GDA Conversion

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GDA Conversion

Postby mrpotter » Tue 03 Jun, 2014 10:51 pm

I've noticed bushwalkingnsw.com uses map references in a format like GDA462568 - which appears to be a co-ordinate system by Geoscience Australia, I'm assuming its used on the LPI maps.

How can I convert these to lat/long?
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Re: GDA Conversion

Postby GPSGuided » Tue 03 Jun, 2014 11:06 pm

Just move it!
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Re: GDA Conversion

Postby mrpotter » Tue 03 Jun, 2014 11:43 pm

Ahh, thanks for that. I've also found this app which appears to do much the same thing

http://www.gracode.com/mapgrids.php

And also this page that talks about zones

http://www.lpi.nsw.gov.au/surveying/geodesy/projections

However I'm still struggling to convert this to a lat/long from GDA462568 - this should be placed on or near Narrow Neck, Katoomba. I assumed 462 referred to the easting and 568 referred to the northing, and added three 0's to make 462000 & 568000. However the result I get with zone 56 is 84°55'08.5"S - 49°09'14.5"E which located on the Antarctic continent. Just a few km's off but seems to be the closest I can figure out
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Re: GDA Conversion

Postby Allchin09 » Wed 04 Jun, 2014 1:06 am

Hey mrpotter,

Have a go at playing around with the Coordinate Tool (thing with the xy in the top centre toolbar) on this mapping website - http://maps.six.nsw.gov.au/. You will need to select "GDA94 - MGA 56" to input grid reference, and select "Geographic" to get a lat/long coordinate. You can also click on the map to get the coordinates at that point.

For GDA462568, the correct eastings and northings are found as follows:

The first three numbers are used to find the eastings, and the next three the northings. They give you how many hundreds of metres east or north your point is. There are also some number that are left out before the threes numbers (as they are pretty much the same anywhere) and after the three numbers (because you usually only give a grid reference to the nearest hundred metres, not nearest metre.

To get your eastings in metres, add a 2 before the three numbers, and two 0s after. To get your northings in metres, add a 62 before the three numbers, and two 0s after.

For example, GDA462568 becomes Easting 246200 and Northing 6256800. Using SIX Maps, that gives me a lat of -33.798558 and long of 150.258552

Hope that help, Dunphy's Pass is a good bit of fun!
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Re: GDA Conversion

Postby mrpotter » Wed 04 Jun, 2014 5:04 pm

Ahh thanks heaps for that information! I've converted a few co-ordinates this afternoon quite successfully.
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Re: GDA Conversion

Postby tom_brennan » Sun 08 Jun, 2014 3:08 pm

A 6 figure grid reference is only unique within a 100x100km square. If you give (or are given) a six figure grid reference, you should normally also give the map name. Otherwise you could end up a good distance away from where you intend!

Out of interest, what do you need the lat/lon for? For most bushwalking purposes it's redundant. The only thing I can think of where it's useful is if you're trying to find a point on Google Maps...
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Re: GDA Conversion

Postby mrpotter » Sun 08 Jun, 2014 5:47 pm

I was marking the points on some prints from Openstreet Maps. While I can appreciate many bushwalkers do use GDA, I'm not aware of any GPS device that uses GDA? Why would you use two sets of co-ordinates? Wouldn't that be confusing? 6 digits only seem useful for less information, which isn't the case when you factor in details of the map used? Is GDA used outside of Australia? Do emergency services use GDA? I know the NSW services do use & accept lat/long co-ordinates, namely its the primary system for NSWAS's rescue helicopters? I really don't understand why GDA is used, is there something I've missed?
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Re: GDA Conversion

Postby icefest » Sun 08 Jun, 2014 6:29 pm

I've set my Garmin GPS to use GDA as it it much easier to transfer my location to a map when using that co-ordinate system as the 1km squares are marked on most maps.
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Re: GDA Conversion

Postby bernieq » Sun 08 Jun, 2014 9:18 pm

mrpotter wrote:I really don't understand why GDA is used, is there something I've missed?
Yes, you have :)
GDA stands for Geocentric Datum Australia so, no, it's not used outside Australia.

