Global Positioning System Reference
In-Depth Information
Google Earth
The most popular geographic information systems (GIS) application, without
a doubt, is the Google Earth personal computer application. It is a product of
the third millennium—a virtual globe that is possible only because of the
reams of digital data made available to us from radar satellites and optical
eye-in-the-sky satellites and from databases containing all sorts of geospatial
data. Most readers will have some familiarity with this computer program.
You may already have zoomed in from outer space and hurtled headlong
toward the earth, viewed in bird's-eye view until you got very close; then the
angle changed, and you appeared to fly across the surface of the earth.
You will by now appreciate the considerable amount of data and data
processing that is required to produce a program like Google Earth. The map
projection that is used when we initially zoom in toward Earth, looking down
from directly above, is called the
general perspective
projection. It is like an
orthographic projection, except that the viewpoint is not infinitely far away
but is instead at a finite distance, one that decreases as we zoom in. In other
words, Google Earth tries to present us with a photographic image of what
we would actually see.
The data are arranged in layers, which declare themselves as we zoom in.
We can choose additional layers—topographical or demographic details such
as roads, political boundaries, store locations. We can change the angle and
zoom in or out diagonally and approach a given location—say, Prague in the
Czech Republic—from different directions, giving us the impression of a 3-D
view. Other data, such as the NASA radar data of the Grand Canyon, are
processed using software that generates relief maps and so creates more
convincing 3-D views: with Google Earth you can fly along the Grand Canyon.
I mention Prague because it happens to be the city that can be viewed with the
greatest clarity: it is imaged with a resolution of 0.1 m (most other cities are
presented at a resolution of 0.3-1.0 m). Rural land is generally provided with
a lower resolution (15 m).
The fusion of several different types of data makes the presentation a little
uneven (for example, a low-resolution strip of land abuts a higher-resolution
strip and looks quite different), but the potential for the future is apparent:
better data will permit improved resolution, with sharper images—and we
won't be able to see the join between two datasets. Indeed, this type of im-
provement is ongoing.
If your computer has a joystick, then Google Earth (which is free) can be
used as a flight simulator. In addition to Earth, we can view the moon and
Mars.
