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I have a google earth route that measures 190.1km in Google Earth.

I then pulled down the .kml file, extracted the points and measured them using Vincenty's formula (ellipsoid earth) and got 190.2.

BUT, if I measure the SAME points along the SAME route to a different distance along that route, I'm way off. Sometimes as much as 2km.

For example, If I want to know the lat and lon of the point closest to 100km, I measure the distance between all the points along the route until the measurement is as close to 100km as it's going to get. When I compare that point to the route in Google Earth, it's around 98km.

Any ideas of why this is the case?

  • This may or may not be relevant gis.stackexchange.com/questions/84885/… - there have been a few Questions about Vincenty's formula on this site before that you may wish to review – PolyGeo Jun 23 '14 at 5:33
  • Thanks for the direction. Turns out I saw that thread when I was doing research on writing the algorithm I have now. I just don't understand how my distances can be the same overall, but so different at certain points along the way. – Will Luce Jun 23 '14 at 5:37
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I believe that the distances in Google Earth assume the earth is a sphere with radius equal to the equatorial radius. (You should check for yourself by constructing and loading simple kml polylines with coordinates, e.g., 0,0 0,90 and 0,0 90,0.) So expect differences on the order of 1 part in 300 in Google's distances compared to the geodesic distance.

To add insult to injury, Google Earth treats the edges of a polygon differently from a polyline. The documentation claims that the edges of a polygon are lines of constant bearing. This is false; they appear to be straight lines on a plate carree projection.

Finally, a challenge for Google Earth users: draw a polygon which approximately outlines Antarctica.

ADDENDUM

Here's the data I get asking Google Earth Pro (version 7.1.2.2041) to compute the distances along simple (hand generated) polylines. The 90 segment data breaks the 90-degree arc into 1-degree segments. Distances are in meters.

0,0 to 90,0 (quarter meridian)
WGS84 geodesic 10001966
GE 1 segment   10001959
GE 90 segments 10001839

0,0 to 0,90 (quarter equator)
WGS84 geodesic 10018754
GE 1 segment   10018754
GE 90 segments 10018627

Clearly this data is inconsistent with a spherical model of the earth (meridian lengths are shorter that equatorial ones). However the discrepancy between the 1 segment and 90 segment results is unforgivable. Even though Google Earth is a very useful tool, you cannot rely on it for any accurate measurements. I understand that Google is at the mercy of the data providers for the imagery and height information. However, this is a completely different issue -- Google Earth just doesn't have a consistent geometrical model of the reference ellipsoid.

This problem was reported to Google via the "earth private beta" mailing list (which is monitored by some members of the Google Earth team) on 2011-07-05.

  • I checked some old correspondence about Google Earth from 2012. I see that GE back then didn't use spherical formulas for distance. However the results weren't consistent with ellipsoidal formulas either unless the flattening was allowed to depend on the position of the points. I will investigate tomorrow. – cffk Jun 24 '14 at 1:40
  • Thanks for looking into this. I had a thought today that might be leading. If the distances between the points are added together from start to finish, all calculations are the same; therefore the distance is the same. A better way to say that may be that the combination of calculations is the same, since every measurement is a separate calculation. But, at any given point along the route, the combination of calculations is different from that of the finished product. Bascially, this goes along with your thought about varying flattening. – Will Luce Jun 24 '14 at 4:54
  • Wow, what great analysis! Thank you. What I'm getting from this is that Google Earth isn't the tool I'm looking for. My goal is to be able to accurately find the point at any given distance along a route. Presumably by adding the distances between the points up to the distance requested. Do you have another tool in mind that might be more useful for this? ArGIS? – Will Luce Jun 24 '14 at 16:09
  • I formulated this into a new question here – Will Luce Jun 24 '14 at 16:39
  • And I answered the new question, providing a pointer to pypi.python.org/pypi/geographiclib as a possible way to address your needs. – cffk Jun 25 '14 at 12:53

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