See also: How to use a Compass at learnorienteering.org
Magnetic declination at National Resources Canada
Map Coordinate Systems Overview
Topo Maps
MAP, COMPASS, AND GRID COORDINATE SYSTEMS
Adapted from Tahoe Backcountry Ski Patrol Manual
Contents
A. How To Use A Map And Compass
B. Grid Coordinate Systems
1. Universal Transverse Mercator (UTM)
2. Public Land System (Township And Range, or T&R)
3. Geographic Coordinates (Latitude And Longitude, or Lat/Long)
a) Quick and Dirty Method
b) Mathematical Method
c) Geometrical Method
d) Lat/Long Ruler Method
C. GPS Uses
D. PRACTICE Problem Answers
A. HOW TO USE A MAP AND COMPASS
Selecting a compass
A good compass with declination adjustment, sighting mirror and clinometer (Suunto MC 2G or Silva Ranger Type
15CL, aka CL515) costs $55$70. Buy a good one. The following discussion assumes you have followed this advice.
Consequently, it does not deal with the procedures and calculations required to correct for declination if your compass
does not have a declination adjustment feature. [If you insist on using a compass without declination adjustment, these
procedures can be explained in the field, but they are confusing and difficult to remember. Don't say we didn't warn
you.]
Declination adjustment
The Magnetic North Pole lies to the south of true North Pole, which causes an error in compass readings known as
declination. The value of declination for a particular area is found on the lower left corner of all USGS maps. For our
purposes, declination of 17° east is sufficient for all maps. To adjust your compass, simply insert the small key into the
screw on the bottom of your compass and turn the screw until the mark at the rear of the orienting arrow lines up with
17° east declination. Now, unlike those who have compasses without declination adjustment, you will never have to
worry about declination again.
Map bearings
Mark (or note) your position and destination on the map.
Place the long side of the compass base plate on these two points so that the cover is towards your destination.
If necessary, the cover of the compass or a ruler can be used as an extension of the base plate.
Turn the bezel until the meridian lines on the compass capsule are parallel with the meridian lines on the map
(or the left or right border of the map) and N on the bezel is on the north side of the capsule with reference to
the map.
The bearing from your location to your destination can now be read at the index pointer (centered luminous
mark near the hinge of the compass).
To transfer this bearing to the field, simply turn your body until the needle is centered on the orienting arrow and the
colors match (red with red and black with black). [After dark, turn your body until the luminous point on the needle lies
between the luminous points on either side of the orienting arrow.] The sighting line through the mirror indicates your
direction of travel.
Orienting (setting) the map
To orient your map (only necessary when in the field), follow the same procedure described above, but leave the
compass on the map and turn the map instead of your body. Once the map is oriented, you can compare field
topographical features with those depicted on the map.
Taking field bearings
open the cover to approximately 60°.
Place the lanyard around your neck and raise the compass to eye level while holding it level with both hands,
and hold it as far from your body as possible so the lanyard is tight.
Close one eye (always close the same eye) and sight through the notch in the cover (mirror), turning your body
until the landmark is precisely above the notch, AND the vertical centerline of the mirror appears to intersect
the axis of the needle.
While maintaining the above position, look into the mirror and turn the bezel until the needle is within and
parallel to the orienting arrow as observed in the mirror.
read the bearing at the index pointer (centered luminous mark near the hinge of the compass).
Finding your location using two or more landmarks
Take a bearing on a known landmark as described above.
To transfer this bearing to the map, place the compass on the map and open the cover wide so the long edge of
the compass intersects the known landmark.
Turn the compass edge around the known landmark until the meridian lines on the capsule are parallel with the
meridian lines (or the left or right border) of the map, and the N mark on the bezel is on the north side (on the
map) of the capsule.
Starting from the known landmark, draw a line along the edge of the compass in the direction of your
approximate location.
Repeat the process using a second landmark approximately 90° from the first landmark.
Your location is the point where the two lines intersect.
For improved accuracy, repeat the procedure using a third landmark.
