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KML Import&Export, Carlson Survey, NAD83 vs WGS84
After a few experiments, it appears that Carlson Survey (I’m using 2023) exports KML/KMZ files from my drawings with NAD83 lat/lon coordinates, and this because I work in NAD83-based state plane coordinates.
My drawing and coordinate files in Carlson Survey are in NAD83 SPC coordinates (various states & various zones). My state plane coordinates are based either on OPUS solutions [recently using NAD83(2011)(Epoch 2010.0)] or RTN off the local CORS network [using the realization of NAD83 the CORS stations are using I presume.]
It’s my understanding that KML files use coordinates in some recent realization of WGS84, and apps that read the KML files assume the coordinates are WGS84 (again, a recent realization) and not NAD83. So there’s a slight discrepancy between what Carlson Survey is exporting and what the apps using the KML files expect. The apps that I use with the KML files are not survey-grade, so I’m not complaining, I simply want to confirm my understanding here.
I primarily use the KML files to give to non-surveyor users to roughly find things in Google Earth and other apps on their phones. (Land conservancy preserve managers are tasked to go check on fences, gates, trail cams, etc.) I also use the KML files exported from deed plots to go scouting for monuments.
Yes, none of this really matters, as NAD83 and WGS84 (recent realizations) are only a few feet apart, which is well within phone GPS errors and probably Google Earth image registration errors.
I want to be sure I understand things somewhat correctly. If I’m approximating things, that’s fine, though I do want to know what I’m approximating.
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It also appears that when I use Points->Adjust Points->Coordinate Transformation, Lat/Lon to Lat/Lon, NAD83 to WGS84, that no transformation is applied, the Lat and Lon in decimal degrees are identical to 8 decimal places (well 10 and 11 places counting the whole degrees) in NAD83 and WGS84.
As far as I can tell, there is no setting in Carlson Survey to specify the realizations of NAD83 and WGS84 used for the conversion, so perhaps Carlson is using the ‘original’ realizations when the datums were virtually identical?
For all of the above (KML export and Coordinate Transformation), in Drawing Setup I have “Grid” chosen for System, Projection set to “State Plane 83”, and Zone set appropriately for the state and location I’m working in for that project. I use reasonable elevations and GEIOD18. I’ve tried setting the “Lat/Lon Datum” to both NAD83 and WGS84 with no change in results.
Cheers,
Tony.
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