[GIS] Convert from WGS84 to GDA94/MGA94 on iOS

australiaconvertcoordinate systemios

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instructions how to convert from a standard lat/lon into those two — and sadly many of the references I did find are out of date or refer to 404 pages.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).

Best Answer

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.

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[GIS] Convert GPS coordinate to ArcGIS x,y point

Basically you want to use the projectGeometry method from the AGSGeometryEngine class to convert from the GPS coordinate system to the coordinate system used by your map. Use the defaultGeometryEngine for this.

Assuming your GPS is giving you WGS-84 coordinates you could do something like this:

CLLocation* gpsLocation = ....

// create a AGSPoint from the GPS coordinates
AGSPoint* gpsPoint = [[AGSPoint alloc] initWithX:gpsLocation.coordinate.longitude
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AGSGeometryEngine* engine = [AGSGeometryEngine defaultGeometryEngine];

// convert GPS WGS-84 coordinates to the map's spatial reference
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AGSPoint* mapPoint = (AGSPoint*) [engine projectGeometry:gpsPoint 
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I'm not familiar with programming in JavaScript but in the OpenStreetMap Wiki there is a section describing "Mercator". Following this link you'll find a sample code snippet to tranform from lat long to mercator. I actually don't know if it's correct because I did not test the code.

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