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MAP DISPLAY FORMATS
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With a vector map there is no image
in the database. Instead an image is created on the fly by
using information from a database. The database itself consists
of coordinates defining points and information on how to connect
those points together to form lines and other objects. In
addition labels attached to the object can provide text to
be displayed along with the object when an image is created.
This sounds complicated but you are probably already dealing
with a database created in exactly the same manner. Consider
a route that you create in a gps. It consists of an ordered
set of waypoints. Each waypoint is defined with a name and
location and the order is used to draw a line connecting them
all together. This is a vector representation of the route
and when viewed on the map/plot screen of your gps it looks
like and is a map of the route. It may seem like a crude map
but this is because you are using only a few points to define
the map. If you had a point for every little turn in the route
then it would look pretty good. This is how the database of
roads in a vector map works. Other objects like lakes work
similarly except that they are a closed system where the lines
come back and close the loop. Then the area inside the closed
polygon is shaded or colored to indicate the object you are
representing and a label may be designed to center itself
within the object.
Since a gps receiver computes a location
it is easy to integrate its location within the locations
of other objects from the database. This is essentially how
a vector mapping program works. It can use mathematics to
compare gps locations with database locations and understand
when a you gps has reached a town or other object. Since the
image is created on the fly by computing a scaled picture
from the database zooming is simply a matter of changing the
scale and redrawing the picture. Panning is accomplished in
a similar manner so that there is no artificial edge to the
map. All of this computation can be very compute intensive
so if the image gets very complicated it can really take a
long time to draw the image. This results in a behavior that
seems sluggish as you try and move the map or zoom out. To
minimize the number of vectors that must be drawn, a computer
database includes attributes assigned to the objects to determine
when they should be drawn based on the scale chosen by the
user. This can decrease clutter on the screen and improve
drawing time. In addition the number of points used to describe
an object can be reduced. This will decrease the size of the
database, increase the drawing speed, but reduce the ability
of the data to accurately reflect every curve and turn in
the road. Some programs are able to describe vectors as curved
objects which can be of great use in describing a tight curve
accurately. The decision on how detailed to make the item
can be varied on a per object basis. In some cases an object
in the database is not described at all but is represented
with an icon which has been predefined for this purpose.
Labels are often located in the database
itself. These can be dynamically added to the vector data
when it is displayed. Some programs use fixed locations for
the label but others may be able to slide the data along the
road so that it fits within the screen boundary. When the
map is zoomed the label data typically does not change in
size which makes it easier to read. As you zoom out the label
may be removed from the display to reduce clutter.
Unlike the case of bitmap drawings
there are no standard formats for mapping databases that are
used by the mapping vendors. Each one has an internal form
that in proprietary and thus one database is designed to only
work with one program. For this reason you will generally
only use the database that was supplied with the program you
bought. Some vendors change the format from year to year to
you can only use maps supplied with that version of the program.
Some programs can use multiple databases but usually only
from the same vendor. In a few cases you may find that the
form of the database is documented or just stored as easily
interpreted text and you can alter or create your own database
entries. Unfortunately because of the size and complications
of the database, even when documented, most users would find
this to be a formidable task. Even within a vendor the database
format can change. For example the database format on the
pc may be different from the way the information is stored
once it is downloaded into the gps. The program performs a
translation during the download.
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