A line from the third point should intersect at approximately the same point
Using the clinometer
We use the clinometer to measure slope angle, a critical indicator of avalanche propensity, and alpha angles, which
indicate whether you are in a potential avalanche run out zone:
Turn the bezel until it reads 270° at the index pointer.
Position the compass so the long edge is parallel with the surface to be measured and the base plate aligns with
the vertical plane (this allows the clinometer pointer to turn freely).
Open the compass cover to approximately 30°.
Steady the compass with both hands and sight along the top edge of the base plate. Looking into the mirror,
read the angle indicated by the clinometer pointer.
B. GRID COORDINATE SYSTEMS
See Map Coordinate Systems for an overview
Patrollers must know how to determine their location on a USGS or Forest Service
map and how to move from one
location to another using a map and compass. Patrollers also must know how to
direct helicopters to a predetermined
landing sites, and be able to use a map and compass to reach such sites.
In order to be able to express their location to various rescue personnel,
patrollers must learn three systems of map
coordinates:
(1) Universal Transverse Mercator
(UTM) which is used by many
search and rescue organizations, (2)
Public Land System (PLS), also called "Township and Range," which is used by the
Forest Service;
and (3) geographic
coordinates (latitude and longitude), which are used by rescue helicopters and
other aircraft.
Coordinates for all three
systems are found on every USGS map we use.
Some search and rescue organizations and air medical services use computer
software to convert among various
coordinate systems, but not all organizations have software or personnel capable
of converting among all three systems.
Consequently, we must be able to express our location in all three grid
coordinate systems.
Most map and compass training will be done in the field. Field training will
concentrate on locating positions and
plotting courses using landmarks and a map and compass. We will not spend much
time on grid coordinate systems in
the field. The following sections describe each of the three grid coordinate
systems used to determine specific map
locations. Examples are based upon the USGS 7.5 minute Quadrangle map.
You should attempt to master this material before you begin field training in
order to gain a more complete
understanding of this critical subject. It may seem overwhelming at first, but
if you go through each system several
times and do the sample problems you should be able to master all three systems.
Instructors will attempt to answer any
questions you have regarding the written material.
1. Universal Transverse Mercator (UTM)
Basis of system
Distance is measured in kilometers north and south from the equator and east and
west from the "principal meridian,"
located in the Pacific Ocean near the International Dateline.
Map marks
UTM grid coordinates are the series of black numbers on the white border of the
map next to the blue grid ticks. For
example, in the upper left margin of the 7.5minute Norden Quadrangle appears
the number 4361000N. The blue grid tick
to the right of this number marks a distance 4,361 kilometers (4,361,000 meters)
north of the equator. In the left end of
the top margin of the map, just below the words "GEOLOGICAL SURVEY" is the
number 727000E. The blue grid tick
to the left of this number marks a distance 727 kilometers (727,000 meters) east
of the principal meridian. Other similar
numbers on the map eliminate the 000m and express the distance only in
kilometers.
Using the map marks to describe a location
Forget all of the superscript numbers. Henceforth, 4361000N will be referred to
as 61, and 728 as 28. To learn how to
describe your location, let's use an example from the Norden 7.5 minute map:
 Locate the summit of Donner Peak in the middle of your Norden 7.5 minute map.
 Draw an imaginary northsouth line from your position to the top or bottom
margin of the map, and note the
blue UTM grid tick numbers on either side of the point where your imaginary line
intersects the map margin.
The numbers should be 31 and 32. [Note that while the blue grid ticks are always
present, often the numbers
are missing and you have to extrapolate them from nearby grid tick references.
For example, at the bottom of
the map, the reference to 32 is missing, and at the top, the reference to 31 is
missing.]
 Do the same exercise using an eastwest imaginary line to the left or right
margin of the map, noting the blue
grid ticks above and below the point where your imaginary line intersects the
map margin. You should fall
between grid ticks 54 and 55.
 Now draw a line between grid tick 31 on the top and grid tick 31 on the bottom
of the map. If you don't have a
straight edge long enough (and you won't in the field), fold over one edge of
the map and use it as a straight
edge to connect the two "31" grid ticks. Be certain that the edge of the map is
flat or your line will be off.
