Google Earth - Imagery Resolution and Accuracy

Imagery Resolution and Accuracy

Most land areas are covered in satellite imagery with a resolution of about 15 m per pixel. This base imagery is 30 m multispectral Landsat which is pansharpened with the 15 m Landsat imagery. However, Google is actively replacing this base imagery with 2.5 m SPOTImage imagery and several higher resolution datasets mentioned below. Some population centers are also covered by aircraft imagery (orthophotography) with several pixels per meter. Oceans are covered at a much lower resolution, as are a number of islands; notably, the Isles of Scilly off southwest United Kingdom were at a resolution of about 500 m or less, although this has since been addressed.

Google has resolved many inaccuracies in the vector mapping since the original public release of the software, without requiring an update to the program itself. An example of this was the absence from Google Earth's map boundaries of the Nunavut territory in Canada, a territory that had been created on April 1, 1999; this mistake was corrected by one of the data updates in early 2006. Recent updates have also increased the coverage of detailed aerial photography, particularly in certain areas of western and central Europe.

The images are not all taken at the same time, but are generally current to within three years. However, with the release of Google Earth 5.0, it has historical images dating back to the 1940s in some spots. Image sets are sometimes not correctly stitched together. Updates to the photographic database can occasionally be noticed when drastic changes take place in the appearance of the landscape, for example Google Earth's incomplete updates of New Orleans following Hurricane Katrina, or when placemarks appear to shift unexpectedly across the Earth's surface. Though the placemarks have not in fact moved, the imagery is composed and stitched differently. Such an update to London's photography in early 2006 created shifts of 15–20 metres in many areas, noticeable because the resolution is so high.

Place name and road detail vary greatly from place to place. They are most accurate in North America, Europe and Australia, but regular mapping updates are improving coverage elsewhere.

Errors sometimes occur due to the technology used to measure the height of terrain; (Note: The following examples no longer exist in the terrain data now used in Google Earth). For example, tall buildings in Adelaide, Australia, cause one part of the city to be rendered as a small mountain, when it is in fact flat. Also, in Downtown Los Angeles, there is a large hill in the approximate location of Bunker Hill, where many of the skyscrapers are located. The height of the Eiffel Tower creates a similar effect in the rendering of Paris. Also, prior to the release of version 5.0 in February 2009, elevations below sea level were presented as sea level, for example: Salton City, California; Death Valley; and the Dead Sea were all listed as 0 m when Salton City is −38 m, Death Valley −86 m, and the Dead Sea −420 m.

Where no 3 arc second digital elevation data was available, the three dimensional images covering some areas of high relief are not at all accurate, but most mountain areas are now well mapped. The underlying digital elevation model has been placed 3 arc seconds too far north and up to 3 arc seconds too far west. This means that some steep mountain ridges incorrectly appear to have shadows extending over onto their south facing sides. Some high resolution images have also been misplaced: an example is the image covering Annapurna, which is misplaced by about 12 arc seconds. Elevation data was recently updated to 10-meter (1/3-arc-second) resolution for much of the United States from the previous 30-meter (1-arc-second) resolution.

In some areas, local government jurisdictions have submitted more finely gridded terrain models through the Map Content Partners program. In March 2010, the County of Marin, just north of the Golden Gate Bridge by San Francisco, California, published a 40 cm gridded terrain surface of 1425 km2 through the program.

The "Measure" function shows that the length of equator is about 40,030.24 km, giving an error of −0.112% compared with the actual value of 40,075.02 km Earth; for the meridional circumference, it shows a length of about 39,963.13 km, also giving an error of −0.112% compared with the actual value of 40,007.86 km.

On December 16, 2007, most of Antarctica was updated to a 15 m resolution using imagery from the Landsat Image Mosaic of Antarctica (1 m resolution images of some parts of Antarctica were added in June 2007); however, the Arctic polar ice cap is completely absent from the current version of Google Earth, as are waves in the oceans. The geographic North Pole is found hovering over the Arctic Ocean and the tiling system produces artifacts near the poles as the tiles become 'infinitely' small and rounding errors accumulate.

Cloud cover and shadows can make it difficult or impossible to see details in some land areas, including the shadow side of mountains.

Some parts of the world have still not been photographed. Very low resolution photos or false color photos indicate that the area has not been photographed. On Google Earth 6.0 it is easy to see areas that are not photographed by simply clicking the Historical imagery option on.