Comments on: Why Are My GPS Positions In The Wrong Place In Google Earth? http://freegeographytools.com/2007/why-are-my-gps-positions-in-the-wrong-place-in-google-earth Exploring the world of free tools for GIS, GPS, Google Earth, neogeography, and more. Wed, 18 Nov 2009 03:05:04 -0700 http://wordpress.org/?v=2.8.6 hourly 1 By: Google Earth & GPS For Cruisers & Boat Owners | http://freegeographytools.com/2007/why-are-my-gps-positions-in-the-wrong-place-in-google-earth/comment-page-1#comment-40 Google Earth & GPS For Cruisers & Boat Owners | Wed, 24 Jun 2009 11:34:09 +0000 http://freegeographytools.com/?p=36#comment-40 [...] There is another issue with GE. It has known differences in GPS measurements when compared to other navigational software like Maxsea and therefore makes it unreliable to use as a primary navigational aide. With this in mind anyone trying to use GE as their primary navigational aide needs their head examined. Do not do it. At the time of writing GE is showing me approx. 20m off my actual position. I don’t need to tell you how significant this error is if you were using it to navigate into a rocky anchorage. There is an article on the difference here. [...] [...] There is another issue with GE. It has known differences in GPS measurements when compared to other navigational software like Maxsea and therefore makes it unreliable to use as a primary navigational aide. With this in mind anyone trying to use GE as their primary navigational aide needs their head examined. Do not do it. At the time of writing GE is showing me approx. 20m off my actual position. I don’t need to tell you how significant this error is if you were using it to navigate into a rocky anchorage. There is an article on the difference here. [...]

]]> By: Leszek Pawlowicz http://freegeographytools.com/2007/why-are-my-gps-positions-in-the-wrong-place-in-google-earth/comment-page-1#comment-39 Leszek Pawlowicz Tue, 16 Jan 2007 16:38:09 +0000 http://freegeographytools.com/?p=36#comment-39 Good point about the terrain. I believe that Google Earth uses 30-meter terrain data for the US, but 90-meter for most of the rest of the world. So in areas of extreme topography, images won't be "stretched" properly to fit the terrain. The Matterhorn in Switzerland was the classic example of this in Google Earth, but GE improved the accuracy of the elevation data there so that it renders more accurately now.<br /><br />Google Earth says that it uses the standard elliptical WGS84 model:<br /><br />http://earth.google.com/userguide/v4/ug_importdata.html<br /><br />But even the elliptical model is too simplistic, and trying to wrap data in the geographic (Plate Carree) on such a simple model of the Earths' shape will introduce additional errors. Good point about the terrain. I believe that Google Earth uses 30-meter terrain data for the US, but 90-meter for most of the rest of the world. So in areas of extreme topography, images won’t be “stretched” properly to fit the terrain. The Matterhorn in Switzerland was the classic example of this in Google Earth, but GE improved the accuracy of the elevation data there so that it renders more accurately now.

Google Earth says that it uses the standard elliptical WGS84 model:

http://earth.google.com/userguide/v4/ug_importdata.html

But even the elliptical model is too simplistic, and trying to wrap data in the geographic (Plate Carree) on such a simple model of the Earths’ shape will introduce additional errors.

]]> By: GearthBlog.com http://freegeographytools.com/2007/why-are-my-gps-positions-in-the-wrong-place-in-google-earth/comment-page-1#comment-38 GearthBlog.com Tue, 16 Jan 2007 13:09:22 +0000 http://freegeographytools.com/?p=36#comment-38 A couple of other points:<br /><br />* 3D Terrain - GE has terrain and the image is stretched over the terrain. The terrain is not detailed enough to be an exact match so the photo will be stretched differently.<br /><br />* The image errors can be attributed to the fact Google uses a spherical WGS84 base model, not elliptical. But, more often it is due to the provider Google got the data from not accurately processing their imagery data. Google is trying to enforce better QA. A couple of other points:

* 3D Terrain – GE has terrain and the image is stretched over the terrain. The terrain is not detailed enough to be an exact match so the photo will be stretched differently.

* The image errors can be attributed to the fact Google uses a spherical WGS84 base model, not elliptical. But, more often it is due to the provider Google got the data from not accurately processing their imagery data. Google is trying to enforce better QA.

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