Add addr:milestone support to Nominatim

[matheusgomesms]

I would like to propose support of addr:milestone from Nominatim: https://wiki.openstreetmap.org/wiki/Key:addr:milestone

It is a way to tag places that are usually in rural areas, by some highway, and have no official addr:housenumber, but use it as a proper address. It is usually stated in a sign/website (and even used by postal letters). Examples:

Highway A-1, km 12 (fictitious)
Rodovia BR-116, km 16.5 (a place that advertises in their website this address, and uses it for postal purposes).

The proposed tagging can be seen in the last OSM link, and Nominatim probably should handle it the same way as it does with addr:housenumber, but also appending 'km' before the addr:milestone value, if it is not in miles.

Rationale for this request: this happens in diverse parts of the world, but very likely people don't use it in OSM for many reasons (not officially approved, lack of support by search engines and apps etc), classic egg/chicken problem that happens in OSM all the time.

In Brazil, we are working towards a full national import of addresses, which contains millions of addresses, including several ones like this. So we are probably picking this tag to be used for those cases, and would like to see Nominatim supporting it.

Finally, we saw that there are at least 4500 cases, worldwide, that have 'km' (or similar) in addr:housenumber, and could benefit from this tagging.

addr:milestone in Taginfo: https://taginfo.openstreetmap.org/keys/addr:milestone#overview

[matheusgomesms]

PS: also probably related to #69

[lonvia]

The simple implementation would be to just interpret the value as an an alternative to addr:housenumber but that doesn't give you the optional unit. I've been looking through the existing usages and it would be good if you clarify the use of this tag before you start importing this tag in a larger number.

First of all there is the question of the unit. Many uses seem to have no unit at all but the wiki page does not define a default. Is it universal that the unit comes before the number? There are at least three objects where the unit is used as a suffix. And then, there are a lot of tags, which use Km. as unit. Does that mean that the expected format is different in other countries? From a data user point of view: must the unit always be there when searching for the address or is it frequently left out?

Then there are quite a few objects with have a addr:milestone and a addr:housenumber, e.g. https://www.openstreetmap.org/node/6817599738. How does that work?

It looks like we need understanding of fractional numbers here. The tagging of this also seems rather diverse. Some use a comma, might just be a tagging error. Some have additional zeros at the end. How do you search for that? Can it be interpreted as a string (meaning the zeros always have to be there) or must it be interpreted as a number (search may or may not have additional zeros).

There are also combinations of addr:place and addr:milestone, e.g. https://www.openstreetmap.org/node/11066782731 That looks suspect.

[GEOtemala]

The proposed addr:milestone=* tag should always substitute entirely the use of addr:housenumber=*.

The use of the prefix "Km." is something that is widely used in general, as this denotes the address is making reference to a milestone. It very common to refer in that form to an address which uses those markers as part of an actual address. This is mostly used as a reference only and does not necessarily read an exact distance (from a milestone) on any highway.

The use of addr:place in combination with addr:milestone has not been discussed before, but I agree on this needing to be further agreed upon in order to establish a common procedure on OSM. I have mixed feelings on that (addr:place) specific tag.

Some places around the world frequently use an additional meter reading to define a location more accurately, and is usually written as Km. 35 + 250 in those cases where the + 250 represent the location (approximately 250 meters after the actual Km. 35 milestone) - something that could be adopted for this proposal as well.

Another common variable (mentioned already above) is the use of a 'period' or a 'comma' to define the decimals & separating the thousands. The US, Australia & Great Britain (as well as many other countries) commonly use the 'period' as a decimal separator, while the EU and yet other countries around the globe use the comma. In order to follow a certain criteria, and since OSM is originally a British creation, I'd suggest using the 'period' as the decimal separator, while the 'comma' would be used to separate the thousands. This would also need to be included on the proposal to avoid even more confusion.