[Tagging] Accuracy of survey

Kotya Karapetyan kotya.lists at gmail.com
Tue Dec 30 21:01:58 UTC 2014

Hi Warin and all,

I am not sure what you dislike in "accuracy". Accuracy is how far the
measured mean value is from the actual value (
http://en.wikipedia.org/wiki/Accuracy_and_precision). However we can start
calling it a "trueness", to follow the ISO definition. If you mean
something else, please explain, and I believe it would deserve a page in
OSM wiki.

Now, I am talking about the absolute trueness. That is, how far a POI is
according to the map from its actual position on the planet. No, I don't
forget that the planet surface is moving relative to the GPS coordinates.
Even more so, there are local surface movements, especially if the survey
marker is located, say, on a bridge or close to an excavation site. It
should be considered when defining this "non-movable" POIs.

Taking into account the inherent precision of the survey marker position
(they are designed to have a well-defined position), it does make sense to
have OSM data for them defined better than for all usual elements.

At the same time, if you are talking about common use, these POIs are of
little interest to "normal" users. So their specific properties will not
disturb anyone.
However, some mappers may be in possession of the surveying tools allowing
them to have better trueness than possible with a GPS, provided that they
have some good reference points. Survey markers are designed just for that.
For these mappers, the absolute location of the survey markers is
important, and I see no reason to prevent them from having it in OSM.

> OSM renders distort road widths according to their classification .. that
is normal mapping for road navigation. If you wanted air navigation then
the actual road width would be better to render, with runways having more

True. However the underlying data is independent from how a specific
renderer represents each element. A street is usually just a line, thus
having no width.


On Tue, Dec 30, 2014 at 12:50 AM, Warin <61sundowner at gmail.com> wrote:

>  On 30/12/2014 6:41 AM, tagging-request at openstreetmap.org wrote:
> Date: Mon, 29 Dec 2014 15:27:23 +0100
> From: Kotya Karapetyan <kotya.lists at gmail.com> <kotya.lists at gmail.com>
> To: Rainer F├╝genstein <rfu at oudeis.org> <rfu at oudeis.org>, "Tag discussion, strategy and
> 	related tools" <tagging at openstreetmap.org> <tagging at openstreetmap.org>
> Subject: Re: [Tagging] Accuracy of survey
> Message-ID:
> 	<CAK2dJ-wHwQAJZ+0-OXjUE9Bhn-w1eLdcYPM4Am4xidn2Fp5PeA at mail.gmail.com> <CAK2dJ-wHwQAJZ+0-OXjUE9Bhn-w1eLdcYPM4Am4xidn2Fp5PeA at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
> Since such reference points are quite common, I would support the idea of
> creating a special tag for them, requiring that they are not moved. However
> we need a clear consensus on how we define the "sufficient" accuracy and
> how the data for such points will be updated.
>  Ultimate 'accuracy'? You do realise that the tectonic plates are moving?
> So your reference points need to include a date so they can be corrected
> for the drift. You'll find that data is available for those survey
> reference points .. together with their precision. Do you want to update
> these points to maintain their 'accuracy'? How often?
> Survey reference points are 'quite common' in built up areas ... but not
> in remote locations. And depending an the age and how precise the survey
> was will have some effect on their 'accuracy'. One surveor in Australia
> forget to allow for the temperature effect on this measurement chain ....
> back when chains were used.
> I disagree with the point of view that an accuracy sufficient for consumer
> GPS devices is sufficient for OSM and therefore there is no problem here.
> Nobody ever declared that OSM is for smartphone users. We are trying to map
> the world, and accuracy should be of primary interest for this project.
> Again the word 'accuracy'.
> Context 1.
> I have advised one mapper in their diary that most, if not all, users will
> be using their data entry with similar equipment to what they have .. so
> any 'inaccuracy' will also be present for the other users. Thus what they
> map should represent what is there and should be usable as a map ..
> considering that the GPS information may be very vague under the tree cover
> present and the local cliffs etc.
> Context 2
> I will be mapping a track that is covered in a few places  .. by an over
> hanging cliff. As such it is not visible by satellite .. nor will the GPS
> track be that 'accurate'. So I'll be mapping it from the available
> information that I have then - a few photos, my track and the satellite
> image. It will take me about a week to traverse the area. No shops etc.
> I would rather have the less 'accurate' representation of what is there
> compared to a blank area. I've plotted one track that goes from one place
> to another (personal knowledge).. where it is not visible on the satellite
> view I've plotted it as a straight line.. I know it is not a straight line
> but it is the best I can do and conveys the information that the track is
> connected, and being straight in that hilly area also conveys that the
> information is a guide. I know there is a similar tack a bit north of this
> track .. but cannot reliably get the entry and exit points .. so have left
> that off as I view it as unreliable for use.  I have come across similar in
> other areas of the world .. but I found the satellite image had better
> information - so 'improved' the information.
> --------------------------
> OSM primary interest?
> 1) to be USEFULL.
>  meaning to have information desired by the user
> sufficient representation and detail  to be able to navigate to a desired
> place.
> Many usefull maps have distortion - to include more details on particular
> objects or to simply emphasise to those objects.
> OSM renders distort road widths according to their classification .. that
> is normal mapping for road navigation. If you wanted air navigation then
> the actual road width would be better to render, with runways having more
> emphasis.
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