Dynamically changing between high and low resolution imagery/terrain

Hello,

In an application I am writing, I am looking to draw high resolution
imagery draped over high resolution terrain. I would like to zoom in
and out.

While zoomed in the underlying "terrain resolution" needs to be high.
While zoomed out it can be down sampled. Perhaps if the viewer is
oriented such that some terrain is near and other is far (near the
ground looking parallel to it) I might need to do something more
complicated.

I would like to employ a similar low/high resolution trick with imagery.

Question: Can someone recommend a library that has already solved this
problem of "high resolution terrain/imagery is available everywhere
but it does not all need to be loaded at this moment due to the
position of the viewer"?

Thank you,

Chris