Sight Plotting: example

At 0927 a morning sight gave a Intercept

4.8’ Away, brg 123 ˚ T.
DR 20 ˚ 10’ N 10 ˚ 15’ E
An observation of the Sun at Meridian
Passage, 1213 hrs, gave a
Lat 20 ˚ 56.1’ N
V/L steaming 340 ˚ T at 18.5 kts
Find the 1200 observed position
Draw a rough sketch so as to be sure of the Directions involved

1213 MP lat

Reciprocal of 340 x
18.5 kts for 13min
1200 MP lat
D’LAT S
1200 DR
D’LAT N
340 x 18.5 kts D’LONG W
for 2h 33min

0927 DR
0927 ~ 1200 = 2 H 33m @ 18.5 KTS = 47.2 ‘
DIST = 47.2 Co = 340 d’lat = 44.4’ N dep = 16.1’ (W)
0927 Dr lat 20 10 N M’lat 20 32.2
D’lat 0 44.4 N d’long = dep
1200 dr lat 20 54.4 N cos m’lat d’long = 17.2’ W

0927 DR long 10 15’ E

d’long 0 17.2’ W
1200 DR long 9 57.8’ E

1213 ~ 1200 = 13min @ 18.5 kts = 4.0M

Dist = 4.0M Co = 340 (reciprocal) d’lat = 3.8’ S
MP lat 20 56.1 N
D’lat 0 3.8 S
1200 lat 20 52.3 N
 57

L
56
A
T
55 I
T
54 U
D
1200 DR E
53

52

51

50
We are now ready to plot the sight(s) on our graph paper

Pick an appropriate scale, in most cases on A4 graph paper this will be

2cm = 1M ( 1cm = 1M would also be acceptable )

Draw your Latitude scale on the side of the page – leave the Departure /
Longitude scale blank for now
From your original sketch try and establish the directions and distances you will
be drawing, so as to have an appropriate start point on your page, in this case
you want your 1200 DR to be about 10 cm from the top right hand corner of the
page

Plot the appropriate bearing and intercept and the 1200 Lat, where they cut, is
the vessels 1200 Observed position
 57
Intercept 4.8 away
L
56
A
T
55 I
T
54 U
D
1200 DR E
53

52
1200 Lat
51

DEPARTURE ( W) 50
7 6 5 4 3 2 1 0
Pot the Departure scale using 2cm = 1M,
and the 1200 DR as zero – as we are
going left from the DR the Departure and
hence D’long will be West
From the Observed position read off the Departure.
In this case it is 6.8 (west)

We need to convert this to D’long using the Mean latitude from our observed position
and the 1200 DR.

Observed Lat 20 52.3 N

1200DR lat 20 54.4 N d’long = Dep
Mean Lat 20 53 .4 cos m’lat

D’long = 6.8 = 7.3’ W

cos 20 53.4

1200 Dr Long 09 57.8’ E 1200 Observed position: 20 52.3 N

D’long 0 07.3 W 09 50.5 E
1200 Long 09 50.5 E
 Star Plot
• Star sights give an actual position using
multiple observations (usually 3 if using
standard sight reduction)
• However the observations are not taken
simultaneously, therefore they need to be
run forward or back to get a position.
• We can use conventional running fix
techniques to do this, but this can be
confusing
 L
A
T
I
T
U
D
E

DEPARTURE
• A simpler method uses a common time
and a common DR as a reference point
and the observations are run forwards or
back from this point
• EG: a vessel observes the following:
1755 – star A brg 180 int 2.0 away
1800 – star B brg 135 int 1.0 towards
1805 – star C brg 090 int 1.0 towards
DR 50 20 N 020 30 W
Course 225 T spd 12 kts
Find the 1800 position
In this case we want to find the position at 1800, using the
information supplied, make up the following table
The run time is the difference between the obs time and the
required time, and is named depending on whether you need
to add or subtract that value to get to the required time
The run distance is based on the course/speed and run time
and takes the same sign as the run time

Star Run Time Run Dist Brg Int

A + 5 min +1 M 180 2 Aw
B - -- 135 1 To
C - 5 min -1M 090 1 To
• On the Graph paper – draw on a latitude
scale and plot the DR – roughly in the
centre of the page
25
24

23 L
A
T
22 I
T
21 U
D
20 E

19
18
17
50°
• Lay off the Course line through the DR
 25
24

23 L
A
T
22 I
T
21 U
D
20 E

19
18
225 ° 17
50°
• Run the appropriate Star back or forward
from the DR as indicated in the table and
plot the bearing and intercept
 25
24

23 L
A
Star T
C
22 I
T
21 U
D
Star B 20 E
Star A
19
18
225 ° 17
50°
• Draw on a Departure Scale – using the DR
as a Reference point and number it
appropriately for East or West
 25
24

23 L
A
Star T
C
22 I
T
21 U
D
Star B 20 E
Star A
19
18
225 °
East 17
0 1 2 3 50°
• Read off the Latitude and the Departure
for the Position
 25
24

23 L
A
Star T
C
22 I
T
21 U
D
Star B 20 E
Star A
19
18
225 °
East 17
0 1 2 3 50°
• From the Plot – Lat 50 21.2 N
• Departure – 1.5 (e)
• D’long = Dep / Cos Mean Lat
• Mean lat = 50 20.6
• D’long = 1.5 minutes / cos 50 20.6
• D’long = 2.35 minutes East
• DR longitude 20 30 W
• D’long 0 2.4 E
• Observed 20 27.6 W

• 1800 Observed Position 50 21.2 N

020 27.6 W