Periplum is defined in the Oxford English Dictionary

Oxford English Dictionary

The Oxford English Dictionary , published by the Oxford University Press, is the self-styled premier dictionary of the English language. Two fully bound print editions of the OED have been published under its current name, in 1928 and 1989. The first edition was published in twelve volumes , and...

 (OED) as having come from the poetry of Ezra Pound

Ezra Pound

Ezra Weston Loomis Pound was an American expatriate poet and critic and a major figure in the early modernist movement in poetry...

, specifically in The Pisan Cantos, Cantos LXXIV to LXXXIV of a larger work known collectively as The Cantos

The Cantos

The Cantos by Ezra Pound is a long, incomplete poem in 120 sections, each of which is a canto. Most of it was written between 1915 and 1962, although much of the early work was abandoned and the early cantos, as finally published, date from 1922 onwards. It is a book-length work, widely considered...

.

A periplum is a map

Map

A map is a visual representation of an area—a symbolic depiction highlighting relationships between elements of that space such as objects, regions, and themes....

 or drawing that that shows how land looks from a point at sea. That is to say that a cartographer often draws maps from a bird-eye view and not from the perspective as the land would actually appear from the crow's nest

Crow's nest

A crow's nest is a structure in the upper part of the mainmast of a ship or structure, that is used as a lookout point.This position ensured the best view of the approaching hazards, other ships or land. It was the best device for this purpose until the invention of radar.In early ships it was...

or deck of a ship. Therefore a periplum would, theoretically, be drawn as if the cartographer were out to sea so that sailors could know which land or port they were approaching (Companion to The Cantos, Terrell; Chinese Characters as Traveling Metaphors in Canto LXXVII of Ezra Pound’s Pisan Cantos, and James Legge’s Book of Poetry 华英诗经 [Huá Yīng Shī Jīng], Tchou).

Pound uses the periplum as a figure to describe the form of the Cantos: not history from a historian's or philosopher's elevated point of view, but rather from the poet's point of view where the poet is a voyager navigating history personally.

As appears in the Pisan Cantos,

Periplum, not as land looks on a map

But as sea bord seen by men sailing.

(E. Pound Cantos LII-LXXI lix. 83)