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31 Cards in this Set
- Front
- Back
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What are the three basic requirements for determining an angle?
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1. Reference or starting line
2. Direction of turning 3. Angular distance (value of the angle) |
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What are azimuths?
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- Horizontal angles measured clockwise from any reference meridian
- measured from the north up to 360 degrees |
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What directions correspond to the quadrants: 1, 2, 3, and 4?
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1 - NE
2 - SE 3 - SW 4 - NW |
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What are bearings?
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- another system for designating directions of lines
- defined as the horizontal angle between a reference meridian and the line |
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How are bearings different than azimuths?
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- azimuths you only measure from the north so you always deal with up to 360 degrees, bearings you can use north and south so you only go up to 90 degrees in either direction
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What are the two main types of bearings?
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Grid bearings
- used when bearings are taken from or to the map Magnetic bearings - used in conjunction with a compass. |
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In what order are bearings written?
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The angle is always preceded by N or S and followed by E or W
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How can you find the values of back bearings if you have the values of forward bearings?
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- Back bearings should have the same numerical values as forward bearings but opposite letters
So if Bearing AB = N44E then Bearing BA would be S44W |
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What are the two main traverses?
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Open traverse: define a path from one point to another (ex. centerlines of roads, pipelines, etc)
Closed traverse: enclose an area (boundaries of properties, etc) |
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What is the formula for calculating the sum of interior angles in a closed traverse?
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Sum of Interior Angles = (N-2)x180
where N = number of angles |
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What is the formula for calculating deflection angles?
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Sum of outside angles - Sum of inside angles = 360
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You are given the following polygon with interior angles and asked to calculate the bearings.
Angle A: 94 degrees 09 minutes Angle B: 41 degrees 42 minutes Angle C: 220 degrees 16 minutes Angle D: 78 degrees 42 minutes Angle E: 105 degrees 11 minutes And you are given the first bearing from AB as being N 63 degrees 25 minutes E Calculate all the other bearings |
AB: N 63 degrees 25 minutes E
BC: N 74 degrees 53 minutes W CD: N 34 degrees 37 minutes W DE: S 44 degrees 05 minutes W EA: S 30 degrees 44 minutes E Make sure to recheck your first angle to make sure you did it correctly |
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How do you correct for interior angles?
For example, what would you do if the actual sum of interior angles was 539 degrees and 49 minutes? There are five angles. |
Based on the equation for interior angles the sum should be 540 degrees.
There is therefore an error of 11 minutes. We must add 3 minutes to the largest angle, and 2 minutes to all others and then check the total again. |
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How do you correct for deflection angles? For example, what would you do if the total sum of deflection angles was 360 degrees and 05 minutes? There are four angles.
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Total should be 360 degrees.
Error is + 5 minutes. Subract 2 minutes from largest angle and subtract 1 minute from the others. |
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How do you convert bearing to azimuth and azimuth to bearing in each of the four quadrants?
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1. Bearing = Azimuth
2. Bearing = 180 degrees - Azimuth Azimuth = 180 degrees - Bearing 3. Bearing = Azimuth - 180 degrees Azimuth = 180 degrees + Bearing 4. Bearing = 360 degrees - Azimuth Azimuth = 360 degrees - Bearing |
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The azimuth of the boundary line is 128 degrees 13 minutes 46 seconds. Convert this to a bearing
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180 degrees - 128 degrees 13 minutes and 46 seconds = 51 degrees 46 minutes 14 seconds
S 51 degrees 46 minutes 14 seconds E |
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The first course of a boundary survey is written as N 37 degrees 13 minutes W. What is its equivalent azimuth?
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360 degrees - 37 degrees 13 minutes = 322 degrees 47 minutes
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What are the three types of north?
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1. True North (TN)
- earth spins on this axis 2. Magnetic North (MN) - compass needle points to magnetic north - its a location in the far north of Canada 3. Grid North (GN) - the vertical lines overlayed on a map point to grid north - not exactly true north In map reading we only use GN and MN |
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What is secular variation?
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- the Earth's magnetic field is slowly changing on time scales that range from years to millenia
- for example. the magnetic declination undergoes changes as a result of magnetic activity - it is important to note that secular variation is changes in magnetic field not magnetic pole |
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-What is magnetic declination or magnetic variation?
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The difference between grid north and magnetic north
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What does it mean if the variation is 3 degrees west? How will this affect map and compass use?
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Magnetic north is 3 degrees west of Grid North, so whenever we calculate a bearing from our map we must ADD the three degrees to use it with a compass, and whenever we take a bearing using the compass we must SUBTRACT the three degrees to use the bearing on a map
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What is grid declination? Convergence angle? And declination?
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Grid declination: difference between grid north and magnetic north
Convergence angle: difference between true north and grid north True Declination: difference between true north and magnetic north |
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How do you use your map to determine declination?
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Check the map and it will have the correction for the year the map was made along with instruction to adjust to the present year.
1.) Calculate the current magnetic declination 2.) The magnetic declination is decreasing 5.9’ per year 3.) 5.9’ * 12 = 70.8’ or 1˚10.8’ (1˚11’)E 4.) Since it’s decreasing minus 1˚11’ from 19˚38’ 5.) Solve 6.) Minus your magnetic declination from your GN Azimuths. |
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How do you convert Grid to Magnetic and when would you do that?
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If bearings are taken from a map they are grid bearings and must be converted to magnetic bearings for compass work.
GMS = Grid to Magnetic - Subtract variation |
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How do you convert Magnetic to Grid and when would you do that?
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- magnetic bearings must be converted to grid bearings for plotting
- MGA = Magnetic to Grid - Add variation |
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How do you box a compass needle?
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The red arrow which points North must be inside the compass box arrow
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How do you find the bearing from you to a mountain peak in the distance using a compass?
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1. Hold compass up and point direction of travel arrow (big one outside the circle) in the direction you want to go
2. Bring compass down to waist level and make sure it is level. Box arrow by moving the dial so the red arrow is inside the box red arrow. 3. Read your bearing at the Bearing Index Point (white line under N) |
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If someone tells you to follow a bearing of 270 degrees from your current position using a compass, what do you do?
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1. Stand up
2. Hold compass at waist level and keep level 3. Move the dial to 270 degrees (or W) at the Bearing Index Point 4. Box the needle by moving your feet to the left or right |
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How do you orient a map using a compass?
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1. Align Direction of Travel Error and Arrow box on compass
2. Align compass on side of map (note needle is not boxed at this time) 3. Grab corner of map and move left or right to box needle so all three arrows are point in the same direction 4. Correct for declination, so if the declination is 13 degrees you would move until the magnetic needle of the compass is about 13 degrees to the right of the Arrow Box (the magnetic arrow should be to the right of the box arrow in the same manner as the diagram on the map) |
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How do you obtain a Bear from you to a peak on a map?
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1. Draw a line from your position to your point of interest on map
2. Align compass on line with Direction of Travel arrow pointing at point of interest 3. Rotate dial to box needle 4. Read your bearing at your Bearing Index Point if it is 290 degrees from where you are standing, if you are facing north, you will have to walk at 290 degrees to get to your point of interest |
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How do you obtain a bear from a peak to you (on a map) using a compass/
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1. Draw a line from your point of interest to your position
2. Align compass on line with direction of travel arrow pointing at your position 3. Rotate dial to box needle 4. Read your bearing at your bearing index point ex. the bearing is 110 degrees from the peak to you |
