[talk-au] Misaligned streets OSM or Bing wrong? - use survey mark?

[Ben Johnson]

On 01/09/2012, at 16:58, Stephen Hope <slhope at gmail.com> wrote:

> On 1 September 2012 11:35, Russell Edwards <russell2pi at gmail.com> wrote:
> 
> I am still curious to know what the positional accuracy of survey markers is meant to be, if anyone can enlighten.
> 
> 
> 
> First question you have to ask is how old the survey is?  Australia as a whole moves north about 7-8 cm a year (from memory), so a 12-13 year old survey has moved about a metre from the start spot.  It's a moving target.  Survey markers stay pretty accurate in relation to everything around them, but not necessarily in relation to some outside measuring source like GPS.

Well THAT brings up some very interesting long-term macro considerations for our project. We are going to need to move EVERYTHING from time to time!

But it won't be quite that simple, because it depends on when each individual node was last positioned. The older the node, the more adjustment it will need. And different land masses move at different rates, so i guess some kind of "continental drift" bot would be applied to local regions as needed.

Japan is one place that comes to mind where shifts are more noticeable.

http://articles.nydailynews.com/2011-03-13/news/29142893_1_geographical-survey-institute-tectonic-plate-axis

I think we need to ask if providers of aerial imagery account for such shifts? I suspect they simply align their images to (outdated?) on-ground survey points. 

I see OSM as forever, and it will become more accurate as technology allows. Continental drift will become an accuracy issue sooner than we think!


> But an even bigger error can be caused by using different projections. I forget which one OSM uses, but using different projections can move a given point 20m quite easily, and a survey marker may well be on a different one.

Can we get a definitive answer on this?

(From memory) I have to use the Merkator projection in JOSM for my circles to appear circular in OSM. It was a little unsettling because it wasn't the JOSM default back when I first installed it anyway.

BJ
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