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Inside or Outside Diameter?

I think most specifications would be of the inside diameter - liquid pipes, guns all use the inside diameter. If the inside diameter is unknown then use an estimate of the outside diameter, leaving any correction to future mappers with more knowledge. Warin61 (talk) 23:54, 12 October 2017 (UTC)

no, specifications are usually about the nominal diameter, which is similar but not identical with the actual inner diameter. —Dieterdreist (talk) 00:47, 13 October 2017 (UTC)
Agree: if this is for a tower or mast, it would be the outer diameter where it represents the support and its capacity to stand the weight of what is put placed on it. If this is for trees, the diameter is specified to be the trunk at roughyly 80 cm of height (smaller trees that are not at least 2 meters high, have no diameter specified, it is too small to be significant and then too variable for what could be in fact a set of branches that are difficult to differentiate or could change at each season.
Alone this tag does not specify where to measure it, it will be specified by another tag where this diameter will be added to add more precision.
Generally this is that other tag that will document how and where to measure the diameter of the feature. The diameter itself is not a mappable feature, it has no meaning alone, and its interpretation will change if it is placed on a node or on a linear way (which may itself be closed in some cases) or on a feature represented by a surface on the map (closed way or multipolygon relation). If a relation is used, it may still represent a linear feature or a surface or volume. The diameter may be used around a node on the map but the vertical extrusion may need a varying diameter (not purely vertical but a cone, or a dome, or some other non-linear variation of the horizontal diameter ); it could be a cable diameter but cables may be more complex with multiple wires in a larger container/support cable. In pipelines, or other underground or aerial conducts, the section of the tube may not necessarily be circular, and the nominal diameter could be measured only on the bottom part, in a tunnel it could be measured on the superior "aerian" part (the lower part being filled to support the basement of a highway or railway), and additional inner structures may limit the usable space inside (including protection materials, or safety space, and it could be the minimum diameter ignoring some areas where there's more space, such as within transport stations or even within tunnels for waterways or waste water.
So in practice we may need additional prefixes/suffixes to specify the measurement, depending on the base feature.
What should be done in this pager is to list the other tagged features using the diameter.
For now it has only been documented here for fire hydrants, but there are many other uses (see the added reference to Wikidata which has a much larger scope than what is on this page for now, and the wikidata is not very relevant here, more restricted in English where it links to "pipe sizes" and in more generic "nominal diameter" in all other languages, but where ther key here is ambiguous as it depends on other features: the wikidata is only approrpaite for only a part of its use and applies only to pipelines, not to the connector of fire hydrants !). — Verdy_p (talk) 02:04, 13 October 2017 (UTC)


For tree trunks, they are usually measured at “breast height”. There’s also the circumference tag for trees. —Dieterdreist (talk) 07:41, 13 October 2017 (UTC)