As some of you may know, after her refit at the end of 1912, early 1913, the Olympic was equipped with 5 compasses on board: One Lord Kelvin's 10'' standard compass with azimuth mirror on the central platform, one Lord Kelvin's 10'' standard compass above the wheelhouse, one Lord Kelvin's 10'' steering compass inside the wheelhouse, one Lord Kelvin's 10'' steering compass for the Captain's bridge, and a 10'' light card compass for the docking bridge. The Britannic was similarly equipped.
The Titanic did not have a compass on top of the wheelhouse. The only standard compass it had was on the central platform. The course that the ship was to keep was the course on this standard compass. The detail of the use of standard and steering compasses on Titanic was explained by Lightoller:
"We have a standard compass and a steering compass. The standard compass is the compass we go by. That is the course that is handed over from one senior officer to another, the standard course. The junior officer goes to the standard compass which is connected with the wheelhouse by a bell, or by a bell push, wire and bell, and when she is on her course he rings that bell continually, showing the ship is on her course with the standard compass. The other officer takes her head inside the wheelhouse from the compass the quartermaster is steering by. The standard course is on a board and the steering compass course is also on a board. Therefore, the quartermaster uses the board that is there for the steering compass. The senior officer of the watch looks to the standard compass board and passes that course along."
The "Courses on board" book used for Olympic in the years following her refit had three column pairs printed in it for the transatlantic part of the voyage. The three column pairs were labeled: Std. comp., Stg. Comp., and W. Comp. Each compass had a column for writing down both the compass course and the compass deviation. There was also a column for writing down the magnetic variation labeled Var. W'ly., and a column labeled True Course. There were also columns for the ship's Lat. N., Lon. W., the Day, Month, Date, Hour From and Hour To for a particular course run. When filling out a given row for a particular run, values were entered into all of these columns.
However, for use in channels, the page only had columns labeled From and To and columns for listing the course on the Std. Comp., and the Stg. Comp. There were no columns for listing compass deviation, and there was no column for the W. Compass. It was only during the the transatlantic part of the voyage that courses and deviations on all three compasses were logged.
My guess, and its just a guess, is that it was the W. Compass was the one installed on top of the Wheelhouse roof. It allowed for steadying the ship on course "by standard" every half hour per IMM rule 253 that was apparently still in effect at that time without having to send a J/O out to the midship platform. I believe that is Dave Brown's main argument for why a standard compass was installed on the wheelhouse roof. It made for easier communication. But the same rule 253 also required that compasses be compared every OOW's Watch (that's every 4 hours) and the comparisons entered into the Compass Comparison Book. This last part of the requirement I believe would involve all three compasses that we are talking about here, thus the three columns printed in the book. Another consideration is that the standard compass on Olympic's midship platform was equipped with an azimuth mirror which was used to take stellar and solar bearings to check the deviation of the standard compass. This was not listed as part of the compass that was put on top of the wheelhouse in the H&W office drawing notebook that specified the compasses used.
What exactly happened after they installed a gyro compass on Olympic I'm not too sure about. Was the central platform compass removed and the one over the wheelhouse then the the only standard on board? The Courses on Board book show the words "W.Comp." crossed out and replaced with the hand written word "gyro". For the channel crossings, the gyro compass values were written in the column for the Stg. Comp. But the "Std. Comp." columns for both channel courses as well as transatlantic courses were still being written in. And the the value written into the "True Course" column always equaled the Std. Comp. course after adjusting it by the variation and std. comp. deviation to a 1/4 of a degree. Thus you may see the true course written as "S 43 3/4 W," while the gyro course was entered with just three digits, e.g., 224, and no fractions.