However, conveniently, GDA94 is equivalent to WGS84 - which is available on all GPSs.

If you look closely at any professionally produced map, you'll see an information panel that contains all sorts of useful/interesting information. For example, how to read and quote grid references, what GDA, WGS and AGD mean, what zones are all about ... and so on.

All good fun !

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Re: GDA Conversion

Postby tom_brennan » Wed 11 Jun, 2014 11:35 am

As Bernie says, GDA (technically GDA94) and WGS (technically WGS84) are close enough for bushwalking purposes. If you need accuracy down to the centimetre, you will need to deal with the difference, but accuracy of < 10m is rarely required for bushwalking. Current difference I think is around 0.5m, but it changes as Australia moves due to continental drift.

GDA94 and WGS84 are datums, which are a model of the earth – that is somewhat curved.

Maps are a translation of the curved earth into a flat surface. Most maps for bushwalking are UTM (Universal Transverse Mercator) projections using a particular datum, and use a Cartesian (XY) co-ordinate system. For Australia, the datum used for projection is now GDA94 (=WGS84), but used to be AGD66, which you'll still see on many maps. The combination of the projection and datum is known as the Map Grid of Australia (MGA), which is why you will often see co-ordinates given as MGA456234 or similar.

It's much more convenient to locate a point on a bushwalking map using a 6 figure grid reference, than it is to locate the same point using Lat/Lon. Try it if you don't believe me. The base unit for the grid reference is the metre, so it makes calculating distances much easier. Maps are also divided into 1km squares, along the grid lines, so points can easily be interpolated.

The main advantage of Lat/Lon is that it is universal (well, ok, it's global!), which is why internally, GPSs and similar store track points in Lat/Lon.

For info on converting grid references, see http://ozultimate.com/canyoning/map_grids.htm
This also has some info on datums and map grids and the difference.
Last edited by tom_brennan on Mon 16 Jun, 2014 12:13 pm, edited 1 time in total.
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Re: GDA Conversion

Postby +dj_hansen+ » Sat 14 Jun, 2014 11:12 am

The difference between GDA94 and WGS84 is actually quite small... millimetre level stuff. Even as a Surveyor, the difference is never regarded for day to day work, and only really when looking at super precise Australia wide Geodetic network level is there issues, mainly due to continental drift and choosing which model of the earth you are using (ITRF). Your GPS stores and works with Lat/Long because this is the native datum for GPS and allows you to perform any number of projections and transformations (AMG66/84) from the original co-ordinates quickly :)
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Re: GDA Conversion

Postby tom_brennan » Mon 16 Jun, 2014 12:31 pm

Is that actually true, +dj_hansen+?

Not that I'm a surveyor, but everything I've read says that GDA94 was pretty much aligned with WGS84 in 1994 (hence the '94'), and since then they've been moving apart due to continental drift. GDA94 is fixed to the Australian continent, which is moving. This impacts things by up to 8cm per year. The figures I saw ranged from 0.72m to 0.89m for 2007, so probably over a metre by now.

There are projections and transformations that will allow you to transform between WGS84 and GDA94, but that doesn't mean that the raw numbers are the same.

See the following links:
http://www.ga.gov.au/earth-monitoring/g ... ences.html
http://www.quickclose.com.au/stanaway07pres.pdf

Of course, the difference is still irrelevant for bushwalking purposes!
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Re: GDA Conversion

Postby +dj_hansen+ » Mon 16 Jun, 2014 4:13 pm

You are indeed correct... but I don't know of anyone (apart from those working with state or nationwide Geodetic (high precision) networks) that takes it into account. These days a high precision Survey station can have 5 or 6 different co-ordinates depending on which Epoch of the ITRF they reference. So yes, GDA94 and WGS84 were the same in 1994 but since then continental drift has moved Australia north west about 1m. So my post was perhaps a little broad & sweeping in saying they are the same.

