This paper is a critical historical analysis of the 1875 map of Santiago, drawn up by Ernesto Ansart. It is the best known and most mentioned map of Santiago of the 19th Century. Scholarly literature has generally considered it a representation of the transformation planned and implemented between 1872 and 1875, by the intendente of Santiago, Benjamin Vicuna Mackenna. Its accuracy as a cartographic representation and its innovative inclusion of urban projects, have also been highlighted. Nevertheless, until now, nor the context of its commission, nor the way in which it was constructed, nor its final outcome, have been taken into consideration. These three major aspects are outlined in this paper, and dependent on them are their analysis and conclusion. As a result, we hold that it was not the main goal of Ansart's map to represent Santiago's transformations, it rather developed from the information intended to build a far more ambitious map, which was never completed. Ansart however used the information gathered to elaborate a unique map of the city, with a completely different objective.