How to Use a Compass with a USGS Topographic Map
METHOD
#1: (these
directions assume your orienting arrow lines up with the North indicator on your
compass dial, meaning the compass has not been adjusted for declination).
A)
Obtain the local magnetic declination for the area represented on your map.
At the bottom of every USGS map is a diagram that displays the difference
& direction between true north (represented as a star), grid
north (abbreviated as “GN”), and magnetic north (abbreviated as
“MN”). Magnetic declination is
the number of degrees and direction between true north and magnetic
north. Because declination
varies over time, it is advisable to get a reasonably current figure.
If your USGS map is more than 15 years old (the declination date appears
in the diagram), here’s an easy-to-use website that gives you only the
information you need:
http://www.geolab.nrcan.gc.ca/geomag/mirp_e.shtml
If
magnetic north is east of true north, the local declination is positive.
If
magnetic north is west of true north, the local declination is negative.
B) Draw a line
on the map that connects your starting point with the destination (your “map
bearing”). Extend the line all the way through the map border (the “neat
line”).
C) Distance
yourself from any nearby metal such as keys, belt buckle, desk, car, fence, etc.
Place the compass on the map so the needle’s pivot point is directly over the intersection of your map
bearing and neat
line.
D) Rotate the
dial until compass ring north agrees with map north.
Read your map bearing
from the compass dial. Make sure
the bearing agrees with your direction of travel – for example, if you intend
to travel due east, the bearing is 90 degrees, not 270 degrees.
E) Do this step mentally – don’t turn the compass dial. If the local
declination is positive, then subtract the declination amount from
the bearing you just derived. If
the local declination is negative, then add the declination amount
to the bearing you just derived.
F)
Turn the compass dial until the figure you calculated in step E lines up with
the index line.
G)
Lift the compass off the map, and with the direction of travel arrow pointing
directly away from you, rotate your body and the compass all in one motion until
the red magnetic needle overlays the orienting arrow.
H)
Site a landmark along this bearing, and proceed to it.
Repeat this step until you reach your destination.
A)
Obtain the local magnetic declination for the area represented on your map.
At the bottom of every USGS map is a diagram that displays the difference
& direction between true north (represented as a star), grid
north (abbreviated as “GN”), and magnetic north (abbreviated as
“MN”). Magnetic declination
is the number of degrees and direction between true north and magnetic
north. Because declination
varies over time, it is advisable to get a reasonably current figure.
If your USGS map is more than 15 years old (the declination date appears
in the diagram), here’s an easy-to-use website that gives you only the
information you need:
http://www.geolab.nrcan.gc.ca/geomag/mirp_e.shtml
If
magnetic north is east of true north, the local declination is positive
If
magnetic north is west of true north, the local declination is negative.
B) Draw a line
on the map that connects your starting point with the destination (your “map
bearing”).
C)
Distance yourself from any nearby metal such as keys, belt buckle, desk, car,
fence etc.
D)
Place the compass on the map so the baseplate is parallel to the line you drew. Make sure the direction of travel arrow points to your
destination.
E)
Rotate the dial until compass ring north agrees with map north.
Do not move the compass when you rotate the dial.
F)
Remove the compass from the map and, with the direction of travel arrow pointing
directly away from you, rotate your body and the compass all in one motion until
the red magnetic needle overlays the orienting arrow.
G)
If local declination is positive, then subtract the declination
amount (turn the dial clockwise). If
local declination is negative, then add the declination amount
(turn the dial counter-clockwise).
H)
Again, with the direction of travel arrow pointing directly away from you,
rotate your body and compass all in one motion until the red magnetic needle
overlays the orienting arrow. Site
a landmark along this direction of travel and proceed to it.
Repeat this step until you reach your destination.
A
compass with adjustable declination allows you to rotate the
orienting arrow independently of the compass dial. If you have such a compass, you may calculate your map
bearing without adding or subtracting the amount of local magnetic declination.
To calibrate your compass in this fashion, rotate the inner liquid
capsule (or turn the screw with the key) until the orienting arrow deviates from
the compass ring’s north indicator by the amount & direction of the local
magnetic declination. For example, if local declination is 10 degrees east of true
north, rotate the inner liquid capsule (or turn the screw with the key) until
the orienting arrow points to 10 degrees east.
If using Method #1, you do not need to add or subtract as
indicated in step E; just make sure compass ring north (not the orienting arrow)
agrees with map north as directed in step D If using Method #2, skip steps G and H; again,
be sure compass ring north (not the orienting arrow) agrees with map north as
directed in step E.
Place
the compass on the map so the baseplate parallels the north-to-south map neat
line. Rotate the dial until
compass ring North agrees with map North. Add
or subtract the amount needed to adjust for local magnetic declination (subtract
if local declination is positive, add if local declination is negative); if your
adjustable declination compass is already calibrated for local
declination, you don’t need to add or subtract, just make sure compass ring
North (not the orienting arrow) agrees with map North.
Holding the map and compass steadily (the baseplate should still be on
the north-to-south map neat line), rotate the map and compass all in one
motion until the red magnetic needle overlays the orienting arrow.
Again, make sure there is no interference from metal when you perform
this (ex: rebar in concrete). Your
map and your compass are now oriented to true north.
Compare the physical features around you with your map to help derive
your location on the map..
Let’s
assume the local declination is 10 degrees east of true north, and the waypoint
you wish to navigate to is directly north of your current location.
If
using a compass without adjustable declination, set your GPS to magnetic
north. Your GPS will indicate a
bearing of 350 degrees must be followed to reach the waypoint.
