Use blitz's collision detection as follows:
PlayerIsOnGround=false For CollisionID=1 to countcollisions(PlayerEntityHandle,STATICSCENE) ;STATICSCENE = collision type assigned to your level IF EntityCollided(PlayerEntityHandle,STATICSCENE)=CollisionEntity(PlayerEntityHandle,CollisionID) then If CollisionY(PlayerEntityHandle,CollisionID)<EntityY(PlayerEntityHandle) then PlayerIsOnGround=true endif next PlayerYVelocity=PlayerYVelocity+AccelerationDueToGravity ;AccelerationDueToGravity=some negative value IF PlayerIsOnGround=True then PlayerYVelocity=0 TranslateEntity PlayerEntityHandle,0,PlayeryVelocity,0