They can't reach 20000 ft or 6000 metres.
Some can fly at 6000 metres, but other models have service ceilings of 4000 metres.
And if you are thinking of flying from low levels to those altitudes, that's just asking for trouble.
Photogrammetry or mapping programs stitch images shot from different camera positions.
Good stitching programs have multiple projection formulas that you can use.
To stitch as the OP requires you would use a program which offers a planar...