The story of Houston's growth is the story of rail meeting water, and here we see that story developing once again after the long war and reconstruction. 
The 1870 census found a population of 9300 souls. When this map was drawn three years later it was pushing 12,000, and would reach 16,513 by the time enumerators made the rounds again in 1880.Houston was growing! 
The image you are looking at here is a bird's eye view, also known as a perspective map. It was a cartographic technique developed in Europe during the Renaissance, and was the rage of late nineteenth century America. 
The artist first used plat maps and surveys to get the lay of the land. Then came the arduous task of walking every street, making sketches of the the buildings and landscape from various angles.Then he adjusted the perspective upward and compiled it all into a cohesive and accurate map of the town. People of the nineteenth century were enthralled. Can you blame them? 

You may be wondering why the view is looking South. That gives you insight into how people back then conceived of the city. 
Buffalo Bayou was the life of Houston. It was how people and goods arrived from the outside world. It was how Texas cotton made it's way to the great textile mills of the North and Europe. 
If you arrived on a steam packet, you would disembark at the foot of Main Street, you would climb the south bank and find the City of Houston spread out before you.