I have some Nad83 data that I want to overlay on Google Maps. This article by ngs.noaa.gov explains translating from nad83 to wgs84 better than anything I have read. If I were to use a tool like the noaa transformation tool to convert a coordinate pair from NAD83 to WGS84 for this purpose, would I want to use an output epoch that matches the approximate date of the imagery? Or is all google imagery georeferenced based on some common epoch independent of the date the imagery was created?

I realize that this datum shift will be small and probably hard to realize on the +/- 1 meter Google photography. But still, I want to make sure I get the best alignment possible.

  • You're right, the difference is likely to be very small. NAD83 is static but WGS84 is dynamic. We're facing a similar problem in Australia with GDA94 to WGS84, the difference is still small but becoming more noticeable, giving birth to the new GDA2020 datum which will be closer to WGS84. Unless you're using your data in a survey you are unlikely to see the difference; I suggest you're overthinking a little and probably overstating the accuracy of Google. How sure are you of the accuracy of the NAD83 point than sub-metre accuracy is a requirement? – Michael Stimson Jun 27 '17 at 23:44
  • Thanks Michael. Google publishes that 90% of points in imagery are within 1 meter and we find that to be the case with the GPS data we import from across the United States. However, data imported from northwestern US is usually off by more than that from southeastern US. I just saw an illustration today showing how that is exactly how the North American plate is rotating. Meaning maybe I need to stop assuming that Nad83 and WGS84 are the same. – benHill Jun 27 '17 at 23:55
  • That may well be the case in very well developed areas but I can personally attest that Google is > 10m different in remote areas of Australia, the trick is knowing which is more accurate: parcel boundaries, roads or Google imagery. GPS, unless you have a really expensive one and setup a base station are usually +/- a few metres (5-10 for cell phones). If you're collecting points by GPS there will be a setting for WGS84, that wont help with your existing points I know but a standard transformation should convert the coordinates without impacting the accuracy greatly. – Michael Stimson Jun 28 '17 at 0:03

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