compass

							GSC 1530
Pace and Compass
 Mapping Exercise
                    Introduction
Today you’ll learn basic
orienteering and how to
make a crude map
The type of map you’ll
make was commonly used
to record mining and
property claims, especially
in the western U.S., during
the 19th century before
more advanced surveying
equipment was invented


                              1822 Michigan Territory Map
               Introduction
The orienteering and
position plotting skills
you’ll learn are still
very useful, especially
if you enjoy hiking,
hunting and boating
Remember: modern
GPS (Global
Positioning System)
equipment may not
always function
properly!
             Introduction
Although many types
of information can be
displayed on maps,
the primary purpose
of any map is to
locate points
(position)
Our technique for
locating points:
“Distance and bearing
from a prominent
marker”
            Introduction
We’ll measure the distance between
points on our traverse via pacing
We’ll measure the bearing (orientation)
between points on our traverse via the
compass
               Pacing
Prior to going outside, determine your
pace length by walking the 50 foot (ft)
course in the hallway
Use a consistent technique for counting
your pace number
Divide the 50 ft length by the number of
paces to obtain your “feet per pace” value
(calculate to two decimal points)
Example: 50 ft/23 paces = 2.17 ft/pace
                  Pacing
To calculate the distance between two
points on your traverse: multiply the pace
count by your previously determined pace
length
Example: assume a 2.80 ft/pace value;
How far did you walk if you counted 100
paces between two points?
Visualize: 100 paces x 2.80 ft/pace = 280 ft
              Pacing
Each person conducts their own pacing
Outside, make sure you maintain the
same pace length you walked in the
hallway and walk a straight line path
between each set of points on your
traverse
      Obtaining Bearings
You and a partner will share a Brunton
quadrant compass
Open the compass and inspect its basic
components with me (switch to document
camera)
              Precautions
Make sure you read the correct end of the
needle (letter N end or white end, depending on
the type of compass you possess)
Strive to hold the compass horizontally level (at
least 1/3 to 1/2 of the bubble resides within the
“bull’s eye”)
Remember: your compass needle must be free
to align itself with the Earth’s magnetic field
   Procedure for Sighting a
          Bearing
Hold compass at waist level as seen in the
following figure, use the adjustable mirror
and sighting arm (pointed toward object of
interest) to align object in the compass
mirror (see figure)
Look into mirror;
sight object of
interest. Visually
extend the line
bisecting the compass
mirror through the
object of interest.
Adjust the mirror and
sighting arm as
necessary. Hold
compass level. Close
one eye (consistently)
to reduce parallax.
Take reading when
needle stops moving.
    Procedure for Sighting a
           Bearing
All readings on a quadrant compass are recorded using
the North (represented by the star) or South pole
positions (00) as reference points
Don’t fret about the reversal of the East – West
designations
You record how many degrees east or west of the pole
positions the “reading end” of the compass needle
points; e.g., North 60 degrees west (N 600 W) or South
30 degrees east (S 300 E)
You should be able to record bearings to a precision of
10
Switch to document camera for more examples
      Further Precautions
Does the “reading end” of your compass
needle point toward the Earth’s magnetic
north or geographic north pole?
Why then does the north arrow on a
common map point toward the geographic
north pole? (see slide)
12/26/05
The magnetic north
pole has shifted
location about 685
miles in the last
150 years (Joseph
Stoner, Oregon
State University)
      Further Precautions
Because the magnetic north pole position
fluctuates, we must be able to correct our
compass bearings to geographic bearings
The variation in bearing between the north
geographic and north magnetic pole is
called the declination – it is measured in
degrees and direction
      Further Precautions
Your position on the Earth’s surface
determines the declination values
Worldwide, declination maps are
periodically revised so we can correct our
magnetic compass readings appropriately
(see figures)
Understanding Inclination and Declination
       Further Precautions
We’ll assume therefore a magnetic declination of
approximately 7o West (2007 MI Aeronautical
Chart)
Prior to your arrival I corrected our compasses
for the local magnetic declination
Important: if your compass was properly
corrected for magnetic declination, then the
bearings you record are geographic bearings –
you’ll use these same bearings for plotting your
map
      Further Precautions
Let’s verify the magnetic declination
correction on your compass – the tip of the
zero pin should point approximately 7o to
the right of 0 (switch to document camera)
           Your traverse
You will map the relative positions of
seven lamp posts marked with colored
flagging tape in the field between B, C, T
and G buildings (see figure)
                                    - You may start your closed loop traverse
                                      at any lamp post

