Hardware Reference
In-Depth Information
points in the room. Instead of building a tracking system,
you can now just place a proximity sensor near each
object, look up which he's near, and read how near he is to
it. You're using a combination of spatial organization and
technology to simplify the task. You can make your job
even easier by giving him visual, auditory, and behavioral
cues to interact appropriately. He's no longer passive; he's
now an active participant in the work.
based lookup at www.geocoder.us , and Worldkit offers an
extended version that also looks up international cities:
www.worldkit.org/geocoder .
Street addresses are the most common coordinates
that are mapped to latitude and longitude, but there
are other systems for which it would be useful to have
physical coordinates as well. For example, mobile phone
cell transmitters all have physical locations. It would be
handy to have a database of physical coordinates for
those towers. However, cell towers are privately owned by
mobile telephone carriers, so detailed data about the tower
locations is proprietary, and the data is not in the public
domain. Projects such as OpenCellID ( www.opencellid.org )
attempt to map cell towers by using GPS-equipped mobile
phones running custom software. As there are many
different mobile phone operating systems, just developing
the software to do the job is a huge challenge.
Or, take a different example: let's say you're designing
a mobile phone city-guide application for tourists that
relies on knowing the phone's position relative to nearby
cell towers to determine the user's position. What do
you do when you can't get a reliable signal from the cell
towers? Perhaps you ask the tourist to input the address
she's at, or the postal code she's in, or some other nearby
cue. Then, your program can combine that data with the
location based on the last reliable signal it received, and
determine a better result. In these cases, and in all loca-
tion-based systems, it's important to incorporate human
talents in the system to make it better.
IP addresses don't map exactly to physical addresses
because computers can move. Nevertheless, there are
several geocoding databases for IP addresses. These work
on the assumption that routers don't move a lot, so if you
know the physical location of a router, then the devices
gaining access to the Net through that router can't be too
far away. The accuracy of IP geocoding is limited, but it
can help you determine a general area of the world, and
sometimes even a neighborhood or city block, where a
device on the Internet is located. Of course, IP lookup
doesn't work on private IP addresses. In the next chapter,
you'll see an example that combines network identity and
geocoding.
Step 2: Know the Environment
Before you can determine where you are, you need to
determine your environment. For any location, there
are several ways to describe it. For example, you could
describe a street corner in terms of its address, its latitude
and longitude, its postal code, or the businesses nearby.
Which of these coordinates you choose depends in part on
the technology you have on hand to determine it. If you're
making the mobile city guide described earlier, you might
use several different ones—the nearest cell transmitter
ID, the street address, and the nearby businesses could all
work to define the location. In this case, as in many, your
job in designing the system is to figure out how to relate
one system of coordinates to another in order to give
some meaningful information.
You can develop your own database relating physical
locations to cultural or network locations if the amount of
information you need is small, or if you have a large group
of people to do the job. But, generally, it's better to rely on
existing infrastructures when you can.
Mapping places to coordinate systems is a lot of work, so
most map databases are incomplete. Geocoding allows
you to look up the latitude and longitude of most any U.S.
street address. It doesn't work everywhere in the U.S., and
it doesn't work most places outside the U.S. because the
data hasn't been gathered and put in the public domain.
Geocoding depends on having an accurate database of
names mapped to locations. If you don't agree on the
names, you're out of luck. The Virtual Terrain Project ( www.
vterrain.org ) has a good list of geocoding resources for
the U.S. and international locations at www.vterrain.org/
Culture/geocoding.html . Geocoder.net offers a free U.S.-
Step 3: Acquire and Refine
Once you know where you're going to look, there are
two tasks that you have to do continually: acquire a new
position, and refine the position's accuracy. Acquisition
gives a rough position; it usually starts by identifying which
device on a network is the center of activity. In the inter-
active installation example described earlier, you could
acquire a new position by determining that the viewer
tripped a sensor near one of the objects in the room. Once
you know roughly where he is, you can refine the position
by measuring his distance with the proximity sensor
attached to the object.
 
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