The Very Basics of Plotting

Bottom line: plotting involves converting field notes of direction & horizontal distance to lines on a map.  Thus there are two factors to address:

• direction

• distance

Direction

• Field notes have bearings recorded.  To plot using a Douglas protractor: (click to expand/collapse)

  • place protractor over map sheet with N properly aligned (i.e. "N is up")

  • centre the the hole of the protractor over your starting point

  • mark the desired bearing at edge of protractor

  • draw a very light line to connect the two points

  • determine the map distance (see below) and draw the line segment

Distance

Converting slope distance (SD) to horizontal distance (HD)

• For those with no desire to understand how/why it works simply use the formula
  HD = SD * COS (slope degrees)
   

• For those who want to know why this works read the next few lines  (click to expand/collapse)

  • why use COS?  (click to expand/collapse)

    • COS is adjacent over hypotenuse ... or ... HD over SD

    • this means COS = HD "as a %" of SD ... COS is used to convert SD --> HD

    • i.e. if COS was 0.92 (or 92%), for every 100m slope we actually travel 92m horizontal

    • since we have SD, COS "converts it" to HD
       

  • Example: SD = 48m, slope=25, 14 degrees  
    HD = SD * COS(14 degrees) = 48m * COS (ArcTAN(0.28)) ...
    working in steps: (click to expand/collapse)

    • COS(14) = 0.970  (means that for every 100m slope we went 97.0m horizontal)

    • SD * 0.970 = 48m * 0.970 = 46.6m HD

Converting Field Measures to Map Measures

• This is simply a "reduction exercise" as ground distances (m) are converted to map distances (cm) (click to expand/collapse)

  • This is a 2 step process (using the HD above and a map scale of 1: 2,000):

  • 1) convert ground measures to cm (i.e. 46.6m --> 4660cm)

  • 2) divide by the scale factor (4660cm / 2000 = 2.3cm)