arcgis desktop - Working with Google Maps/ArcMap lat/long

I know a bit about lat/long, projected and geographic coordinate systems etc., and a bit about geodesy and datums.

I have a lat-long in deg/min/sec provided to me by someone else (point location = northern Italy). I have a map in ArcGIS in the same region and I'm constructing a grid over it for some analysis, centred on the lat-long point above.

In ArcGIS, I'm working with projection Roma 1940 Gauss Boaga Ovest - the data frame and map layer are both in it - and when adding the XY point also in this projection, it ends up pretty much where it should be (but not exactly)

The thing is, when putting these coordinates into Google Earth, the point is around 25 or 30 metres south east of where it is in ArcGIS. The provider is saying the Google Earth location is where it should be.

So, where does the difference derive from?

I understand Google Earth is a WGS84 geographic coordinate system so I tried a point file in WGS84 and added the XY and it still ends up in the slightly incorrect position in ArcGIS as outlined above. Also have tried various combinations of WGS84 and Roma 1940 Gauss Boaga Ovest and can't get it right.

Would the Roma 1940 datum skew it slightly or is it nothing to do with that?

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I see that Google Maps actually uses WGS84 Web Mercator projected coordinate system, which has its own problems.

Would ArcGIS actually be more accurate then?

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I asked the provider and they said the coordinates provided have come from Google Maps.

I've tried to reproject etc. and nothing will change the position from the red dot (see left of image). I've since discovered Google Maps is a WGS_1984_Web_Mercator projection, based on a sphere. I've attempted to create a custom transformation of datum in ArcMap from 'GCS_WGS_1984_Major_Auxiliary_Sphere' to 'GCS_Roma_1940', no change to dot position when entering the coordinates.

As you can see, the Google Maps on the right is in a slightly different position, by roughly 35m, to the ArcMap image on the left. I'm pretty much out of ideas.

Answer

Due to various vagaries in the ArcGIS software, I'm going to tell you to use different coordinate reference systems. That will let you access the predefined transformations and hopefully make your life easier.

Use WGS_1984_Web_Mercator_Auxiliary_Sphere instead. This one uses WGS 1984 for the geographic CRS (aka datum). The one you're using, WGS_1984_Web_Mercator, uses a sphere (GCS_WGS_1984_Major_Auxiliary_Sphere) and there are no predefined transformations for it.

Next, use Monte_Mario_Italy_1 which is the same, just a different name. Pick Monte_Mario_To_WGS_1984_4 for the transformation.

Or if you want to use Roma_1940_Gauss_Boaga_Ovest, you'll have to define a custom transformation that's a copy of Monte_Mario_To_WGS_1984_4.

FROM roma 1940 TO wgs84
Method: Position Vector
X axis translation: -104.1
Y axis translation: -49.1
Z axis translation: -9.9
X axis rotation: 0.971
Y axis rotation: -2.917

Z axis rotation: 0.714
scale difference: -11.68

If you define a custom transformation in ArcMap, you may have to define it FROM wgs84 TO roma 1940. If so, change the signs on all parameters.