by looking over their shoulder.

Reading a bearing

• imagine we want to call in the bearing to some object (maybe we've seen a clue)

• sight on the object
• align the needle by rotating the bezel while looking at needle in mirror
• read the bearing from the direction of travel on the bezel

• we have to both sight on the object and align the needle at the same time
• adds one more thing to check we are doing all at once

• pick an object in an approximately north or south direction
  • should be at least 1 km away
  • should be narrow and clearly distinguishable: the peak of a roof, side of a building, power pole, etc.
  • keep the students close together; even better if they can line up facing object one above the other along the side of a hill
  • take a bearing but don't tell the students what you get
  • have the student read the bearing of the same object
  • compasses are only accurate to 1 degree; one compass might be out one degree one way while another is out one degree the other way
  • ideally student values should all be within 2 degrees of each other; if not work with outliers to get more accurate while other students take bearing of other objects
• move on to harder objects in approximate east or west direction
• choose a closer object
• objects that are closer are harder to accurately read bearing because it depends on where you're standing (left or right)
• sight on object you're talking about, not one that is between you and it

Shooting a bearing

• imagine we've been told to head off in some direction

• set the direction of travel on the compass
• align the needle while looking in the mirror, by rotating our body
• determine what object you're sighting on
• we have to both sight on the object and align the needle at the same time

• keep the students close together
• give a sample bearing near south (say 160 to 200 degrees) or near north (340 to 360 or 0 to 20 degrees)
• have students shoot bearing and identify object they can see
  • students are likely to identify objects at varying distances

• best to choose object at distance you want to travel
  • if you choose object much further (e.g. mountain peak) walking left or right will not change the direction to your sighted object because it's so far away
• if you can see all the way to you destination, choose the furthest object you can see

• have students repeat with sample bearing near east (70 to 110 degrees) or west (250 to 290 degrees

3/ Pacing

• Step is one foot to the opposite
• Pace is one foot to the same foot (two steps)

• counting your pace

• measure your pace in 10m

• calculating distance from paces

• calculating paces from distance

4/ Exercises: bearings

• hand-out: Pacing and Traverse

5/ Advance

As they come up in discussion, describe:

• back bearings

• offsets