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Using Google Maps I get coordinates around the hundreds values (in an Australian location). For example:

enter image description here

However when I transform coordinatess from epsg:28356 to epsg:3857 (Google Map's projection), I get results in the ten's of millions.

enter image description here

Does anyone know why this is?

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    The coordinates in your screen capture are in degrees latitude/longitude (EPSG:4326). – user30184 Apr 18 '17 at 7:04
  • Thank you for the detailed answer. I'm still very confused. Would you know a way to convert the resulting epsg:3857 coords into degrees? Or is not possible due to the lose in accuracy? – Brad Woods Apr 18 '17 at 7:34
  • Change the coordinate system on the right side "Output coordinate system" by pressing the button named "Change" and you can do the conversion into EPSG:4326 directly. – user30184 Apr 18 '17 at 8:28
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The coordinates Google maps show you are geographical coordinates. As you can see they use degrees as units. That's why the value is between -180 and 180. Google uses WGS 84 to calculate distances and show coordinates on the map. But the map itself is projected with the Web Mercator system (3857), but that is not the EPSG of Google Maps when it comes down to coordinates and distances.

The issue why they are using two different systems is because of some general problems: WGS 84 is nice for calculating distances worldwide. But they are in degrees and not meters or feet. Also it is not well suited to show a map of a 3-D earth on a 2-D map. Therefore you need a projection (https://en.wikipedia.org/wiki/Map_projection).

The problem with the Web Mercator is that it can't be used for calculating distances as they differ quite a lot from the real distances. Usually a map projection is only suitable for a smaller distinct area on the earth. That's why you have for example UTM zones(https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system). But the Web Mercator tries to show the whole earth in a single projection. But that is only a compromise for presentation.

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Google Maps is transforming the coordinates so it can display them in latitude, longitude (decimal degrees).

The coordinates are being transformed from EPSG:3857 (WGS 84 / Pseudo-Mercator, units = meters) to EPSG:4326 (WGS 84 / geographic, units = degrees)

For example, here is the coordinates from your screenshot transformed from EPSG:4326 to EPSG:3857.

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