[OSM-dev] Difference between osm and osmChange formats

[Sergey Galuzo]

Thanks for excellent info.

As far as special format for diffs... Why do we have it as opposed just to using full history format for diffs? Same data...

Thanks,
Sergey.

-----Original Message-----
From: Frederik Ramm [mailto:frederik at remote.org] 
Sent: Wednesday, June 01, 2011 6:34 AM
To: dev at openstreetmap.org
Subject: Re: [OSM-dev] Difference between osm and osmChange formats

Hi,

On 05/31/11 23:43, Sergey Galuzo wrote:
> Obvious differences between full and change are explicit 
> create/modify/delete tags.

We have full history OSM files and normal OSM files (both use .osm extension and <osm>...</osm>), and we have simple diffs and replication diffs (both use .osc and <osmChange>...</osmChange>).

Normal OSM files never have a "visible" attribute, and they contain at most one version of every object.

Objects in full history OSM files contain a "visible" attribute, and these files may contain different versions of the same object, with at most one having visible=true. If there is a version with visible=true, it will be the highest numbered version of that object. They do not necessarily contain a full sequence of versions, i.e. you could have a full history file that only has versions 2 and 5 of something.

(There are also .osm files as written by the JOSM editor; these do not have a "visible" attribute but may have an "action" attribute which, if set to "delete", has a similar effect. This is a niche case.)

The diffs contain <delete>, <modify>, and <create> blocks. A version in <create> is usually 1. (Current implementation means it is *always* 1 but this could be changed; it is not specified in the API that the first version of something must always be 1.) A version in <create> or <modify> can never have visible=false.

If you have full replication diffs then one diff can contain the same object multiple times, with different version numbers. It is possible for the same object to appear in the <create>, <modify>, and <delete> section of the same replication diff. Versions are not guaranteed to be sorted, i.e. you could first encounter version 6 in a <delete> block and later encounter version 5 in a <modify>.

Since multiple edits are conflated in a simple diff, it is possible that a simple diff contains an object as version 2 in a <modify> block but the consumer has never before seen that object (because the version 1 that would have been in a <create> has happened in the same diff interval as the modification to version 2).

In short, your assumption

var changeTag = !Visible ? "delete" : Version == 1 ? "create" : "modify";

is mostly true but it depends very much on the context in which you want to use it; there's a lot of things to trip you over.

Bye
Frederik

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