Nguyen Hue Street was a canal (and was the location of Ben Thanh market until 1911); the walls of Saigon's citadel, burned to the ground in 1859, were still intact; the area around present day Ben Thanh market was nothing more than swampland and the cathedral dominated the skyline (the Opera House wasn't built until 1897).
Considering Favre drew this without the benefit of a birds-eye view is pretty impressive.
You can view a larger version of the map here.
For a ground level comparison, this panorama shows the city around the same time (though we're not sure of the exact year):
