kml export

[John Thompson]

The kml placemarks exported by J-Field are about 4 feet northwest of where they should be, at least in my neck of the woods.

I'm not sure if this is a coordinate transformation bug, a rounding error, or a feature. Anybody else seen this?

 

[Shawn Billings]

Soundslike the difference in WGS84 and NAD83. How are you finding the difference? What are you comparing to?

 

[John Thompson]

Soundslike the difference in WGS84 and NAD83.

That's what I was thinking.

I found the difference when I compared the exported kml of the surveyed points with another kml file of the design points. Scared me when I first saw it. Then I exported the design points as kml and they were right on top of the surveyed points, and 4' NW of the pre-J-Field design points. Then I created a new job and manually typed in a point 40°00'00"N, 102°00'00"W and exported it. Google Earth puts the exported placemark at 40°00'02"N, 102°00'05"W.

 

[John Thompson]

I changed the CS to WGS84(ITRF2008) and manually typed in a point 40°00'00"N, 102°00'00"W and exported it. Google Earth puts the exported placemark at 39°59'60.00"N, 102° 0'0.01"W. My previous test had NAD83(2011) as the CS. I guess that explains it.

 

[Mikhail Drakin]

Kml files are always saved in WGS84/EGM96 geoid.

 

[John Thompson]

I suppose one can only ignore the difference between NAD83 and WGS84 for so long...

For years, I have used Carlson Survey (2016 with IntelliCAD 8.1) to import and export kml to/from Google Earth. In Drawing Setup I can sepcify Lat/Lon Datum as NAD83 or WGS84. But if I use my own LDP, it forces me to use WGS84, and if I use SPC, it forces me to use NAD83. As far as I can tell, Carlson actually exports the kml as NAD83 regardless of the setting. It has always matched the GE imagery fine, so I didn't care. I can often pick well-defined points off of GE and get coordinates that match surveyed NAD83 coordinates within a foot.

I think that means that the GE imagery is actually referenced to NAD83, not WGS84. That is, until Google updated the photo earlier this year. The new photo is shifted about 4' NW of the old photo (2013) and it matches (within a foot or so) the kml exported from J-Field. So it looks like Google's new photo is on WGS84 and Carlson has a bug to fix.

 

[Shawn Billings]

Many software developerstreat them as equivalent. For size and shape of the earth, they practically are. For location they are not. I'd be surprised if Google is paying close attention to what reference frame the images come to them. Some may be NAD83 and some ITRF. And some may be too loose to matter, and this is what has allowed developers to treat them as equivalent for so long.