Mapa is not the only exception, actually! Words like clima, dogma, sistema, teorema all are masculine and end in -a. (plus some words that refer to people, like guardia or atleta, that can be masculine or feminine).
And there's also words like águila or hacha, which are feminine for all purposes except which definite article you use with them, because the two a's right next two each other in 'la águila' sound bad. Leading to constructions like "el águila calva" where the article is masculine and the adjective is feminine.