Like you say... irrelevant for Bushwalking purposes :) and maybe I'm just a rough Surveyor ;)
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Re: GDA Conversion

Postby YoungCodger » Wed 07 Jul, 2021 1:25 pm

Allchin09 wrote:For example, GDA462568 becomes Easting 246200 and Northing 6256800. Using SIX Maps, that gives me a lat of -33.798558 and long of 150.258552


Thanks.

I'm trying to do the same thing as OP - efficiently convert GDA123456 to decimal lat/long. Your explanation helps with the first step, but how can one quickly convert the Eastings and Northings to decimal lat/long? This works https://geodesyapps.ga.gov.au/grid-to-geographic but it's pretty fiddly and slow. If anyone has a more efficient method to convert, please let me know!

I've learned how to create detailed topo maps in QGIS, which is much more useful than a physical map in terms of the amount of detail you can get for a specific scramble. Plotting important coordinates directly into QGIS / other GPX apps would be very useful for creating a reference before adventures.
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Re: GDA Conversion

Postby wildwanderer » Wed 07 Jul, 2021 2:00 pm

I use orux maps. It converts between all the lat / long formats at the touch of a button. [create waypoint, then click on the Grid Ref in the waypoint creator and there is options to display all the lat long, MGRS, UTM etc conversions]

Im sure there are online converters floating around on the net somewhere also
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Re: GDA Conversion

Postby tastrax » Wed 07 Jul, 2021 8:07 pm

YoungCodger wrote: Plotting important coordinates directly into QGIS / other GPX apps would be very useful for creating a reference before adventures.


In QGIS its pretty easy as there is an 'advanced digitising' menu where you can simply add the coordinates in the CRS format of your layer

In a GPS you can normally change the Position format (Garmin) to the coordinate system of your choice (say Lat Long), then add a waypoint and simply edit the numbers (in Lat Long format). You can then change the coordinate system back to what you desire (GDA94) and the gps will do all the transformation for you.
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Re: GDA Conversion

Postby tom_brennan » Wed 07 Jul, 2021 10:15 pm

YoungCodger wrote:I'm trying to do the same thing as OP - efficiently convert GDA123456 to decimal lat/long. Your explanation helps with the first step, but how can one quickly convert the Eastings and Northings to decimal lat/long? This works https://geodesyapps.ga.gov.au/grid-to-geographic but it's pretty fiddly and slow. If anyone has a more efficient method to convert, please let me know!

I've learned how to create detailed topo maps in QGIS, which is much more useful than a physical map in terms of the amount of detail you can get for a specific scramble. Plotting important coordinates directly into QGIS / other GPX apps would be very useful for creating a reference before adventures.


Do it graphically. GDA123456 means nothing unless it's applied to a particular map (eg Katoomba 1:25k)

If you're in NSW, you can use https://maps.ozultimate.com/ to plot waypoints or a track directly on to the map, and then download it in GPX format. GPX is in lat/lon.

You can do the same thing in QGIS, but maps.ozultimate is probably easier for the lay person.
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Re: GDA Conversion

Postby YoungCodger » Fri 09 Jul, 2021 10:30 pm

Thanks all. Someone showed me today that Avenza's settings allow me to display GDA coordinates (perfect for reading bushwalking notes), and now I just learned that I can set a QGIS project to GDA coordinates also. Solved!
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Re: GDA Conversion

Postby caelyx » Wed 14 Jul, 2021 6:33 pm

One additional option, in part for the benefit of anyone who finds this page later via Google.

For converting between grid, lat/long and other systems, I find this webpage invaluable: https://www.earthpoint.us/convert.aspx
Put in a valid coordinate in some format, and it gives you a large number of other formats for that location. Particularly useful when converting between links like decimal degrees and degrees/minutes/seconds, etc.
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