                                    - Walk in either a clockwise or counter-
                                      clockwise direction

                                    - Record the bearing (consensus) between
                                      successive posts on your traverse; each
                                      partner records their pace count between
                                      the posts

                                    - Carefully record your bearings and pace
                                      count information

                                    - Finish your traverse at the origin
                                      point (walk a closed loop)




= lamp post marked with colored flagging tape
         Plotting Your Map
First convert your pace counts between lamp
posts to land distances and then map distances
We’ll draw our map using a scale of 1 inch = 50
feet (1 inch on the map is equivalent to 50 feet
on the ground surface) (see board example)
Be careful when converting calculator decimal
values to map measurements if your ruler
subdivides each inch into sixteenths
Example: 3.5 inches (calculator) = 3 5/16 inches
(ruler)
        Plotting Your Map
Two methods for proper conversion of the
calculator decimal component to
sixteenths of an inch:
e.g., 3.5 inches: cross-multiply; 0.5 = 5/10,
5/10 = x/16, 80 = 10 x, x = 8
Or, multiply the decimal component by 16;
0.5 x 16 = 8
So 3.5 inches (calculator) = 3 8/16 inches
(ruler)
        Plotting Your Map
Place a geographic north arrow and scale
(verbal and bar) on your map
Place your initial lamp post point on your
map sheet (with forethought) as a
“prominent marker”
Use the protractor to determine the
bearing between successive lamp posts,
use the calculated map distances to
determine the offset between the lamp
posts (see figure)
N




1 inch = 50 feet (ft)   1

    0     50 ft
        Plotting Your Map
The protractor will be used to mimic (one
hemisphere at a time) the face of your
compass
Mentally (or physically) replace the
protractor’s numerals with those seen on
the compass face (switch to document
camera)
           Plotting Your Map
To plot the traverse points:
1) Place the protractor’s center point atop the prominent
marker point; align the protractor’s vertical axis parallel
to the map’s north arrow
2) Determine the bearing orientation between the
prominent marker and the next lamp post on your
traverse
3) Place the next point, along this bearing, the calculated
map distance between the two posts (view document
camera example)
4) Repeat this procedure until you plot all seven lamp
post positions; flip the protractor’s orientation to plot
southerly bearings
         Important Note
Your map will actually contain 8 points
representing the 7 mapped lamp posts
Two number one points will appear – the
one you placed on the map as the initial
prominent marker and the number one
point fixed on the map when you plotted
the distance and bearing data gathered on
your traverse from lamp post seven back
to lamp post one (see figure)
N




                            Error of closure



1 inch = 50 feet (ft)
    0      50 ft        1
          Important Note
The error of closure is one measure of the
accuracy of your mapping
An error of closure < 35 ft is reasonable for
a first mapping attempt
   Addendum - Triangulation
You’ll also determine the location of another
lamp post (marked with a different colored
flagging tape) via a technique called
triangulation
This is the same technique used in the GPS
system
No pacing is utilized; your position is located by
taking bearings to three (tri) known points
This technique is particularly helpful if you are
uncertain of your location (see figure)
            Numbers equal ground elevation above sea level
              Sight bearings to three known map points



       Known point 3
                                                       Known point 2




Known point 1
    Triangulation Directions
After you finish gathering data for the closed
traverse, locate the triangulation lamp post (It
lies within the closed traverse boundaries.)
Sight bearings to three carefully chosen lamp
posts on your traverse
FROM those lamp post points on your map draw
bearing lines that are opposite (reverse) of the
readings you recorded
The exact, or approximate, intersection of the
three bearing lines locates the triangulation point
(see figure for further explanation)
N                           Example: if you sighted a
                            N 300 E bearing TO your
                            post number four, then you
                            draw a S 300 W bearing line
                            FROM post number four



                                       Reverse bearing
                                       line




                                     Error of closure



1 inch = 50 feet (ft)
0          50 ft              1


    = triangulation point
          Final Comments
Each person submits a finished map
Your final map contains: a north arrow,
scale, closed traverse points (plus error of
closure point), the triangulation point
(label) and your name
On the rear of your map sheet, answer
(complete sentences) the questions on
the second page of your instructions
handout (number 8).
Good luck!