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Radar plotting tips, radar plotting made easy

 

Radar Plotting Formulas

Time to C.P.A. = "A" to C.P.A. x Plotting Interval
 
Distance "O" to "A"

 

"W" to "O" = Own ships speed x Plotting interval
 
60

 

Course Line = "W" though Centre of "A"

 

Targets speed = Distance from "W" to "A" x 60
 
Minutes from "O" to "A"

 

 
        

Aspect = The angle of the course your going and from the centre of the screen to "A" (Draw a line from the centre of the plot to "A" and measure the angle between this line and your course) (red if you would see targets red sidelight and green if you would see his green sidelight)

 


Tidal problems using Radar
Drift = Distance from "A" to "W"
 
 
Set = Direction from "A" to "W"
 
Rate of Tide = Distance from "A" to "W" x 60
  Minutes from "O" to "A"  
Basic Radar Plot
Warning !!!!!!!!!!!!!!!!!!! Watch to see if you are using Relative or True Compass Bearings.
Using a relative bearing is how you would take a compass bearing of another vessel.
Yellow Plot is the targets first plot
Red Plot is the targets last plot
1
2
3
4
5
6
7
8
(1) Find the course your going, and draw a line for your course
(2) Draw a line where the first plot is and mark the distance, call this "O" (Drawing 1 above)
(3) Lay off 2nd bearing course and distance (Drawing 2 above)
(4) Lay off 3rd bearing course and distance, call this "A" (Drawing 3 above)
(5) From "O" draw a line through "A" and carry the line well past the centre line on the screen (Drawing 4 above)
(6) Using your course line, transfer this onto "O" and pull the line down the sheet (Drawing 5 above)
(7) This line is where "W" goes
(8) To find the distance you would find "W" use this formula
 
own ships speed x plotting interval
 
 
60
 
(9) Measure this distance and make a mark, call this "W" (Drawing 6 above)
(10) From "W" draw a line through "A" (this is the other vessels course and speed) (Drawing 7 above)
(11) From the centre of the screen - draw a line to meet the "O" to "A" line so they meet at 90 degrees
(12) This is called C.P.A. (Drawing 8 above)
Aspect
This is the angle between your vessel and the targets vessel and what colour of sidelight you see
From the course line your steering draw a line onto plot "A", the angle between the 2 lines is the aspect and if you saw a green sidelight then it would be (E.g."Green 048 degrees) and if a red sidelight then (E.g. Red 056 degrees)
In this case, the Aspect would be roughly "RED 015 degrees"
Alteration of Course
(1) Make Basic radar plot
(2) Find out the amount of degrees your going to alter course
(3) The "W" to "O" is the course your steering just now, lets assume your going to alter 90 degrees to starboard, using a protractor with the zero degrees of the protractor on the "W" to "O" line - measure off 90 degrees to starboard and make a mark
(4) From "W" draw a line towards the mark you just made
(5) With a set of compasses measure the original "W" to "O" and draw an arc towards and past the new line
(6) Call this Mark "O one" (this is your new course) "W" to "O one"
(7) From "O one" draw a line through and well past "A"
(8) Make a new C.P.A. by drawing a line from the centre of the screen onto the new "O one" to "A" line so the line joins at 90 degrees
(9) Measure C.P.A. line
Change of your vessels speed
(1) Make Basic radar plot
(2) Find your new speed
(3) On the "W" to "O" line if you reduce your speed then this line will get shorter
(4) On the "W" to "O" line if you increase your speed then this line will get bigger
(5) Use this formula to find the distance you need to measure back from "O" to get "W"
 
"W" to "O" = Own ships speed x plotting interval
 
 
60
 
(6) From "W" measure this distance and call it "O one "
(7) From "O one" draw a line through "A" and go well past the centre line of your screen
(8) From the centre of the screen draw a line so it meets the new "W one" to "A" line at 90 degrees
(9) This is your new C.P.A.
Change of course and speed
(1) Make Basic Radar Plot
(2) Find out your new speed
(3) Use this formula to find the distance you need to measure back from "O" to get "W"
 
"W" to "O" = Own ships speed x plotting interval
 
 
60
 

(4)From "W" measure this distance and call the new mark "O one"
(5) Now find out the amount of degrees you have to alter course
(6) The "O one " to "W" is the course your steering just now, lets assume your going to alter 90 degrees to starboard, using a protractor with the zero degrees of the protractor on the "O" to "W" line - measure off 90 degrees to starboard and make a mark
(7) With a set of compasses measure "O one" to "W" and draw an arc towards and past the new line
(8) Call this Mark "W one" (this is your new course) "O one" to "W one"
(9) From "W one" draw a line through and well past "A"
(10) Make a new C.P.A. by drawing a line from the centre of the screen onto the new "W one" to "A" line so the line joins at 90 degrees
(11) Measure the new C.P.A.

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