Maps played an extensive role in colonialism, as Bassett would put it "by providing geographical information in a convenient and standardised format, cartographers helped open West Africa to European conquest, commerce, and colonisation". However, because the relationship between colonialism and geography was not scientifically objective, cartography was often manipulated during the colonial era. Social norms and values had an effect on the constructing of maps. During colonialism map-makers used rhetoric in their formation of boundaries and in their art. The rhetoric favoured the view of the conquering Europeans; this is evident in the fact that any map created by a non-European was instantly regarded as inaccurate. Furthermore, European cartographers were required to follow a set of rules which led to ethnocentrism; portraying one's own ethnicity in the centre of the map. As J.B. Harley put it, "The steps in making a map – selection, omission, simplification, classification, the creation of hierarchies, and 'symbolisation' – are all inherently rhetorical."