[Tagging] Accuracy of survey

Friedrich Volkmann bsd at volki.at
Tue Dec 23 22:59:58 UTC 2014

On 23.12.2014 17:57, Tom Pfeifer wrote:
> I would consider that a non-issue as you said, for those reasons:
> - When it comes to GPS traces on objects that don't move (*), the
>   beauty of crowdsourcing is on our side. The collection of
>   traces over a longer time creates a cloud of traces which
>   form a Gaussian bell curve, in density, over the ground truth.
>   Thus a junction of two road traced again and again is still
>   a good reference point to calibrate aerial imagery.

There are no GPS traces for pipeline markes. There are traces for roads and
paths only. These traces can bear a systematic error due to reflections
(e.g. under a cliff).

Even if you collect plenty of GPS traces with no systematic error, these
still cannot beat a theodolite triangulation.

> - We are getting access to increasingly better geo-referenced
>   aerial imagery, thus mapping can now use different sources
>   and calibrate between them.

In places where GPS is most inaccurate, e.g. in a gorge covered by woods,
aerial images are inaccurate too, and most of the ground details are not

Friedrich K. Volkmann       http://www.volki.at/
Adr.: Davidgasse 76-80/14/10, 1100 Wien, Austria

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