midtoad's blog

Problems with tracklogs displayed in GoogleEarth

There's a problem with flight tracklogs (at least for my part of the world) displayed in GoogleEarth. They are not overlaid in the correct position on the satellite image. Nor are any of the roads in the Canada Roads overlay, either. But worse, as you drag the satellite image around on the screen, the tracklog moves around as well, either increasing or decreasing the error in its location.

I posted the following text on the Oz Report forum, and on the Google Earth forum:

JD, turning on Canada Roads helps me to undersrtand the problem. The roads are displaced 380m meters to the northeast of where they should be on the satellite image. so there's a real problem with GoogleEarth, and this problem is also present in GoogleMaps.

At first I thought that my tracklog was perfectly lined up with the road-lines in GE, but then I noticed something peculiar. As I grabbed the satellite image and moved it around on the screen, my tracklog also moved somewhat- but independently of the position of the roads and of the satellite image! When I put one end of my track in the centre of the screen, the end of the tracklog was directly aligned with my trip destination in the satellite image. But as I moved that end toward the edge of the screen, it moved further and further away from where it was supposed to be. there's a real problem with the calibration of the satellite images and the roads relative to the tracklog!

And yet... I took a GPS waypoint at an intersection right outside my residence, and then located that intersection in GE. When I created a Placemark in GE, the coordinates of the placemark exactly matched the coordinates of my GPS waypoint. So it appears that individual coordinates are not wrong (which would explain why Davis says his Big Spring takeoff spot is okay), but rather the scaling of tracklogs is wrong. As you change scale or move a tracklog on the screen, its placement error also moves.

Someone want to try making a tracklog in their neighbourhood and then uploading it to GE? I wonder if the error is worldwide or just confined to some areas, like Calgary?

BTW, I have converted my GPS tracklog to .kml file on both PC and Mac, with identical results. On the PC, I uploaded from the GPS to G7ToWin, saved as .igc file, then opened the file in MaxPunkte and saved as .kml. I also opened the .igc file with igc2kml and converted the format, and the result was the same as with MaxPunkte. On the Mac, I opened the .igc with Flight Track, saved as .gpx, then converted to .kml using LoadMyTracks. Same results using the Mac OS X version of GE as using the Windows version, so we know the problem is with GE, not any of the conversion tools.

the attached pair of images show a screen capture of GE when the tracklog is in the center of the screen, and when it is in the lower-left. Notice how the tracklog has shifted both with respect to the satellite image and with respect to the roads overlay.