In order to map a city spiritually we need a signage system which will indicate spiritual events and encounters, their location, nature and duration. SInce it seems unlikely that government bodies will provide such a system, perhaps the best thing is to supply the signage on stickers. These can be placed on buildings, lampposts, taxi seats, escalators and other sites of spiritual encounter. They can also be placed on magazines, CDs, DVDs in record stores and newsagents to indicate the spiritual potential of the contents [rather like parental advisory stickers for lyrics]. Perhaps other consumer goods too. Bread and wine obviously. But also other foods, clothing items, gadgets and furniture.
This will enable us to read the city, spiritually; to follow the scent trails of God. But it would help to have maps. To make these each sticker could have a barcode. Teams of 'angels' would comb the city with barcode readers to add new sites to the official maps via a database. It would be possible to monitor for spiritual hotspots and deserts, parts of the city filled with presence and those with no spiritual value whatsoever.
Clearly there are possibilities for self-directed pilgrimage, armed with spiritual maps. But rather than the single route of traditional pilgrimage, the city becomes a navigable 'pilgrimage field' [cf. 'story fields'] with many potential routes [just as ghost walks and rock tours coincide among the same streets and buildings].
Here are some possible signs. You can no doubt think of others.
I've chosen the colour yellow to represent the divine because connotations of light, gold etc are fairly universal in the world's religions. It also makes the signs harder to read [contrary to normal signage practice] - as is spiritual presence.