Geography Reference
In-Depth Information
language that specifies a set of features (placemarks, images, polygons, three-dimensional
models, textual descriptions, etc.) for display in Google Earth.
The resolution and currency of the photographic images varies across the Google Earth's
surface with key urban areas in Western countries often having a resolution as high as 15-30
cm with rural areas and less developed countries typically having a resolution of 15 m or less
(Nourbakhsh, 2006). The average refresh cycle of the images is around 18 months, although
this is shorter in some key urban areas. Indeed, because different images are mosaiced to-
gether from a variety of sources, a single city or region may have consecutive images taken
from different months which can be noticeable if adjacent images show different weather
conditions. Not all cities are depicted in high resolution and Google's database contains
more imagery of the United States than elsewhere (Trimbath, 2006). This US bias is evident
in place name searches and the default setting to US customary units of measurement rather
than standard international units. There are also some problems with the accuracy of pho-
togrammetric measurements obtained using Google Earth's tools (Goodchild, Chapter 2).
In July 2005 MSN launched MSN Virtual Earth (also known as Microsoft Local Live),
which is similar to Google Earth but does not yet cover the whole planet (Biever, 2005).
MSN Virtual Earth also includes oblique photographs, called the 'birds-eye' view, of several
US and UK cities. Figure 7.1 is a comparison of Google Earth and MSN Virtual Earth for the
same area - a rural landscape a few miles southwest of Bristol, UK. The resolution for this
area in MSN Virtual Earth is very high, allowing a clear view of a farm and out-buildings. The
corresponding photograph from Google Earth shows a much lower resolution image with
the farm barely visible in the mosaic of fields. The difference in the resolution of the images
is a good example of the inconsistency in aerial photography provision by internet portals,
with different portals sourcing images for the same location from different companies.
This raises reliability and data quality issues when using images from internet portals for
research and decision-making in planning and also poses potential problems surrounding
data standards and ownership.
The added value of the oblique photographs is demonstrated in Figure 7.2, which shows
a property in Bristol orthogonally and obliquely from the four cardinal compass directions.
Photo-interpretation is often easier using an oblique photograph because the profile view
Figure 7.1
A comparison of differences in resolution of images between MSN Virtual Earth (left)
and Google Earth (right): a farm 5 miles to the southwest of Bristol, UK, October 2006 (source:
author screenshot from http://earth.google.com and http://maps.live.com)
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