Attention cartographers and experts in geomatics and geodetics

[erick]

Help! Please.

I’m looking at the coordinates for a city in the WGS84 coordinate system. For example, using http://epsg.io/4326 (http://epsg.io/4326) and choosing "Coordinates on a Map" and then inserting “Sydney New South Wales, Australia” for "Place or Address". It locates Sydney and points to Longitude 151.2067 and Latitude -33.8675 (rounded to four decimals). This puts me on George Street, close to Martin Place.

So where do I find the definition of this coordinate location with respect to the city of Sydney (in this example)? This looks to be close to the GPO? Melbourne seems to be the GPO. Brisbane – I’m not sure. Adelaide is Victoria Square. Perth – I’m not sure.

I’ve searched high and low and I cannot find any definition for the coordinates for a city – particularly those associated with WGS84 and generally for all cities in the world.

However, I understand there might be various ways of determining what constitutes the “coordinates” or centre of a city. Perhaps it is:
· geographic (centre of gravity of the spatial footprint)
· population (centre of gravity of the city’s population)
· geometric (centroid) – whatever that means?
· a specific location – Town Hall, Post Office?
· the astronomical observatory which is the origin of coordinates from which the country was triangulated
· perhaps even the zero kilometer point – the point from which all distances from the city are measured

Obviously the choice of definition would depend highly of the context of the requirement.

I just need to find a definition that this website (selecting WGC84 coordinate system) is using.

Can anyone help?

Thanks
Eric

[koputai]

I always understood the centre of Sydney to be the GPO.

However, measuring distance by road (zero kilometre point you mentioned) to/from Sydney is measured from an obelisk in Macquarie Place. It's an interesting place to visit, with a number of very historical items in the park.

http://en.wikipedia.org/wiki/Macquarie_Place

Cheers,
Jason.

[Wavytone]

Eric that location is where the GPO used to be - and where miles from Sydney were once measured.

The post was originally in the footpath in George outside the GPO. It was moved to Macquarie place some time in the 1980s when the GPO closed.

[koputai]

The obelisk has been in Macquarie Place since it was erected in 1818, and Sydney roads have ALWAYS been measured from there, not the GPO.

[Wavytone]

Got a reference to that Jason ? There certainly was a milepost in George at at Martin place, and I may be able to find a photo of it.

[Paul Haese]

To answer your question Eric. The geodetic co-ordinates seem have been selected arbitrarily for the example you have given. It is the same for Adelaide, Clayton bay or anywhere in Australia. For Tehran you would need to first find out what is considered to be the local centre of any city or township and then get the co-ordinates from there. If you move the box with your cursor it will update the co-ordinates for you.

The current system is pretty accurate and down to 200mm I think. Survey marks will be accurate to 1mm or so generally as those are used for horizontal and vertical control in a given datum. However, for GPS 200mm is within the standard deviation as I understand it. Its been about 27 years since I thought about this stuff (ex cartographic technician and surveyor in the Australian Army), so I might be a bit rusty.

[koputai]

Yep.

"The Obelisk, erected in Macquarie Place in 1818, is the geographic centre point of nineteenth century Sydney, the Colony, and the network of nineteenth century roads throughout NSW. It was erected in this location by Governor Macquarie to mark the place from where all public roads were to be measured and has continued to perform this function over most of the history of European settlement in Australia, for over 190 years."

Source:
http://www.baseline.nsw.gov.au/heritage/heritageProperty/14

Cheers,
Jason.

[mithrandir]

If you look at the HDOP and VDOP figures from your GPS the number is measured in units of the nominal accuracy of the device. Most of mine are only quoted to give a figure within 2 or 5m, but since I'm not surveying that's good enough.

[Paul Haese]

Not talking about mobile GPS units Andrew. Survey instruments I am referring to here. Those are much more accurate.