Plotting a Bearing onto your map Why we plot bearings Where am I? - PowerPoint PPT Presentation

Plotting a Bearing onto your map • Why we plot bearings • Where am I? • Location by resectioning • Where is the ____ I can see in the distance? • Location by intersection • Using straight line course legs

Location by Resectioning � a.k.a Triangulation N 70° ? ? A single bearing, sighted to a peak, resulting in two possible locations along a trail.

Location by Resectioning N N 70° X 305° A bearing to a second peak confirms the location on the trail.

Location by intersection Using bearings sighted from two or more known locations, to find an unknown location.

An example ? ? GNMN 5° Where are we along the shoreline? Where is the cave we can see across the lake?

We sight a bearing to a known cabin on the map, with a result of 60° M It’s NE of our location.

30 40 20 50 0 1 0 6 0 360 70 350 80 340 90 3 3 0 0 0 1 320 110 310 120 300 130 290 140 0 8 2 1 5 0 270 160 260 170 250 180 0 0 4 2 9 230 200 1 220 210 GNMN 5° Center your protractor on the known location.

0 10 350 20 340 3 6 0 3 30 3 0 320 40 310 50 300 0 6 290 70 280 80 2 7 0 9 0 260 100 250 110 0 120 4 2 230 130 220 140 210 1 5 200 0 160 190 170 1 8 0 GNMN 5° Align the protractor with Grid North.

Our bearing was sighted relative to Magnetic North. � We want to plot it relative to Grid North. � We need to convert it. Bearing to plot on the map measured from GN MN Grid North 60M° + 5° = 65G° 65° 5° 60° Bearing measured with our compass from Magnetic North

0 10 3 5 3 0 3 6 0 4 0 2 0 330 30 320 0 4 3 50 65° 1 0 300 60 2 0 9 7 0 280 0 8 2 7 0 9 0 0 6 100 2 0 1 5 1 2 0 240 120 230 1 3 0 0 140 2 2 210 150 0 1 0 6 190 1 2 0 7 0 1 8 0 GNMN 5° Mark the converted bearing at the edge of the protractor.

65° l a v r e t n i 0 e m l i m 1 0 . 0 2 1 0 0 0 www.maptools.com 2 0 0 0 1 3 0 0 0 1 0 m s e e t e l m i r 4 i 0 n 0 0 t e 0 r v a l GN MN 5° Use a straight edge to draw the bearing line. Remember the cabin was NW of us, so we want the line to the SW of the cabin.

65° Forward Bearing Back Bearing X GNMN 5° The bearing plotted back towards our location defines our location where it crosses the lakeshore.

65° 65° g g n n i i r r a a e e B B d d r r a a w w r r o o F F g g n n i i r r a a e e B B k k c c a a B B Back Bearing GN Forward Bearing MN 5° 130° A bearing to a second cabin helps confirm our location. But it also shows us standing in the lake!

45° Forward Bearing 65° 65° 65° g g g g n n n n i i i i r r r r a a a a e e e e B B B B d d d d r r r r a a a a w w w w r r r r o o o o F F F F Back Bearing g g g g n n n n i i i i r r r r a a a a e e e e B B B B k k k k c c c c a a a a B B B B Back Bearing Back Bearing GN Forward Bearing Forward Bearing MN 5° 130° 130° A third bearing reveals an “uncertainty triangle.” Some of our bearing have small errors.

You can use your compass � as a protractor

Plotting a bearing with a baseplate compass 340 N 20 320 40 300 60 280 8 0 W E 20 100 0 6 2 0 120 4 40 2 220 140 200 160 S 60 80 N 0 4 20 3 320 4 0 300 60 280 8 0 W E GNMN 100 0 6 2 1 240 2 5° 0 0 140 2 2 200 1 6 0 S Set the dial to the desired bearing (using the map’s north reference) Align the compass with the north reference lines.

N 340 20 320 4 0 300 60 0 80 8 2 W E 1 260 0 0 120 240 0 1 2 4 2 0 200 160 GNMN S 5° Keeping the compass aligned with the Grid North lines, move it until an edge touches the know point.

N 0 20 4 3 320 40 0 60 0 3 280 8 0 W E 100 0 6 2 120 240 140 220 1 200 6 0 GNMN S 5° Draw the bearing line along the edge of the compass.

Compass Adjusted to Read Grid North with Magnetic North 5° to the East Align these lines with Grid North N 340 20 320 4 0 300 6 0 0 80 8 2 W E GN 260 1 0 MN 0 120 240 5° 140 0 2 Arrow aligns with 2 200 160 S Magnetic North NOT Grid North This compass, adjusted 5°E. declination, would provide bearings relative to Grid North. No conversion necessary.

Plotting Compass Bearings � near Sharktooth Peak Sharktooth Peak Map You are hiking along a trail, north of Coyote Lake. You are not exactly sure of your location, and decide to sight some compass bearings determine your location. � Sharktooth Peak � 59°M � Silver Peak � � � 87°M � Cockscomb � � � 136°M

First let’s get our north reference sorted out. USGS provides us a declination diagram dated 1982. Probably too old to be accurate today. � Google “declination calculator”, and find the current magnetic declination for this map. Hint:Use the lat/lon from one of the map’s corners.

http://www.ngdc.noaa.gov/geomag-web/#declination

Now let’s look at the Grid North lines on the map Grid North varies with location. But it doesn’t change over time So the Grid North information I measured 1.4° � between in the 1982 declination diagram True and Grid North should still be correct. � But the angle it makes with True North (the edge of the map) doesn’t look like a 1/4° to me! I don’t think this kind of error � is common. But it’s a good idea � to check your maps � when you first get them.

Our compass bearings are measured from Magnetic North. We want to plot on the map, measuring from Grid North. MN To convert from a Magnetic bearing 13.2° 1.4° to a True bearing… GN � Mag. + 13.2 + 1.4 = Grid Bearing to Peak, measured from True North or Bearing to Peak, measured from Grid North Bearing to Peak, measured from Magnetic North Mag. + 14.6 = Grid round to Mag + 15 = Grid

Convert your bearings to work from Grid North. � Plot them on the map. You are hiking along a trail, north of Coyote Lake. You are not exactly sure of your location, and decide to sight some compass bearings determine your location. � Sharktooth Peak � 59°M � Silver Peak � � � 87°M � Cockscomb � � � 136°M

Silver � Sharktooth � Peak � Peak � Cockscomb � 87°M 59°M 136°M

You are somewhere east of Big Margaret Lake. � Plot some bearings to find your exact location 0°M 341°M View North 264°M View West 138°M 186°M View South

Note that small errors change the location where the bearings intersect when the bearings are either very close to each other or when they are about 180° apart. � Ideal targets are separated by 45° - 135°

From your campsite at Frog Lake you can see what looks like an old mining cabin in the distance. � Curious about its location, you take a bearing with your compass. � 345°M � The next day you camp at Rock Creek Lake. Again you can see the cabin and take a compass bearing. � 66°M � Where is the cabin? Should you take another bearing before you hike to the cabin?

Location by resectioning is often taught, � but is seldom used in the field It’s rare to find 3 identifiable features all of which are on your map sheet. � It’s more common to use just one bearing and combine it with other information, like being located on a trail or other identifiable feature. � Often you do not have enough information to detect errors in plotting.