I seem to remember that some of the charts Fred Noonan used, with his notations on them, got sent back to the US. Somehow, I've always assumed these were part of a series and that if one took them and walked into a library or antique book dealers shop and said "Would you please get me the whole set", they'd just go and get it. If for some reason they couldn't find the South Pacific section, at least the chart Fred Noonan used would give scale, which in turn would help determine the approximate size of Nikumaroro, which would then tell if Nikumaroro would just be depicted as a blob with no distinct contours or in greater detail. Obviously, that's rather naive and it doesn't work that way. Did aviators in the 1930s use "custom-made" charts? Did early airlines have their "own" (private) charts (if so, are there maybe any of these perserved in the Pan Am archives, as Fred worked for Pan Am?)
The chart Noonan used for the South Atlantic crossing, Natal to Dakar, was not an aeronautical chart. It was a U.S. Navy Hydrographic Office chart called "North Atlantic Ocean, Southeastern Sheet."
The charts Fred (and Harry Manning) used for the Oakland-Honolulu flight in March 1937 were created by Pan Am specifically for aeronautical use. He used "Pan American Airways System, Pacific Division, California - Hawaii. (East - Half)" and (West-Half). A notation on the "East-Half"chart says "Compiled from United States and foreign sources. U.S.C & G.S. Chart 5000." (U.S.C & G.S is U.S. Coast & Geodetic Survey)
The chart the 14th Naval District used to manage the Earhart search has the same scale and general appearance as the "Pan American Airways System, Pacific Division, California - Hawaii" charts. It covers from Honolulu (where the "Pan American Airways System, Pacific Division, California - Hawaii (West-Half)" chart stops, to Guam and has the same scale and style as the Pan Am charts. I'd bet dollars to donuts that it's the next map in the Pan Am series.
Eventually Pan Am created aeronautical charts for the Hawaii-NewZealnd/Australia route which would cover the Phoenix Islands but I don't think they were available yet in July 1937 or else the Navy would not have had to draw in the lat/long for McKean and Gardner Islands. The first Pan Am survey flight to New Zealand left San Francisco the same day Earhart left Oakland for Honolulu , March 17, 1937.
So if Pan Am Aeronautical charts were not yet available for the South Central Pacific, what charts were?