"The Explored Galaxy" was a map of charted space in the Alpha Quadrant. Canopus, represented here by Canopus III, was located between the Tholian Assembly and Romulus. Both locations were identified in DS9: "The Search, Part II", "Call to Arms" as being located in this quadrant. This chart was also seen in several Star Trek: The Next Generation and Star Trek: Deep Space Nine episodes set in the 24th century, from the year 2364 to 2370. These were: TNG: "Conspiracy", "The Measure Of A Man", "The Emissary", "The Mind's Eye", "The Game" and DS9: "In the Hands of the Prophets", "Cardassians".
In the script of "Where No Man Has Gone Before", this planet was referred to as "a Canopus planet", not "the Canopus planet". Canopus Planet might be Alpha Carinae II, Alpha Carinae III, Alpha Carinae V, or another planet in the system not identified in the canon.