NavList:

See my prior posts about the Polhemus computer:

As an addendum to my previous post, I forgot to point out that the central meridian on both the plotting sheet and the Polhemus computer were 119º 15' W and the central parallel was 34 º N although that should have been clear from the context.

I also forgot to show how the final fix coordinates were determined. The latitude is easy, just read it off the central meridian scale and remember, for the plotting sheet, to divide by 4 since I multiplied the scale by 4 at the beginning. To determine the longitude you do the reverse of the process used to plot the A.P.s, set the scale to 56º (34º above the center parallel) and read straight down from the fix to where it strikes the diagonal scale and that is the longitude. On the plotting sheet do the same and place one leg of the dividers at that intersection and measure the distance from that intersection to the center of the plotting sheet  on the vertical scale, again dividing by 4. See figure 26.

In addition to the plotting disk we just used, the Polhemus comes with 6 other disks on which are drawn the graticle for 0º,  25º,  35º,  45º 55º,  and 65º latitudes for a Lambert projection at a scale of 1:5,000,000, a common scale used on the GNC series of aeronautical charts which allows you to use it at any latitude. (You use the 0º again for polar grid navigation.) Since the graticle is marked with latitude and longitude you just plot the A.P. on the graticle and read out the longitude also on the graticle, see figure 27 through 29. Figure 28 shows the disk for 65º by itself and figure 29 shows it mounted on the Polhemus base.

The Polhemus was used by the Air Force but the Navy also used similar devices such as the Mk5 and Mk6 plotting boards which are used in a similar fashion although they do not have the computer functions on the other side to do the in flight celnav calculation Figure 30 is a picture of a Mk6A plotting board. The Polhemus is 8 and a half inches in diameter while the plotting board is 12 inches across and is much heavier since it incorporates a storage compartment inside.