Dial 350 degrees on your compass. With
the direction-of-travel arrow pointed directly away from you, turn
your body & compass in one motion until the red magnetic needle overlays the
orienting arrow. Site a landmark
along that bearing, and proceed. You
will be traveling directly north toward the waypoint.
If
using a compass with adjustable declination, set your GPS to true
north. Your GPS will indicate a
bearing of zero degrees must be followed to reach the waypoint.
Adjust the declination on your compass so the orienting arrow points to
10 degrees east. Dial zero degrees
on your compass. With the
direction-of-travel arrow pointed directly away from you, turn
your body & compass in one motion until the red magnetic needle overlays the
orienting arrow. Site a landmark
along that bearing, and proceed. You
will be traveling directly north toward the waypoint
TO DETERMINE
PERCENT OF SLOPE AND ANGLE OF SLOPE:
Percent of slope is determined by dividing the amount of elevation change by
the amount of horizontal distance covered (sometimes referred to as “the
rise divided by the run”),
and then multiplying the result by 100. The
“run”
assumes you’re traveling on an idealized flat surface – it does not
account for the actual distance traveled once elevation change is factored in.
Example: let’s assume your climb gains 1,000 feet in altitude
(the rise) and the horizontal distance as
measured on the map is 2,000 feet (the run).
1,000 divided by 2,000 equals .5
Multiply .5 by 100 to derive percent of slope:
50%
Example: let’s assume your climb gains 500 feet in altitude
(the rise) and the horizontal distance as
measured on the map is 3,000 feet (the run).
500 divided by 3,000 equals .166
Multiply .166 by 100 to derive percent of slope:
16.6%
Example: let’s assume your climb gains 700 feet in altitude
(the rise) and the horizontal distance as
measured on the map is 500 feet (the run).
700 divided by 500 equals 1.4
Multiply 1.4 by 100 to derive percent of slope:
140%
Angle of slope represents the angle that’s formed between the run (remember it’s an idealized flat surface that ignores
elevation change) and your climb’s angular deviation from that idealized flat
surface. To calculate this, you
divide the rise
divided by the run, and
then obtain the inverse tangent of the result.
Example: let’s assume your climb gains 1,000 feet in altitude
(the rise) and the horizontal distance as
measured on the map is 2,000 feet (the run).
1,000 divided by 2,000 equals .5
Press the INV button on your calculator (sometimes
called 2nd function)
Press the TAN button on your calculator
Your angle of slope is 26.5 degrees
Example: let’s assume your climb gains 1,000 feet in altitude
(the rise) and the horizontal distance as
measured on the map is 1,000 feet (the run).
1,000 divided by 1,000 equals 1
Press the INV button
on your calculator (sometimes called 2nd function)
Press the TAN button on your calculator
Your angle of slope is 45
degrees
If
using a compass without adjustable declination, make sure
the direction of travel of arrow is pointing directly away from you.
Now rotate the compass dial until the red magnetic needle overlays the
orienting arrow. Observe the
reading at the index line. If local
magnetic declination is positive, then add the necessary
amount. If the local
declination is negative, then subtract the necessary
amount. The number at the index
line after adding or subtracting is the true direction you are facing.
If
using a compass with adjustable declination, make sure the
direction of travel arrow is pointing directly away from you.
If you haven’t done so already, adjust the declination so the orienting
arrow deviates from the compass ring’s north indicator by the amount &
direction of local magnetic declination. Turn
the compass dial until the red magnetic needle overlays the orienting arrow.
The number at the index line is the true direction you are facing.
HOW DO I POINT MYSELF TOWARD A SPECIFIC DIRECTION?
In this case, you know the number of degrees, but don’t know what
“aspect” to face (what direction should I face?).
Let’s assume you wish to face true north
(zero degrees).
Compass without adjustable declination:
turn the compass dial until zero is at the index line. If local
declination is positive, subtract this amount from zero on the
compass ring (turn the dial clockwise).
If local declination is negative, add this amount to zero
on the compass ring (turn the dial counter-clockwise).
With the direction of travel arrow pointing directly away from you,
rotate your body and compass together in one motion until the red magnetic
needle overlays the orienting arrow. You
are now facing true north.
Compass with adjustable declination:
turn the compass dial to zero. If
you haven’t done so already, adjust the declination so the orienting arrow
deviates from the compass ring’s north indicator by the amount & direction
of local magnetic declination With
the direction of travel arrow point directly away from you, rotate your body and
compass together in one motion until the red magnetic needle overlays the
orienting arrow. You are now facing
true north.
WHY AM I ADDING FOR ONE SITUATION, BUT SUBTRACTING FOR
ANOTHER?
Let’s assume the local magnetic declination is 10 degrees
east of true north (a positive declination).
Therefore, the needle always points to 10 degrees.
Let’s also assume that although you don’t know what direction
you’re facing, it coincidentally happens to be true north. If you rotate the
dial until the red magnetic needle overlays the orienting arrow (implying, “What
direction am I facing?”), the reading at the index
line will be 350 degrees; you must add 10 degrees to the 350 to
determine the true direction you are facing.
Conversely, if you dial zero on the compass (implying “What
direction should
I face to be pointed toward true north?”), then rotate your body and
compass together in one motion until the red magnetic needle overlays the
orienting arrow, the direction you’ll face is 10 degrees, even though zero is
dialed at the index line. You must subtract
10 degrees by turning the dial clock-wise to a reading of 350 degrees,
and then rotate your body & compass again until the red magnetic needle
overlays the orienting arrow.
If you’re in an area where magnetic declination is positive, here’s an easy way to remember:
WHAT DIRECTION AM I
FACING?
A = ADD