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I have a mobile app (iPhone/iPad) for recording data and taking notes, including photographs, and a government client needs GPS coordinates attached to each photo.

They require coordinates in the GDA94 and/or MGA94 specification, but I can't find any concrete instructions how to convert from a standard lat/lon into those two — and sadly many of the references I did find are out of date or refer to 404 pages.

Is there a simple way to do the conversion? It doesn't need to be accurate since I'm just using a smartphone GPS chipset. But I'd like something with better than 1m accuracy.

As far as I can tell, WGS84 and GDA94 were identical in 1994 but due to continental drift are around 1.4 metres apart now (iOS GPS coordinates are in WGS84 aren't they? I couldn't find any definitive documentation for that either).

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    GeoScience Australia has a page which performs the calculations for you, but unfortunately doesn't show the formula used. There's an email address on that page so you could try asking for further info. – Stephen Lead Jul 8 '14 at 2:34
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    GA has updated their GDA technical manual (PDF). It says to see Dawson and Woods (2010) which is luckily online. It has a time-dependent 14-parameter tfm, coordinate frame flavor. I think the spreadsheet in the answer only does GDA94 to grid, and doesn't include the ITRF/WGS84 conversion (it doesn't ask for any epoch/time information). – mkennedy Sep 15 '15 at 16:34
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There's another page on the GeoScience Australia website at http://www.ga.gov.au/scientific-topics/positioning-navigation/geodesy/geodetic-techniques/calculation-methods#heading-4 which contains a link to an Excel file.

If you download this file, then un-hide rows 8-39, you may be able to unpick their formula and apply it in your app.

enter image description here

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  • Also un-hide the Constants and Parameters sheet – Stephen Lead Jul 8 '14 at 2:41
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I have recently had to perform the same transformation. I picked apart the formulas in the spreadsheet at http://www.ga.gov.au/scientific-topics/positioning-navigation/geodesy/geodetic-techniques/calculation-methods#heading-4 and have put together a Matlab function which can handle this, which is also vectorised to help chug through as many coordinates as needed at once..

I just thought I would leave a link to it on the file exchange in case it was useful for anyone searching something similar to this.

https://au.mathworks.com/matlabcentral/fileexchange/57304-convmga94-lat-long-

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You can use the ArcGIS SDK for iOS to reproject coordinates from WGS-84 to GDA94. The SDK is a free download from developers.arcgis.com/ios and docs are found online here.

Converting points, lines or polygons is a simple operation using the AGSGeometryEngine class. This code snippet, converts a CLLocation object called location from WGS-84 to GDA94:

AGSGeometryEngine* engine = [AGSGeometryEngine defaultGeometryEngine]; AGSPoint* wgs84Point = [AGSPoint pointWithLocation: location]; AGSSpatialReference* gda94SR = [AGSSpatialReference spatialReferenceWithWKID: 4283]; AGSPoint* gda94Point = (AGSPoint*) [engine projectGeometry: wgs84Point toSpatialReference :gda94SR];

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  • The 4283 ID for GDA94 comes from this list of supported coordinates systems located here. You can read more about it here – progrmr Jul 11 '14 at 14:49
  • ArcGIS seems to require having their logo clearly visible somewhere inside the app, which is a bit much for a small use case like this. Also I can't find any details about how much their licensing fees are. – Abhi Beckert Jul 14 '14 at 23:19
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If you're just using the smartphone GPS you probably do not need to worry about the conversion. The current difference between GDA94 and WGS84 is of the order of 1 metre, but you'd be unlikely to get a GPS location any more accurate than 4 metres in my experience. If differential GPS was being used however, it might matter.

But if you are intent on converting as accurately as possible anyway, then you need to first convert the lat/lon values to cartesian earth-centred X,Y,Z coordinates, then apply some matrix operations: scale, rotate, and offset using parameters computed for the current year. Finally you need to convert back to lat/lon.

You will also need the 14 parameters which are given on page 17 of this set of slides: http://www.quickclose.com.au/stanaway07pres.pdf

AusDatumTool is able to do these conversions if you want something to test your results against. ( http://www.binaryearth.net/AusDatumTool )

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  • It's 1 metre now but the data is being archived by a government department and possibly used again 300 years later. – Abhi Beckert Apr 7 '16 at 21:54
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Here's a windows app I wrote to do the job.

2GDA94.zip

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We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

  • Would you be willing to share the source? – George R Oct 12 '16 at 0:18

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