Next, do the same exercise to connect grid ticks 32, 53 and 54. [We strongly
suggest that for training
purposes you draw the entire UTM grid on your Norden 7.5 minute map.]
 Note that the lines you have drawn have created a square box around the Donner
Peak reference point. [In the
future, you only have to create this "box," and it is unnecessary to draw
complete lines between the grid ticks.]
Notice that the lines you have drawn are not parallel to the edges of the map,
but are slightly skewed. This is
because the UTM grid system is not based upon True North, but your map is. From
now on, you will focus on
the "box" you have just created.
 Within the box draw a line, parallel to the left and right sides of the box,
from the summit of Donner Peak to the
nearer of the top or bottom margin of the box. [In this case, the bottom
margin.]
 Divide the distance along the bottom margin into ten equal parts, and record
where your line intersects the box
margin to the nearest tenth. You should find that your line intersected the
bottom margin of the box 8 tenths of
the distance from grid line 31 (the left side of the box) to grid line 32 (the
right side of the box), and thus you
should record 318. [It is actually 31.8, but the decimal point is eliminated.]
[Try using your UTM grid reader
to speed this process.]
 Next, draw a line, parallel to the top and bottom margins of the box, from the
summit of Donner Peak to the
nearer of the left or right margins of the box. [In this case, the right
margin.]
 Again, record where the line falls to the nearest tenth. You should find that
the line intersects the right box
margin 3 tenths of the distance from grid line 54 to grid line 55, and thus you
should record 543.
 Then simply combine the two three digit numbers into one sixdigit number,
giving the eastwest coordinate
before the northsouth coordinate. In our example 318 and 543 would produce "UTM
318543 on the 7.5 minute
Norden Quadrangle." (Note that we reference the particular map because you might
get a slightly different
reading using a 15minute map of the same area.)
PRACTICE PROBLEM 1: Try determining the UTM location of the triangle marking the
tablet atop Anderson Peak,
located roughly 1 3/4 inches from the bottom margin and 6 1/2 inches from the
right margin. See answer 1 on last page
of this appendix.
2. Public Land System (Township and Range)
Basis of system
Figures 2 and 3 of this appendix describe the Township and Range system used by
the Forest Service. Because we
patrol pursuant to volunteer agreements with the Forest Service and use their
radio system, we must know how to
communicate in a language the Forest Service's radio dispatcher understands.
Once you understand the basic Township
and Range system, return to this page to learn how to more precisely express a
particular location using this system.
Map marks
Township, Range and Section boundaries are marked in red on USGS maps. If the
line is dashed, it means its location is
approximate. Township (T.) and Range (R.) designations are found along the
margins of the map wherever there is a
Township or Range boundary (as contrasted with a Section boundary). Township and
Range designations also appear in
the body of the map where appropriate to mark a deviation in the boundary.
Section numbers appear in red in the center
of each Section. For example, the Peter Grubb Hut is located in Section 2, T. 17
N., R. 14 E.
Using the map marks to describe a location
To describe a particular point within a section:
Find the Township and Range, and the section
Sivide the Section into four equal quarters, also know as quadrants.
Subdivide the quadrant in which you are located (or wish to describe) into
four subquadrants.
Describe the tract in which you are located in terms of the subquadrant,
quadrants, Section, Township and
Range. This method describes your location in a 40acre tract (a quartermile by
quartermile square).
For example: the Peter Grubb Hut is located in the "Southwest 1/4, (of the)
Northeast 1/4, Section 2, Township
17 North, Range 14 East."
Because this is too vague a description for someone trying to find you, when
dealing with the USFS, you should give this
description, but also give a UTM or geographical coordinate depending on which
rescue agency you expect to respond.
It wouldn't hurt to give all three.
Alternatively, you can measure the distance from the east or west boundary and
the north or south boundary of the
section, using the scale at the bottom of the map:
Use a ruler, piece of paper or a stick to measure from the left or right (east
or west) boundary of the section to
your location. Record the distance in feet.
