Taking a look at our diagram we are sailing due south on a course of 180 degrees at a speed of 08 kts. From a DR position at 0900 we take a bearing on a shore buoy. The buoy is easterly to our observation rendering an LOP of 100 degrees. What is important is that we are somewhere on that line but "exactly" where we don't have that information. The DR position only indicates a position based upon Speed-Distance-Time…. It's very important that you understand that concept…All DR's will be marked along our course line.
At precisely 1000 hours I still have the buoy in sight and take another compass bearing. From my observed position my LOP to the mark is 025 degrees which I plot on my chart. The next thing I do is to chart a 1000 DR position. Since I estimate my speed at 08 kts and I traveled 60 minutes, I traveled a distance of 08 nautical miles. I measure eight miles off my chart and indicate on my track line a 1000 DR.
The 1000 DR tells me where I should be. My 1000 LOP tells me that I am somewhere along the bearing line.
This is all simple enough. Don't confuse the concepts. What I do is merely pick up the 0900 LOP and move it….Where Do I move it? I move it to the 1000 DR. "When two LOP's intersect you have a fix". The 1000 DR tells me where I should be but my FIX tells me where I actually am. This is a running fix.
Obviously in the last hour wind and sea have thrown me slightly off my course line. Coincidentially This line passing through The DR and the Fix we know to be 100 degrees. The reciprocal course is 280 degrees… so observation would indicate we are being set to the west…"by slightly north we haven't made quite the distance as our DR". If we therefore measure the DR distance to The Fix with our dividers and measure it 0.25 miles we now know that wind and current is 280 degree set 0.25 drift. Generally there are no problems with plotting once you get accustomed to labeling properly.