When Britain gained formal control over what is now Guyana in 1814, it also became involved in one of Latin America's most persistent border disputes. At the London Convention of 1814, the Dutch surrendered the United Colony of Demerara and Essequibo and Berbice to the British, a colony which had the Essequibo river as its west border with the Spanish colony of Venezuela. Although Spain still claimed the region, the Spanish did not contest the treaty because they were preoccupied with their own colonies' struggles for independence. In 1835 the British government asked German explorer Robert Hermann Schomburgk to map British Guiana and mark its boundaries. As ordered by the British authorities, Schomburgk began British Guiana's western boundary with Venezuela at the mouth of the Orinoco River, although all the Venezuelan maps showed the Essequibo river as the east border of the country. A map of the British colony was published in 1840. Venezuela protested, claiming the entire area west of the Essequibo River. Negotiations between Britain and Venezuela over the boundary began, but the two nations could reach no compromise. In 1850 both agreed not to occupy the disputed zone.