Do the same from the top or bottom (north or south) boundary of the section.
e.g., the Peter Grubb Hut can be described as being in "Section 2, Township 17
North, Range 14 East, at a point
1500 feet West of the Eastern boundary of the section and 2400 feet South of the
Northern boundary of the
section." A mouthful, but they will find you.
PRACTICE PROBLEM 2:
Try describing the location of Anderson Peak in both
"Township and Range" methods.
See
answer 2 on last page of this appendix.
FIGURE 1: EXAMPLE OF TOWNSHIP & RANGE PUBLIC LAND SUBDIVISIONS
A. Townships are numbered north and south of the base line.
Ranges are numbered east and west of a principal meridian.
The designated abbreviation would read: "Township 3 South, Range 45 East."
FIGURE 2: Continuation of EXAMPLE OF TOWNSHIP & RANGE PUBLIC LAND SUBDIVISIONS
B. Each Township is 6 miles square, and
divided into 36 Sections. Each Section is 1
mile square.
C. One Section contains 640 acres which can be
subdivided into smaller tracts.
D. A Section can also be subdivided into (16)
40acre tracts which can be designated by
cardinal directions.
E. A legal description of this parcel of 40 acres
(beginning with the smallest designation first) will
read: "SE ! NE !, Section 36, T. 3 S R. 45 E."
1/16 of section, or subsub quadrant
3. Geographic Coordinates (Latitude and Longitude)
Basis of system
Geographic coordinates use the degree as the basic unit of measurement, with
subdivisions into minutes and seconds.
The origin of this system is off the west coast of Africa, where the Greenwich
meridian (0° longitude) intersects the
equator (0° latitude). Latitude is measured north or south of the equator up
to 90° at the poles, and longitude is measured
west or east of the Greenwich meridian up to 180° in the Pacific Ocean. The
contiguous states, Hawaii and most of
Alaska are in latitude north and longitude west. Therefore, "north" and "west"
may be dropped when giving geographic
coordinates in the U.S. In the U.S. latitude and longitude cannot be confused
with each other because the range of
numbers used to describe each are so different. However, by convention latitude
is normally given first.
Map marks
Each corner of a USGS map contains a latitude and longitude reference. Latitude
references are on the left and right
margins and longitude on the top and bottom margins. An easy way to remember
which is which is that San Francisco is
at (roughly) latitude 38, so all of our latitude references will be near 38. [Or
remember that in the U.S. the longer
number is for longitude (3 digits versus 2 digits for latitude).
The border of each USGS 7.5 minute map is divided into three 2.5 minute
sections, marked by a minute reference (or
minute and 30 second reference if it falls between minutes) and a black tick
extending into the body of the map. For
example, onethird of the way up the left or right hand margin of the 7.5 minute
Norden Quadrangle is the reference
17'30", which is really 39°17'30". Similarly, onethird of the way across the
top or bottom margin from the left border is
the designation 20', which is really 120°20'.
Using the map marks to describe a location
Plotting approximate latitude and longitude on a topo map in the field is a
relatively simple process. There are two ways
to do it, mathematically and geometrically. Fortunately, on the maps we use the
two methods are virtually identical, and
produce a third method which we call Quick and Dirty (Q&D). We will describe the
Q&D method first, but remember
that it will only work on our maps. [Note: The easiest method involves use of
the Lat/Long Scale we provided.
Skip
ahead to p. 15, subsection (d) and learn this method first, then come back and
learn the other methods.]
For all three methods, first use a straight edge to transfer the point you wish
to describe to the nearest horizontal and
vertical edges of the map, as you would in calculating UTM coordinates. You must
now estimate the linear distances
between your points and the nearest lesser latitude or longitude references on
the borders of the map and express those
distances in minutes and seconds.
All three methods are based on the fact that the distance between geographical
coordinate ticks on the map are 2'30", as
described above. Because there are 60" in one minute, 2'30" equals 150".
Consequently, if we can divide distance
between geographical grid ticks into 150 equal parts, we can describe the points
we have transferred to the map borders
to the nearest second.
a) Quick and Dirty method
Fortunately, on our maps, the distance between geographical grid ticks for both
latitude and longitude is easily divisible
into 150 segments. The distance between grid ticks on the top and bottom borders
(longitude) is approximately 150 mm,
so each millimeter equals 1 second. The distance between geographical grid ticks
on the left and right borders (latitude)
is approximately 7.5 inches, so each inch equals 20 seconds, and each 1/4"
equals 5 seconds.
