The 1867 Luso-Brazilian’s Covenant has allowed an exchange of maps and memories that has resulted in the formation of two major cartographic collections, essential to both countries and to several territorial policies undertaken by the respective States. The maps choice, the process of letters reproduction, the later use of the cartographic collections, implies in the understanding of the collections assembly from their purposes for which they were intended from the beginning. The best-known result of the Covenant has been the formation of two map libraries too much important for both countries. For Portugal, the maps that came with D. John VI to Rio de Janeiro in 1808 and returned after the exchange, have served to initiate the collection of current Portugal’s Geographic Institute. For Brazil, letters and memoirs have served to shape a collection that would be the basis on which would be structured the Itamaraty Map Library. A careful analysis of how Duarte da Ponte Ribeiro has chosen the maps in more than one Portuguese file, and the way of doing it favouring the problems related to the Brazil Empire’s frontiers with the neighbouring countries, gives the clue to the understanding of which were the purposes of this choice that were put through from the first approaches made by the Brazilian diplomat, until the beginning of the negotiations for the covenant establishment, which has been developed from the year of 1863 until the year of 1867. The exchange, however, does not concern only the maps that the Brazilian diplomacy have wanted to bring from Portugal, but concerns in an effective way of that was offered to the Portuguese government and it could be useful in the production of a more accurate chart of the kingdom like that one performed by Filipe Folque - the man at the other border of the covenant – to the lusitanian territory administration policies and to the frontier disputes with Spain. The weaving process of the covenant, their purposes not always cleared by both countries, the different characters involved in it, the technical cartographic processes developed and the results obtained for some specific issues such as the maps production based on the letters collection drifted by the covenant are, in our sight, examples that can clarify the relationship between: the cartographic knowledge contained in those collections, diplomacy and territorial policies put through on both sides of the Atlantic.