To go from one place to another, the two key things to know are angle to the target, and distance to the target. To put it simply, you start from a known point, then take the most direct route which is a straight line. Your compass will help you find and stay on that line, and you will keep track of the distance you travel to get there. This will be cover more in lesson 8.
OK, lets start with plotting on your map. To follow with this lesson, you should have four things:
1. Map - any map should do. If you click on the map image, a full normal sized version is available for you to print and use to follow along.
3. Protractor - any protractor will do, but I recommend finding one made for map reading.
4. Straight edge - your protractor will usually serve this purpose just fine.
In this case I'm using a military map with a scale of 1:50,000 and a east declination of 3.5o.
1. Start by placing a point on the map where you want to start. I'm going to start at Mount Olive Church in the town of Pitkin, LA.
2. Place a point where you want to go. I'm going to walk to the Microwave Tower on highway 463.
3. Now draw a straight line starting at the church that goes through the microwave tower and beyond.
4. Place the protractor on the map centered on the start point, ensure that 0o points north.
5. Look out to the edge of the protractor to determine at what azimuth the line passes through. The answer is: 342o.
6. Use the straight edge to measure the distance from start point to target. The answer is: 2".
7. Now the math comes in. The azimuth we measured on the map is a grid azimuth. If we were to walk out into the woods at an azimuth of 342o we would be 3.5o off because of the GM (grid to magnetic) angle. to compensate we must do some math. The general rule of thumb is this:
o Easterly GM angles: Grid to magnetic subtract, magnetic to grid add.
o Westerly GM angles: Grid to magnetic add, magnetic to grid subtract.
o A cool way to remember this is the saying: Every Great Man Shall Make Good Ale. E: G-M=S/M-G=A. Easterly: Grid to magnetic = subtract, magnetic to grid = add. Once you remember the easterly, the westerly is the exact opposite.
O.K., now your asking how do I know if a GM angle is westerly or easterly? Look at the declination diagram. If the magnetic line is to the left (west) it's westerly, and if its to the right (east) it's easterly. Look at this diagram. The left GM angle and the center GM angle are westerly, and the right GM angle is easterly.
Now on to the math. Take the grid azimuth and subtract the GM angle to determine the magnetic azimuth.
So the angle on our compass we would have to follow is 338o.
8. Now we need to know how far to walk. When we measured the distance we got an answer of 2". We take that measure to the coordinate scale and measure it against the scale. The coordinate scale on a map usually has 100 meters/kilometers and .10 miles/miles. See the picture below for an example.
The coordinate scale is used by measuring all whole miles, then using the remainder on the .10 mile scale; or by measuring all whole Kilometers, then measuring the remainder on the 100 meter scale. In our example, the distance would be measured like so:
So we would have to walk 1.6 miles. If we measured the same distance against kilometers, it would be measured like so:
So we would have to walk 2.5 kilometers.
9. Now we now have a leg to walk that is 338.5o and 1.6 miles long. We will use this information later in the lesson.