To determine geographical location, simply find your position in seconds, then
convert to minutes and seconds as
necessary. For longitude, simply measure in millimeters along the top or bottom
border from the next lowest mark to
your intersecting point. This is the number of seconds to add to determine
longitude. For latitude, simply measure the
number of inches from the next lowest latitude mark to your intersecting mark
and multiply by 20 (seconds per inch) to
get the number of seconds to add to the next lowest latitude reference.
To determine the latitude and longitude of the Peter Grubb Hut (PGH):
first draw lines to intersect the nearby top and left borders of the map.
the distance from the next lowest geographical grid tick (20', or 120°20')
to the mark you placed across the top
border of the map is approximately 120 millimeters.
120 mm x 1 second/mm = 120 seconds. 120 seconds ÷ 60 seconds/minute = 2.0
minutes.
add the 2 minutes to the 120°20' (next lowest longitude mark) and the
longitude of the Peter Grubb Hut is
120°22'.
To determine the latitude of the Peter Grubb Hut:
measure the distance from the nearest latitude mark below your transferred
mark to your mark. This distance is
approximately 6.25 inches.
6.25 inches x 20 seconds/inch = 125 seconds.
to convert the seconds to minutes and seconds, divide your answer by 60
seconds/minute. 125 seconds ÷ 60
seconds/minute = 2'5".
add the 2'5" to the latitude mark you measured from (20', or 39°20').
39°20' + 2'5" = 39°22'5".
Consequently, the Peter Grubb Hut lies at latitude 39°22'5", longitude
120°22' determined by the quick and dirty method.
[Note that we don't give the full designation of "latitude 39°22'5" north,
longitude 120°22' west." Because everything in
the Northern Hemisphere is "north" latitude, and almost everything in the United
States is "west" longitude, we drop the
"north" and "west" designations when giving geographic coordinates in the U.S.]
Adding an elevation reference (in this
case "7800 feet") is helpful but not essential.
b) Mathematical method
The Q&D method is actually the mathematical method without long division
calculations. With the mathematical
method, you simply measure the distance between the next lowest geographical
grid tick and your mark, and divide that
distance by the distance between geographical grid ticks:
first, measure and record the distance between geographic grid ticks on the
map. E.g., the distance between
longitude marks on the top or bottom edge of the Norden 7.5 map is about 150 mm.
then, measure from the nearest geographical grid tick to the right of where
your point bisects the top or bottom
map border of the map.
take that figure, in millimeters, and divide it by 150 mm, then multiply by
2.5, the number of minutes between
geographical grid ticks. This will give you a figure in minutes.
to convert your answer to minutes and seconds, simply multiply the fractional
portion of your answer by 60
seconds/minute.
then add your answer to the longitude of the nearest geographical grid tick to
the right of your point (i.e., the
nearest grid tick below your point).
Use the same process to determine latitude, but note that the distance between
geographical grid ticks on the left and
right borders of the map is approximately 190 mm rather than 150 mm.
To determine the latitude and longitude of the Peter Grubb Hut, first draw lines
to intersect the nearby top and left
borders of the map:
the distance from the 20' (120°20') grid tick to the mark you placed
across the top border of the map is
approximately 120 millimeters.
120 mm/150 mm x 2.5 minutes = 2 minutes.
add the 2 minutes to the 120°20' (nearest longitude grid tick to the
right) and the longitude of the Peter Grubb
Hut is 120°22'.
To determine the latitude of the Peter Grubb Hut:
measure the distance from the nearest latitude mark below your transferred
mark to your mark. This distance is
approximately 158 millimeters.
158 mm/190 mm x 2.5 minutes = 2.08. To convert the fractional portion of your
answer to seconds, multiply it
by 60. .08 minutes x 60 seconds/minute = 4.8 seconds. Round it to 5 seconds to
produce 2'5".
add the 2'5" to the latitude mark you measured from. 39° 20' + 2'5" =
39°22'5".
