I was initially surprised they don’t just do this on device. A height map of the city the user is in (say San Francisco) at only one float per square meter though is still fairly large. With a land mass of 121 sq km for San Francisco, that’s 121 million floats or about 500 MB.
The actual area of interest surrounding the user is probably at most 1 km^2, but that’s still 1M points or about 4MB (which can be compressed, and easily quantized with say 16-bit float instead) which I really wouldn’t want some app downloading on demand. It’d be interesting to know how many map tiles they need for a good solution though. 256x256 meters with 16-bit floats is just 128 KB. That seems within the realm of acceptable, especially if it reduces the error in pickup.
The range of heights is quite constrained to architecture. So 16 bit integers in meters would save half the data over floats. Then your bitmap of heights could have compression applied. A whole flat roof or road would be very compressible. Trees would be the worst.
The actual area of interest surrounding the user is probably at most 1 km^2, but that’s still 1M points or about 4MB (which can be compressed, and easily quantized with say 16-bit float instead) which I really wouldn’t want some app downloading on demand. It’d be interesting to know how many map tiles they need for a good solution though. 256x256 meters with 16-bit floats is just 128 KB. That seems within the realm of acceptable, especially if it reduces the error in pickup.