On 01/09/2012, at 16:58, Stephen Hope <slh...@gmail.com> wrote:
> On 1 September 2012 11:35, Russell Edwards <russell...@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I suspect they simply align their images to (outdated?)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
_______________________________________________
Talk-au mailing list
Talk-au@openstreetmap.org
http://lists.openstreetmap.org/listinfo/talk-au