Consequently, the Peter Grubb Hut lies at latitude 39°22'5", longitude
120°22' determined by the mathematical method,
the same answer we obtained using the Q&D method.
c) Geometrical method (see Figure 3):
If you can remember how to do long
division or have a pocket calculator with
you, the mathematical method is quickest. However, if the calculations give you
a headache, try this method.
To determine latitude or longitude geometrically:
draw a line from the nearest reference point below your mark at an angle
approximately 30° inward from the
map border and approximately the distance between that point and the next
highest geographical reference
point. [See line AB on the map excerpt on page 13 of this appendix.] The exact
angle is not important, nor is
the length of the line, but it should be approximately the length of the
distance between longitude grid ticks on
your map.
consider this line to be divided into 150 equal increments. The figure 150
represents the number of seconds
between geographical grid ticks on a 7.5 minute map (2.5 minutes x 60
seconds/minute = 150 seconds). On the
Norden 7.5 map the distance is approximately 150 mm  extremely convenient for
our purposes. Consequently,
measure 150 mm along your 30° line and mark that point (see point B on the
excerpt).
connect that point (B) with the next highest longitudinal mark on the border
of your map (point C) by drawing a
line between those two points (line BC).
now draw a line parallel to your last drawn line from your intersecting mark
(point X) to the 30° line (see line
DX). Be certain that lines BC and DX are parallel.
next, measure along the 30° line from the beginning (point A) to the point (D)
where your last drawn line (DX)
intersects the 30° line (point D). This measurement (line AD) is the number of
seconds to be added to
determine the correct longitude. Because in this case 1 mm = 1 second, you don't
have to do any calculations to
convert from millimeters to seconds.
To determine latitude, use the same method, but a different base measurement.
The 150 mm trick won't work because
the distance is approximately 190 mm. However, as we learned in the quick and
dirty method, it is also approximately
7.5 inches, so each inch equals 20 seconds, and each 1/4 inch equals 5 seconds.
Consequently, make all latitude
measurements in inches rather than millimeters.
To determine the geographic coordinates of the Peter Grubb Hut by the
geometrical method, first draw lines from the hut
to intersect the nearby top and left borders of the map. To determine longitude
(points X and Y on the map excerpt):
draw a line from the 20' mark on the top border (just above the red section 29
number) diagonally downward to
the left at an angle of approximately 30°. Draw it all the way to the left
border of the map. Then measure along
this line 150 mm from where you started and make a mark (point B).
connect this point with the upper left corner of the map (the next highest
longitude grid tick). [See line BC.]
draw a line parallel to your last line from the Peter Grubb Hut mark on the
top border (point X) to the 30° line.
[See line DX.] Be certain that lines BC and DX are parallel.
measure along the 30° line from the 20' mark (point A) to the point where
the second parallel line meets the 30°
line (point D). It should be approximately 120 mm. [Sound familiar?]
120 mm x 1 second/mm = 120 seconds ÷ 60 seconds/minute = 2 minutes. Add the 2
minutes to 120°20' and
you have 120°22' for Peter Grubb Hut longitude.
To determine Peter Grubb Hut latitude, employ the same process:
draw the 30° line upward to the right from the 20' (39°20') mark
(point E) on the left border of the map.
measure off 7.5 inches on the 30° line and draw a line from that point (F)
to the upper left corner of the map
(point C). [See line CF.]
draw a line, parallel to your last drawn line, from the Peter Grubb Hut
intersecting point on the left border (point
Y) to the 30° line. [See line GY.] Be certain that lines CF and GY are
parallel.
measure the distance from the 20' mark (point E) to this point on the 30°
line (point G) in inches  it should be
approximately 6.25 inches. [See line EG.]
6.25 inches x 20 seconds/inch = 125 seconds.
125 seconds ÷ 60 seconds/minute = 2'5".
add 2'5" to 39°20' and you get a latitude of 39°22'5".
So the geographical coordinates of the Peter Grubb Hut are latitude
39°22'5", longitude 120°22' as determined by the
geometrical method. This is the same answer we got by using the other two
methods.
