The bounds for EPSG 3857 are:
xmin: -PI * 6378137,
ymin: -PI * 6378137,
xmax: PI * 6378137,
ymax: PI * 6378137
which result in the following bounds:
- The extent is a square. And, the unit is in SI meter.
- OpenLayer sets this bound for you by default when you work with a data source that is in EPSG 3857.
- If you regularly perform (sizeable) map caching or map tile generation - knowing where the bounds are lets you have fine control over the tools you use, and thus planning, monitoring, concurrent/parallel generations, and etc.
- If you write codes that convert coordinates to map tile indexes and vice-versa - you need to know where the bounds are.
- However, do note that the valid geodetic bounds (i.e., latitude and longitude) that can be projected into EPSG 3857 are [xmin, ymin, xmax, ymax] = [-180, -85.06, 180, 85.06].