[OSM-newbies] R: Road Maximum speed in UK units

Dermot McNally dermotm at gmail.com
Sat May 2 14:08:57 BST 2009

2009/5/2 Fabrizio Carrai <fabrizio.carrai at gmail.com>:
> In  my opinion, the speed limit should be recorded in the local speed unit.
> During the driving I would be able to check my speed vs the limit reported
> on the traffic signals, whatever their units are.
> No conversions have to be done.

This is an old topic and a divisive one, but regardless of my personal
opinion of how this should be tagged, you're taking too narrow a view
of the purpose of the tag. If I interpret your point correctly, you're
assuming that the purpose of the tag is to somehow make you aware of
the limit in force to allow you to compare it manually against your
actual speed. To achieve this, clearly a simple number for human
interpretation will suffice.

However, the real gain from tagging speed limits is that a GPS device
(which knows your current speed) can know whether you are currently
within the limit or not - and to do this requires clarity of units for
the tagged speed. That is, an application using the tag data has to be
able to make one of the following assumptions:

1. All values are in a predetermined unit (earliest usage and wiki
documentation assume this and specified km/h. Disadvantage is that mph
values need to be approximated in km/h, representing a loss of
accuracy and a separation from what is "on the ground")

2. All values are in the unit legally employed in the country in which
they are recorded (so applications need accurate border information,
something we don't yet have. In addition, any changes in legal units
necessitate immediate retagging. Limits that are dual signed [height
limits, say] create a tagging dilemma).

3. All values stored as number only are assumed to be in a default
unit (km/h for maxspeed) with the option to add a unit suffix where
you wish to represent a different unit. Snags: all processing requires
pre-parsing of values, pure numeric analysis no longer works without a
normalisation stage. Mappers will vary widely in style of suffix
recording (space or no space, 'mph', 'm.p.h.', 'MPH', 'mp/h'....)

None of the approaches are perfect, but any usable approach must allow
a machine to "understand" the limit, not just present it to a human.


Iren sind menschlich

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