How does a G.P.S coordinate help me to know
where I am on a paper map?
Your G.P.S manual probably did not tell you how to use the G.P.S with a conventional paper
map. The manual probably did tell you how to enter a Waypoint and find your way back to
somewhere that you have already been. Yet to know where you are on a map or to be able to find
your way to somewhere you have not yet been they sell electronic maps. Personally I still prefer
the paper maps because I can see all of the surrounding area at a glance. Furthermore some of
my paper maps contain detail that is not on my electronic map. Fortunately I have learned how to
know where I am and how to locate anything on a paper map using my G.P.S.
To most of us reading a coordinate like the one to the right here
does not really help us to know our location on a map. However
you can use a G.P.S coordinate like this to find your location
on a paper map.
(a) Take two grid lines that cross each other near where you think you might be on the map. Take
note of the bearing assigned to each grid line. Take the degree, minute and seconds of the North
and West bearing and enter it as a Waypoint into your G.P.S receiver.
(If your G.P.S is set to read hundredths of a minute then you must convert any seconds on the
map to hundredths of a minute. In other words 30” seconds on the map will read .50 for your G.P.S
receiver.)
(b) Now check your G.P.S receiver to find the bearing and the distance to that Waypoint.
(c) Next we will figure out what 180 degrees is from the bearing given by the G.P.S to the Waypoint.
For example lets say that the G.P.S says that Waypoint is 360 degrees (Due North) and 3.5
miles away. Now if you have to travel 360 degrees due North 3.5 miles to reach that Waypoint
then where are you in relation to it on the map? You are 3.5 miles 180 degrees due South of the
Waypoint. Whenever you want to know where you are on the map pick two grid lines that cross
each other on the map. Then enter them into the G.P.S receiver as a Waypoint. Then check your
G.P.S to find out where the Waypoint is. You are always 180 degrees and the same distance that
is given away from that Waypoint.
One hundred and eighty degrees is a half circle. So 180 degrees will always be exactly the
opposite direction. There is a couple of ways to know what 180 degrees is from any given bearing.
The easiest way for me is to look at my compass. My Silva type 20 compass has an index line that I
set on the bearing for the direction I want to travel. Directly across from this index line is another
index line that shows me 180 degrees or the opposite of the direction of travel. Lets say that the
G.P.S said that the Waypoint I entered where the two grid lines crossed was 36 degrees 1.5 miles.
I would simply set my compass for 36 degrees as if I was going to travel to that Waypoint. I would
then look at the index line directly across from 36 degrees that would be set on 216 degrees.
I would then know that I was 216 degrees 1.5 miles away from the Waypoint I entered where the
two grid lines crossed
Your compass may not have an index line telling you the opposite of any given bearing.
In that case figure it out with your solar powered calculator.
Take whatever bearing your G.P.S says is the direction to the Waypoint where the two grid lines
cross and do the following:
Take any bearing that is between 1 degree and 180 degrees and add 180 to it.
For example if the G.P.S gives me a bearing of 36 degrees to that Waypoint I would add one
hundred and eighty to it. 180 + 36 = 216 degrees. That is 216 degrees is the opposite direction of
36 degrees.
Take any number that is between 181 degrees and 360 degrees and subtract 180.
For example if the G.P.S gives me a bearing of 216 degrees to that Waypoint I would subtract one
hundred and eighty from it. 216 -180 = 36 degrees. That is 36 degrees is the opposite direction of
216 degrees.
I keep my G.P.S set on Magnetic North for ease of use with my compass. Since I keep my G.P.S
set to read Magnetic North the 216 degrees is a Magnetic North bearing. Maps however are lined
up with True North. Therefore the declination for your area must be included to pinpoint your
location on the map, if your G.P.S is set to read Magnetic North.
(d) Take the amount of declination for your area. Add or subtract this to the reversed 180-degree
bearing given by the G.P.S to the Waypoint. In other words we first figured out what 180 degrees is
from the bearing given by the G.P.S then we add or subtract the declination from that bearing.
For example lets say I am back in Idaho in the woods outside Post Falls. The G.P.S said that
the bearing to the Waypoint I entered was 1.5 miles 36 degrees. I do not want to go to the
Waypoint rather I want to know where I am on the map.
I want to know the opposite of 36 degrees. Therefore I add 180 degrees to the 36 degrees
that gives me a bearing of 216 degrees. (36 + 180 = 216) The amount of declination must still be
added to the 216 degrees. In Post Falls Idaho there is a 19-degree declination that must be added
to the Magnetic North bearing. (216 + 19 = 235) So I am 235 degrees 1.5 miles from the Waypoint.
It is not enough for me to know that I am 235 degrees from the Waypoint. I also need to know
the exact angle of 235 degrees from that Waypoint to my location on the map. This is where a
protractor becomes handy.
As explained in the next step I would take my military protractor center it over the Waypoints’
two grid lines. I would measure the 1.5 miles from my map scale and line up the 235 degrees on
the protractor and locate myself on the map.
(e) Use a Military protractor, Declitractor or other 360-degree protractor with your ruler to find your
location on the map.
Unlike the regular protractor that we have all used in school these devices have a complete
circle or square measuring 360 degrees. The 360 degrees rotate clockwise. There are cross hairs
in the center that you can line up directly over the Waypoints’ two grid lines on your map. The
angle we want runs from the center of the cross hairs to the degree marked on the edge of the
protractor.
I also have a plastic ruler I use the standard side for measuring distance. I would place my
ruler on the mile scale in the map legend to know how many sixteenths of an inch are in the
distance to the Waypoint. Then I would lay my ruler across my military protractor. I then have a
straight edge that is lined up with the cross hairs and the degree line. The ruler is also my
measuring device to measure the distance from the Waypoint to my location. In this way I can
pinpoint my location on the map in relation to the Waypoint.
In my example in (d) the cross hairs of the Military Protractor would be lined up with the two
grid lines on the map where the Waypoint was established. The 235-degree line and the cross
hairs on the protractor are then lined up using the ruler. With the ruler I measure the 1.5 miles
from the cross hairs on the protractor to my location along the 235-degree bearing. The point
where the 1.5 miles ends is my location on the map.
The book can teach you much more than what you just read in the example here
and it is easy to do!
