Meridian Hill and Meridians West

While looking idly at Parkman's A Half-Century of Conflict, chapter "France in the Far West", I noticed

This meeting [of a French expedition with the Comanches] took place a little north of the Arkansas, apparently where the river makes a northward bend, near the 22d degree of west longitude.

(Footnote 1, p.. 577 in the Library of America edition.) That would the 22d degree using Washington, D.C., as the prime meridian: using Greenwich, it is about the 99th meridian. I have long known that Meridian Hill has its name from the period when Americans drew the prime meridian through the White House. But I hadn't before noticed any figures of longitude using the Washington meridian.

The footnote interested me, and I dipped here and there into the two volumes of France and England in North America looking for figures of longitude, without much luck. The map in the Library of America edition of Parkman's The Conspiracy of Pontiac uses the Greenwich meridian. I'm not sure why Parkman would have used the Greenwich prime meridian for his earlier book and Washington for his later.

On the other hand, a map of 1837 reproduced in The Sword of the Republic: The United States Army on the Frontier 17834-1846 does use the Washington meridian. Perhaps Parkman's source for the location of the meeting used the reckoning by Washington, and Parkman copied it without adjusting. Judging from the atlas, the meeting was pretty close to where Fort Dodge, Kansas, now is.