Put a filled map in an item frame and it stops being a picture of the land and becomes the land — raised into a solid model built out of blocks, wearing the map's own colours.
That is the whole mod. There is no new item, no new recipe, nothing to learn. You craft a map, fill it in and hang it up exactly as you always have. It just stands up now.

Optional, and detected automatically - if you do not have it, nothing about the mod changes.
Elevation normally comes from loaded chunks, which is the right rule while a map is filling in but leaves an already explored map flat forever: by the time it is on your wall, the ground it shows is long unloaded and there is nothing left to measure.
With Distant Horizons installed, the mod falls back to its record of terrain you have already seen, so old maps stand up on their own. It is only ever used for ground nothing else can read, and the moment you walk that terrain again the exact measurement replaces it.
One caveat worth knowing: Distant Horizons only knows what it has itself seen or generated. If you explored those maps before installing it, its record is empty too, and the terrain appears once it has generated that area.
Maps already hanging on a wall are picked up too. Stand near them and they fill in by themselves; you do not have to take anything down.
Needed on both sides: the server records the elevation, the client draws it.
The model is worked out once per map and rebuilt only when the terrain actually changes, so a finished map on a wall costs nothing to keep looking at - drawing it is pure copying, with no decisions taken per frame.
Neighbouring ground standing at the same height is merged into single large surfaces rather than drawn column by column, which on flat country such as ocean or plains is the difference between thousands of pieces and a handful. The hidden half of every surface is discarded as well, so what reaches the graphics card is roughly half of what is there.
On the world side, elevation is only recorded for maps someone is holding or standing near, only from chunks that are already loaded, and a finished map drops out of the sweep entirely. The mod never forces terrain to generate.
Feedback and bug reports are genuinely useful at this stage - it is an early release and the rough edges are known ones.