At 2:07 you appear to line up the compass with the graph depicting the magnetic declination to set the declination. The USGS (map publisher) specifically warns against such practice. That angle is NOT drawn to scale. If it is found to be accurate, it's solely by accident. PLEASE DO NOT USE THE DIAGRAM BUT DO CALCULATE THE REQUIRED DECLINATION NUMERICALLY. Good vid and yes, the audio needs a bit of attenuation.
The military maps and the US Military do teach you to orient a map to magnetic declination. I guess they figure it is accurate enough for field viewing/terrain association. The precise method is to place N at the Index Line then rotate the bezel to the degrees of declination (MN to TN). If you have a west declination turn the bezel left. If East turn the bezel right. Place the compass edge along the map edge. Rotate map and compass until red is in the shed. If the edge of the map is not accessible then use the Eastings. You will be out the amount of convergency (degrees between TN and GN). This is usually a cpuple of degrees.
I have never understood why most instructions (not all do) say to use back bearings to find your location on the map. As long as the lines you draw to find your location passes over the two land marks you took bearing to. The lines will be at the same place on the map, and they will cross at the same location. The only difference whether you use the two forward bearings 290˚ & 60˚, or the two back bearings of 110˚ & 240˚, is the direction the compass is pointing when you draw the lines. The lines are in the same place. I can see you would get an incorrect location fix , only if you drew the lines from where you think you are, and ignore the fact the lines would not be crossing the two land marks. Those would be parallel bearing lines, and incorrect. I even asked a friend, a professional surveyor, if he could think of any error that would creep in from using forward bearings, and not back bearings, and he could not think of any. Lines drawn with forward or reverse bearings would cross or intersect at the same point. As long as the two land marks are on the drawn lines. Additionally if you use forward bearings you eliminate the possibility of making a math error, when converting the forward bearings to reverse bearings. You would only need to make sure you converted the magnetic north compass bearings taken with the compass, to the correct grid north (or true north) bearings to be measured on the map. otoh, , making such location measurements shouldn't ever be needed to to be done. However, if you know why people say to use back bearings, I'd be pleased to hear and be convinced why it would be necessary or preferable to use them.
The only practical reason I can think of is that drawing a forward bearing line on the map away from the landmark along the azimuth you shot, and then extending that line back toward your location may require (depending on your map scale and the distance of the landmarks) a longer straightedge than is needed to only draw the reverse azimuth line from the landmark towards your location. I can see that it would be preferable not to have to juggle a ruler and a compass, if the 6" straightedge on your lensatic will do. But I am also curious to know if anyone has any other reasons why back azimuths are usually recommended over the method TyJee28 describes.
Fyi Your resection calculations are incorrect. You included the 1 degree 50 minutes into your Grid Magnetic Angle. The convergency angle of 1 degree 50 minutes must first be subtracted from the magnetic declination (TN to MN). Rounded off you end up with a GMA of 14.5 degrees.
I enjoy your videos. Fyi...there is no need to do a back azimuth during resection or modified resection. You must be used to using a Commenga compass. With a baseplate compass you shoot your azimuth to the mountain then place the front edge of the compass on the mountain. You rotate the compass until N is facing up the map and the inner meridians lines are aligned with the Easting lines of the map. There is no need for computing a 180 azimuth. Your technique works but is unnecessary. All good.
At 2:07 you appear to line up the compass with the graph depicting the magnetic declination to set the declination. The USGS (map publisher) specifically warns against such practice. That angle is NOT drawn to scale. If it is found to be accurate, it's solely by accident. PLEASE DO NOT USE THE DIAGRAM BUT DO CALCULATE THE REQUIRED DECLINATION NUMERICALLY. Good vid and yes, the audio needs a bit of attenuation.
The military maps and the US Military do teach you to orient a map to magnetic declination. I guess they figure it is accurate enough for field viewing/terrain association. The precise method is to place N at the Index Line then rotate the bezel to the degrees of declination (MN to TN). If you have a west declination turn the bezel left. If East turn the bezel right. Place the compass edge along the map edge. Rotate map and compass until red is in the shed. If the edge of the map is not accessible then use the Eastings. You will be out the amount of convergency (degrees between TN and GN). This is usually a cpuple of degrees.
Easy snd to the point.. thank you sir
I have never understood why most instructions (not all do) say to use back bearings to find your location on the map. As long as the lines you draw to find your location passes over the two land marks you took bearing to. The lines will be at the same place on the map, and they will cross at the same location. The only difference whether you use the two forward bearings 290˚ & 60˚, or the two back bearings of 110˚ & 240˚, is the direction the compass is pointing when you draw the lines. The lines are in the same place.
I can see you would get an incorrect location fix , only if you drew the lines from where you think you are, and ignore the fact the lines would not be crossing the two land marks. Those would be parallel bearing lines, and incorrect. I even asked a friend, a professional surveyor, if he could think of any error that would creep in from using forward bearings, and not back bearings, and he could not think of any. Lines drawn with forward or reverse bearings would cross or intersect at the same point. As long as the two land marks are on the drawn lines.
Additionally if you use forward bearings you eliminate the possibility of making a math error, when converting the forward bearings to reverse bearings. You would only need to make sure you converted the magnetic north compass bearings taken with the compass, to the correct grid north (or true north) bearings to be measured on the map. otoh, , making such location measurements shouldn't ever be needed to to be done.
However, if you know why people say to use back bearings, I'd be pleased to hear and be convinced why it would be necessary or preferable to use them.
The only practical reason I can think of is that drawing a forward bearing line on the map away from the landmark along the azimuth you shot, and then extending that line back toward your location may require (depending on your map scale and the distance of the landmarks) a longer straightedge than is needed to only draw the reverse azimuth line from the landmark towards your location. I can see that it would be preferable not to have to juggle a ruler and a compass, if the 6" straightedge on your lensatic will do. But I am also curious to know if anyone has any other reasons why back azimuths are usually recommended over the method TyJee28 describes.
Good video
Fyi
Your resection calculations are incorrect. You included the 1 degree 50 minutes into your Grid Magnetic Angle. The convergency angle of 1 degree 50 minutes must first be subtracted from the magnetic declination (TN to MN). Rounded off you end up with a GMA of 14.5 degrees.
Needs better audio.
Thanks for the vid.
I enjoy your videos. Fyi...there is no need to do a back azimuth during resection or modified resection. You must be used to using a Commenga compass. With a baseplate compass you shoot your azimuth to the mountain then place the front edge of the compass on the mountain. You rotate the compass until N is facing up the map and the inner meridians lines are aligned with the Easting lines of the map. There is no need for computing a 180 azimuth. Your technique works but is unnecessary. All good.