PRACTICE PROBLEM 3:
Can you describe the location of Anderson Peak using
geographic coordinates? Try all three
methods to see if you can arrive at identical answers. See answer 3 on the last
page of this appendix. If you didn't get
the correct answer, please review the text and try again. This material may bend
your brain, but it is extremely important
to be able to precisely describe your position in the field.
FIGURE 3: EXAMPLE OF LATITUDE AND LONGITUDE USING GEOMETRICAL METHOD
d) Lat/Long Scale Method: Skewed Scale To Find Position
To determine the longitude and latitude of a point using the special
longitude/latitude scale, the scale cannot
be laid parallel to the map border because in this orientation the scale does
not match the interval and is too
long. The scale must be turned to an angle to find a solution. The following
procedure describes how to turn
the scale to find the latitude and longitude of a point.
The 7.5 minute map has tick marks at 2'30" intervals along both the horizontal
and vertical axis. To carryout
this procedure, straight lines must be drawn across the map and down the map at
these 2'30" intervals. See
Figure 4 for an example of correct positioning of the scale to find Longitude.
Note that the ruler has one side
for measuring distance labeled "USGS 7.5' MAP SCALE" and one side for
Latitude/Longitude measuring.
Longitude
1. On the longitude side of the scale align the first (right) tick mark of the
scale on a 2 minute 30 second
(2'30") line.
2. Align the last (left) tick mark of the scale on the adjacent 2'30" line so
the scale is straddling the
point to be located. The numbers on the longitude scale need to be facing right
side up.
3. Slide the scale up or down the map (keeping the tick marks on the 2'30"
lines) until the skewed scale
aligns with the desired point on the edge of the scale. The first and last scale
tick marks must still be
aligned the with the 2'30" lines.
4. Each tick on the scale has a pair of twodigit numbers that stand for seconds
(e.g. 00•30). The first
number of the pair allows you to count up the scale starting at 0 seconds., and
the second number of
the pair allows you to count up the scale starting from 30 seconds. If the
longitude line to the right
that is being measured off ends in 30" (e.g. 120o 17' 30"), then count on the
scale starting with the 30
minute values. If the longitude line being measured from (to the right) is an
even minute (e.g. 120°
20' 00"), then count on the scale using the numbers starting with 00 minute.
5. The longitude position is the sum of the values counted on the scale plus the
value of the longitude
line measured from.
Latitude
1. On the Latitude side of the scale align the first (bottom of scale) tick mark
of the scale on a 2'30"
line.
2. Align the last (top of scale) tick mark of the scale on the adjacent 2'30"
line straddling the desired
point to be measured. The numbers on the latitude scale need to be facing right
side up.
3. Slide the scale across the map (keeping the tick marks on the 2'30" lines)
until the scale aligns
skewed with the desired point on the edge of the scale. The first and last scale
tick marks must still
be aligned with the 2'30" lines.
4. Similar to the longitude, on the Latitude edge of the scale, each tick on the
scale has a pair of twodigit
numbers that stand for seconds (e.g. 00•30). Count using the pair of numbers
similar to
Longitude.
5. The latitude position is the sum of the values counted on the scale plus the
value of the longitude line
measured from.
C. GLOBAL POSITIONING SYSTEM (GPS) USES
See Using a handheld GPS
D. PRACTICE PROBLEM ANSWERS:
 "333488 on the 7.5 minute Norden Quadrangle."
 "Southwest 1/4, Southeast 1/4, Section 3, Township 16 North, Range 15 East"
or, using the alternative method:
"Section 3, Township 16 North, Range 15 East, at a point 2350 feet west of the
eastern section boundary and 750 feet
north of the southern section boundary, at about 8700 feet elevation."
 "Latitude 39°15'33", longitude 120°17'46" at about 8700 feet elevation."
[Remember, we always give latitude first,
and don't have to express "latitude" and "longitude".]
If you missed any of these, reread the pertinent sections and try again.
Links:
Map Coordinate Systems Overview
How to use a Compass at learnorienteering.org and Magnetic declination at National Resources Canada
Sample map
