NavList:

As part of the research for an upcoming expedition, I am trying to ascertain how accurate were nineteenth century calculations of compass error when derived on land-based exploring expeditions. These calculations combined declination and compass error and in this case were expressed as variation. They were vital for turning explorers field books (based on magnetic bearings) into maps (orientated to true north)

My main worry is the errors in the Nautical Almanac for the period, which have come to light on this site. An example calculation is shown below. I wonder if one of the list members could run the calculation though modern software and check for accuracy. The explorer is taking a bearing to the sun at sunrise using a magnetic compass. Longitude is approx 132º 20' East although this was not recorded in the field book calculation:

Example 1

Date:                July 3rd 1856

Latitude:            15º 18'  South   

Azimuth:           N64º  0' E Magnetic      

(Sun Centre at sunrise)

Altitude:            0º 0'

Sun Declination: 23º 0' north

Amplitude Sun: 23º 50'

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Calculation:

Amplitude Sun:  E 23º 50' North

                           90º 00'

                        N 66º 10' East True

Az                    N 64º 00' East Magnetic

Var                        2º 10' East Variation

This is an exact copy of the explorer’s field book entry. Declination and amplitude came, presumably, from the Nautical Almanac.

Example 2

This example was not worked in the field book but would appreciate an answer.

Date:                March 1^st, 1856

Latitude:            19º 28' 05" South          

Azimuth:           N98º  0' E Magnetic      

(Sun Centre at sunrise)

Altitude:            0º 0'

Long was approx 127º 40'  east although not recorded in the field book.

Any solutions would be greatly appreciated.

The dates are for local time in northern Australia, not the date at Greenwich

Kieran Kelly

Sydney

Australia