Resection

Sometimes your location is a given. Your starting off at a well defined trail head, a parking area, etc. At these times it is best to compute your starting leg from this site (legs will be covered more in lesson 8). Sometimes you aren't sure exactly where you are, or may want to get a precise bearing to verify your location.

You could do this by moving to a known point like a road intersection, a bridge, a mountain top, etc. Or you could do what is known as a resection. A resection is computing your location by determining the back azimuth from two or more known points that you can see.

It really isn't very hard, and a GPS operates off a similar principle except that it uses satellites as known points instead of terrain features. Someone doing a resection can be just as accurate to his/her position as a person using a GPS, and can sometimes be faster.

To start off, you need to identify two man made or natural terrain features. In this lesson we will use the map from lesson 5. For our exercise we will assume we can see the Microwave Tower on highway 463. First we use our compass to determine the azimuth from our position to the tower - which is 332^o. This is a magnetic azimuth, so we must convert this to a grid azimuth. Since we have a easterly GM angle of 3.5^o we will add the GM angle:

332^o

+3.5^o

335.5^o

Now we have a grid azimuth from us to the tower, but we need to get the azimuth from the tower to our position. To do this we compute the back azimuth. The rule of thumb for a back azimuth is if the azimuth is < 180^o then add 180^o; and if the azimuth is >180^o then subtract 180^o. This is because there can never be an azimuth greater than 360^o or less than 0^o. Since our azimuth is greater than 180^o, we will subtract 180^o:

335.5^o

-180^o

155.5^o

Now we need another known point to use, so we'll assume we can see the intersection on the hill at Benchmark 62. We see that hill at 25 . Now we just need to do the same conversions:

25^o

+3.5^o  (GM angle)

28.5^o

+180^o (Back azimuth)

208.5^o

Now we go to our map and using our protractor, we draw lines starting from our two targets. Where those lines intersect - that's where we are!

That is a normal resection.

Modified Resection

There is a simpler way to do an intersection if you are on a linear feature like a riverbank, a ridgeline, a trail, a road, etc. This is known as a modified resection.

To do a modified resection, you find one or more known points you can see from where you are. Then you determine the azimuth, convert, and compute the back azimuth. Where the line from the known point intersects with the trail, road, ridge, etc. your on - that's where you are!

Here is an example. Your walking along Sixmile Creek and can see the Microwave Tower on highway 463 at an azimuth of 81^o. Do the math:

81^o

+3.5^o (GM angle)

84.5^o

+180^o (Back azimuth)

264.5^o

and you find yourself here: