That kind of mapping already exists. One of the links I posted earlier ( Boomtown, Flood Town ) shows the 100 Year and 500 Year Flood Plain on a regional scale. Also note the dots showing buildings that have historically flooded that are outside of those flood plains. With GIS it would be easy to incorporate that into the flood plain maps. The USGS supplies free DEM data ("Digital Elevation Model") at a 10 meter map grid for the entire lower 48 states. While this was originally derived from 7.5 minute topo maps, it is continually being updated as better data ( LIDAR for example) becomes available. I know that NOAA has also been doing high precision DEM mapping for Texas coastal areas.

So tell me Blacktop, do you think a staged evacuation would have been worse than what is happening now? Would it be worse to be stuck in a massive traffic jam on higher ground North of Houston, rather than being stuck on a rooftop in the rain? Right now we are seeing a massive extremely chaotic evacuation using boats, helicopters, air mattresses, kayaks, etc.

Yes, there will be an element of chaos in any large scale evacuation. However, experience from wildfire areas shows that it is possible to do. Realistic advanced planning helps a lot. Remember the "7P" principle. Much better to get at least some of the most threatened people out ahead of the flooding, than to mount a massive ad hoc rescue effort after the water rises and the roads are blocked.