[OSM-talk-ie] Tagging townlands in OSM-Ireland

Dermot McNally dermotm at gmail.com
Wed Sep 10 22:41:52 BST 2008


Hi Paddy,

Paul has covered pretty much all the points, I think, but just in case...

2008/9/10 Patrick Casey <pcasey at compuserve.com>:

> I now understand why one or two places in 'my' area in the
> Tubber-Ruan-Crusheen triangle aren't where they should be on the map (e.g.
> the village of Ruan). May I take the liberty of moving them a few hundred
> yards to their correct sites  or does a committee have to vet changes. I'm
> new to OSM and not yet sure of the protocol and the last thing I want to do
> is contravene some existing convention or practice. I'm very conscious of
> the fact that in a project like this the participants need to conform to
> standards to prevent anarchy. Anything else and the OSM becomes useless.

I'll emphasise two things here - firstly, OSM is in every real sense
an anarchy. Consider the fact that anybody can map and tag anything in
any way they choose. Though it's certainly true that norms of
behaviour are important enough that most mappers will go along with
consensus (like agreement on how to tag townlands, with a view to a
useful treatment on rendered maps).

Secondly, you should absolutely move stuff you know to be misplaced,
in particular place nodes. I should have added to my first name that
the GNS data source is pretty inaccurate, so my import was conducted
with the expectation of cleanup by those who knew better. However, you
should be mindful of the fact that some data sources will disagree,
based on reasons like the following:

GPS inherent tolerances
Yahoo and Landsat imagery never calculated perfectly (Yahoo generally
OK, Landsat terrible)
Mapping slip-ups.

So in cases where there's a chance that the other mapper knows
something you don't, it can be worth talking before you edit. But
clearly your case is a clear-cut one.

> If I interpret your tenor correctly there would be no objection if I were to
> "re-baptise" e.g. Coolbane as a locality. I would restrict such changes to
> places that I know well and have walked over and can vouch for the fact that
> they consist of a few scattered houses or farms or cabhals and not tiny
> groups of habitations that one might call a hamlet. Would that be OK ?

I absolutely encourage you in this. As I said, I took this approach in
the NI import based on the experience of the RoI one.

> Some of the lakes in 'my' area seem to have been very roughly drawn (or my
> GPS was faulty). For example, the footpath to Dromore Castle from the car
> park (which I walked with the GPS) appears to cross part of the lake. There
> is a New Testament example of walking on water but I haven't yet managed to
> emulate it. Is it possible that those lakes were traced from a relatively
> low-resolution satellite image ? I'm talking about small errors here, e.g. a
> couple of hundred yards.

This is expected, and while it's worth correcting the lakes, it may be
worth thinking about the best method of correction:

Most Irish lakes have been auto-traced from aerial imagery, mostly
landsat satellite imagery in fact. Landsat calibration with real life
is poor enough to place lakeside roads under water, but useful enough
to trace first and move as opportunity presents. I've always had the
sense that it can be off by as much as 100m. What this means is that,
where your local lake has a rough outline and is poorly aligned, its
shape may very possibly be fairly good. This being so, it may be
better to move the entire lake based on local measurements rather than
(or rather, before) just editing your nearby bank (since that will
require similar surgery all around the lake's edge). Moving the whole
lake can be easily accomplished using JOSM (and probably fairly easily
in Potlatch too). In JOSM, where there are accurately surveyed roads,
it's sometimes even possible to adjust such traced lakes by first
calibrating the landsat image in your editor - as long as you can see
the roads, you can usually line it up. Then you can move the lake
without even resorting to your own measurements.

Obviously, if you are willing or able to walk the perimeter of a lake,
that works too.

> I couldn't understand your statement ".....it's a bad idea to tag for the
> renderer....".  Could you clarify that, please ?

It's a kind of motto at OSM. An often-quoted example is that of
Americans, who may be used to paper maps that display freeways in
orange. Nonetheless, they shouldn't tag them as secondary roads, which
just happen on the UK-written OSM renderers to be shown in orange.
They should instead tag them as motorways and perhaps produce a
localised renderer if the colour scheme annoys them.

And now, the begging :)

I know that the area you named is entirely on the wrong end of Clare.
That said, one of the very few OSM gaps in the national road network
is on the Kilrush end of the N68. I don't suppose you ever drive that
way with a GPS logger switched on? Traces or actual mapping would be
extremely useful. Even those parts of the road that are mapped are, in
places, traced from calibrated Landsat, so a resurvey is overdue.

Happy mapping!
Dermot

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