Measuring Angles Direct Method

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Measuring Angles Direct Method
Mr. Conrado Agm 230
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Angles

• An angle defines the rate at which two lines or
  planes diverge from a common point.

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Angles

• All angles have three parts
• Backsight The baseline or point used as zero
  angle.
• Vertex Point where the two lines meet.
• Foresight The second line or point

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DD DMS

• Two systems are used for recording angels
• Decimal Degrees (DD) Fractions of a degree are
  expressed as a decimal.
• Degrees-Minutes-Seconds Partial degrees are
  expressed as minutes () and partial minutes are
  expressed as seconds ()

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DMS

• One degree equals 60 minutes
• One minute equals 60 seconds

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Adding Subtracting DMS

• Is more difficult in DMS method.
• Ie. Add 20 - 45 25 to
• 30 - 24 33
• 69 and 62 are not possible and therefore must
  be reduced
• Step 1 subtract 60 from the seconds until
  remainder is less than 60. Add the number of
  whole 60s to the minutes.
• Step 2 is subtracted from the minutes until the
  remainder is less than 60. Add the number of
  whole 60s to the degrees.
• The result is the correct angle

• 20 45 27
• 30 24 35
• step 1 50 69 62
• 60
• step 2 50 70 02
• 60
• 51 10 02

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Direct Method of Measuring Angles

• Requires an instrument
• To measure vertical angles the scope must be able
  to rotate up and down
• Transits
• Theodolites
• Total Stations
• Precision is a major difference
• Levels usually limited to one degree
• Total Station and Theodolites capable of less
  than 20 minutes

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Angle Measurement Steps

• Center the instrument
• Zero set the angle scale
• Align the scope on the backsight
• Turn the angle
• Record the angle

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Centering the Instrument

• For complete accuracy the instrument must be
  centered over the vertex of the angle.
• This is done by suspending a plumb bob from the
  center of the instrument usually over a surveyors
  nail or stake.
• If done incorrectly the potential for error is
  great.

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Centering the Instrument

• Procedure
• Most instruments allow for slight adjustments
  after mounting by loosening the instrument from
  the tri-pod or the leveling screws.
• To set the tripod use the plumb bob through the
  mount center as a guide.
• Mount and level instrument leaving base slightly
  loose for later adjustment.

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Centering the Instrument

• When centered the base is tightened and
  instrument re-leveled.
• Re-check plumb bob for center
• If wind in present use a wind block to lesson
  wind effect.

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Zero Setting the Instrument

• After the instrument is centered the angle scale
  can be moved to zero or any other starting angle.
• Loosen the scale lock
• Align scope on backsight
• Rotate scale until desired angle is set
• Snug scale lock

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Aligning the Instrument of the Backsight

• When shooting longer distances very small scope
  movement may result in large swings of the
  crosshairs.
• Use the tangent screw to make fine adjustments to
  increase ease and accuracy.

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Turning the Angle

• Now rotate the scope until the crosshairs are
  aligned with the second scope.

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Record the Angle

• All instruments measure angles but different
  makes use different scales.
• Most manual instruments use a vernier scale.

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Reading Angles David White Instrument

• The 360 horizontal circle is divided into
  quadrants 0-90
• The circle is marked by degrees and numbered
  every 10 degrees
• The horizontal circle is referred to as the
  degree scale
• The instrument vernier divides each degree on the
  circle into 12 equal parts of 5 minutes each. (60
  minutes per degree)

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Reading Angles David White Instrument

• The vernier is marked in minutes starting at 60
  in the center and reading both left and right to
  30. The 60 mark is both 60 and zero.
• Up on the vernier scale is the same direction as
  up on the degree scale.

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Reading Angles David White Instrument

• Example
• Note point at which 60 on the vernier touches
  degree scale. If 60 coincides exactly with a
  degree mark there are no fractions of a degree.
• If 60 on vernier does not coincide exactly with
  degree mark, your degree reading is the line
  which 60 has just passed plus a fraction of the
  next degree.

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Reading Angles David White Instrument

• Example continued
• Start at 60 on the vernier and read up until you
  find a minute line that coincides exactly with a
  degree line.
• Each line on the vernier represents 5 (5, 10,
  15, 20, 25, 30)
• Example 71 20

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Reading Angles David White Instrument

• Example continued
• If no minute mark up coincides read the vernier
  scale back.
• When reading back from 30 to 60 the 60 mark is
  now 60 not zero. Therefore your reading will be
  30, 35, 40, etc.
• Example 71 45

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Assignments

• LAB
• How to measure horizontal angles with a level.

• HOMEWORK
• Read chapter 7 Landscaper Surveying