There's a typo/error in Good Omens that gets used a few times. It's the use of the word 'ordinance' instead of 'ordnance' in reference to maps.
They mean different things:
or•di•nance (ôr dn- ns)
n.
1. An authoritative command or order.
2.
A custom or practice established by long usage.
3.
A Christian rite, especially the Eucharist.
4.
A statute or regulation, especially one enacted by a city government.
ord•nance (ôrd n ns)
n.
1. Military materiel, such as weapons, ammunition, combat vehicles, and equipment.
2. The branch of an armed force that procures, maintains, and issues weapons, ammunition, and combat vehicles.
3. Cannon; artillery.
In the case of the map, it should be 'ordnance' as the Ordnance Survey was orginally set up for military purposes.