Clark's 'Latitude'

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Clark's "Latitude" for Fortunate Camp

In his journal entry for August 18, 1805, Clark recorded, "The Latd of the forks agreeable to observation is 43°30'43" North." This is a surprising entry not only because the latitude that Clark recorded is nearly one and a half degrees (100 miles) too far south of the actual site, but also because none of the other journals mention a latitude being obtained at Fortunate Camp before the 19th—one day after Clark had left on his reconnaissance of the Salmon River. Considering the intense level of activity that must have occurred after the captains were reunited on the 17th, and their parleys and trading with the Shoshone, it is most unlikely that either captain found time to make a meridian observation. But, supposing Clark did make an observation of the sun's altitude on the 17th (or on the 18th before he set out for his Salmon River exploration), wouldn't Lewis have mentioned it, recording at least the observed altitude of the sun? And later, when Lewis made his own observations and found Clark's latitude to be in error, couldn't he have made a comment about it similar to the one he made about Clark's observation on August 15th at Rattlesnake Cliffs?

Clark's entry for the 18th clearly states that the latitude of the forks came from an observation. But what if an observation was not actually made at Fortunate Camp. Suppose Clark estimated it, taking the distance he traveled south from Rattlesnake Cliffs, converting it to degrees, minutes and seconds and then subtracting that from the latitude he calculated for Rattlesnake Cliffs? Although calling the derived latitude an "observation" would clearly be stretching the meaning of the word, it is still a justifiable usage.

The latitude Clark had calculated for the Rattlesnake Cliffs, unfortunately, was too far south by about one degree (69 miles), as Lewis pointed out—probably after making his own observations at Fortunate Camp. If Clark's latitude for Rattlesnake Cliffs had been correct, he could have calculated the straight-line distance he had traveled south from the Rattlesnake Cliffs, converted that to degrees, minutes and seconds, and subtracted them from the latitude at Rattlesnake Cliffs. This means, however, that Clark would have needed to plot his river survey data before making the calculation, and it is unlikely that he had time to do so by the morning of the 18th. But supposing that the unlikely happened, and that Clark had indeed plotted his survey data, he would have found that the forks lay only 15½ miles south of the Rattlesnake Cliffs. Converting this distance into latitude would have placed the forks only about 13'30" of latitude south of Rattlesnake Cliffs, not 30 minutes, and his calculated latitude for the forks of the Jefferson would have been about 43° 47' 18". Therefore it is highly improbable that the latitude Clark recorded in his journal for the 18th came from a calculation like that described above.

There is another possible source of Clark's erroneous latitude, but it involves some compounded errors plus the fact that the captains didn't always record events on the date that they happened. On Clark's map, east of the indicated site of Fortunate Camp is the notation: No. 43 Latd. 43°30'43" (the figure in minutes can be interpreted variously as 30', 35', or 38', but most likely it is 30).1 A line through it indicates that Clark wished to delete it. Underneath the deleted latitude he wrote: 44°35'28.1", which is the average of the four observations that Lewis made between August 19th and August 21st.

Based on the drafting style and the notations, it seems reasonable to suggest that Clark plotted Atlas Map 65 (from the three forks of the Missouri to three forks of the Jefferson), Atlas Map 66 (from the three forks of the Jefferson to Fortunate Camp) and Atlas Map 67 (at least that part to the divide) while waiting at Fish Weir Camp for Lewis. On August 27th, Clark received a message from Lewis to meet him at the "upper" Shoshone camp. In view of the language barrier, Lewis must have written a note for the young Shoshone messenger to deliver. Did that note contain the latitude of Fortunate Camp from one of Lewis's observations? Lewis's observation for the 20th yielded a latitude of 44°39'43". The 9 of 39' easily could have been misread for a 0, making it 30', but the 4 of 44 would be difficult to mistake for a 3. Even if the minutes of latitude were misread from a poorly written numeral, it still takes another condition to produce the change in the degrees. One answer might be that Clark, thinking of his own observation of August 15, with its latitude of nearly 44° north, may have thought that Lewis meant 43° and changed it. Not knowing of his error, he then may have added the 30'43", thus recording 43°30'43" on his map and in his journal, but back-dating the journal entry to the 18th because that's where he was at the time. This is a difficult interpretation to defend, but it could have happened.

When Lewis and Clark rejoined on August 29th Lewis corrected Clark's understanding of the latitude, and Clark dutifully changed it on his map. Why didn't he also change it in his journal? It is likely that the journals already had been put away for safe-keeping in preparation for the resumption of the journey, and Clark forgot to change it later. At any rate, despite the uncorrected latitude in Clark's journal, Clark's map bears Lewis's calculated average latitude of Fortunate Camp from four observations.

Now, what about those observations?

This page is supported in part by a grant from the National Park Service CCSP Program.