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Re: GPS coordinates and Google Earth

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GPS coordinates and Google Earth
23 June, 2014 12:30

In a discussion on an earlier thread a question came up concerning the accuracy of the relation between GPS tracks and their projection onto Google Earth.
I mentioned that the angle at which satellite images were taken was one reason for this discrepancy.

I made a more systematic test this spring. I had both a Garmin GPS unit and a digital camera (SONY DSC-HX50V) that recorded GPS coordinates. With the memories fresh in my mind of where I took photos, I could then map my photo GPS coordinates onto Google Earth. This would allow me to compare the Google Earth/satellite images for how accurate those image overlays were to the GPS coordinates.

Example One: Anopoli acropolis

This is a capture from the Google Earth 8/16/2010 satellite image.

It can be seen that the satellite image fits quite accurately to the GPS coordinates of specific structures.

Here is a capture from the Google Earth 10/14/2013 satellite image.

The displacement is obvious. Even though the coordinates are exactly the same, the satellite image displaces the pinned structures as much as 15m ‘north’ of their true position.

Example Two: Kali Laki to Ag Pavlos route

However, the situation is more complicated than one year’s image being better/more accurate than another year’s image. Since the satellite images are taken at angles that vary to a greater or lesser extent from the vertical, this means that different sections of one and the same image can differ in how accurately they map onto the GPS coordinates.

Here is a comparison of the 8/16/2010 and the 10/14/2013 satellite images including both my GPS track and the locations of photos that I took.

From a distance they seem to be accurate enough. But when we take a closer look we can see some interesting differences from year to year.

The following is a close-up of the start of the route.

I know precisely where I turned on my GPS: while sitting under a tree. Here is the photo (#415) I took of where I was sitting.

You can see that the 2010 satellite image fits more accurately with my GPS and the Google Earth coordinates.

And yet, when we take a close look at the finish of the route, the situation is reversed.

The GPS track on the 2013 satellite image faithfully reproduces where I walked: coming down just west of the chapel and stopping under the big trees. On the 2010 image the track is incorrectly shown as entering behind the chapel.

I also took a photo (#444) of the flower-covered hillside just west of the chapel. Again, this position is correctly given on the 2013 image, but incorrect on the 2010 image.

I don't know what conclusions can be drawn, other than to say that the coordinate readings of specific structures/locations taken directly from the Google Earth images have an error of as much as 15m.

Edited 2 time(s). Last edit at 23/06/2014 12:35 by Mike.
Re: GPS coordinates and Google Earth
23 June, 2014 13:55

Hi Mike,

Here is what I found:

There are at least two reasons for the accuracy errors:

1. GPS Error – Even though GPS units give positions to a precision eight decimal places in latitude and longitude, corresponding to an accuracy of about a millimeter or so, the true accuracy with which you can measure a position with GPS is far poorer than that. Consumer GPS unit manufacturers commonly quote an position accuracy figure of plus or minus 15 meters; while modern units can usually achieve better accuracy in real-world use, there is still likely to be some error in the position you measure with GPS. I’ll be covering techniques and tools for maximizing the accuracy of your GPS measurements in a future series of posts; suffice it to say that I took pains to get as accurate a measurement of that position with my GPS as possible.

2. The positions in Google Earth don’t necessarily coincide with the correct position on Mother Earth. In my neighborhood, an exact geographic location shows up offset about 30 feet in Google Earth.

So don’t expect to use GPS with Google Earth (or Google Maps) for high-precision positioning and mapping – it’s just not accurate enough for that.

From: [freegeographytools.com]

And from another source:

Availability of data that make users in different disciplines use Google Earth in positional data extraction encourage specialists to carry out researches in order to test and evaluate positional Google Earth extracted data.
From the measurements carried out in year 2012 and results obtained above, it can be concluded with:

* Google Earth positional accuracy is not fixed but varies from time to another. This may be referred to the process of updating Google Earth by replacing the original images by a better resolution images.
* In spite of changing the positional accuracy positively to be better, but the negative change may be expected.
* Reliability of Google Earth extracted positional data can be supported by making some sort of field check.
* Horizontal accuracy of Google Earth in Khartoum area (Sudan) is about 1.80m. This accuracy can successfully be used to derive planmetric maps with medium and small scales.
* Moreover, height information can be extracted from Google Earth was estimated with 1.73m accuracy. This accuracy enables specialists to have topographic information and assist to produce contour maps with 1:50,000 scale and smaller.
* Google Earth represents a powerful and attractive source of positional data that can be used for investigation and preliminary studies with suitable accuracy and low cost.


Full PDF: [www.ijmse.org]

All the best,

webmaster Sfakia-Crete.com

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