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MAP DISPLAY FORMATS
by Dale DePriest - All rights reserved.
For digital computer use there are
basically two display formats used for maps. These are bitmap
maps and vector maps. This article focuses on exactly how
these imaging technologies differ and when you may want one
versus the other. Each of these map forms can be augmented
with a poi database as well.
A bitmap is not a map in the way we
use the word in geodetics. It is an image of the pixels on
a computer screen, or other digital display device. The image
is formed out of individual pixels that when arrange in a
rectangular form to indicate a recognizable image for us.
We can view this image as a picture but, like the pictures
we see in the newspaper, if you look really close you will
see that the image is nothing more than a bunch of dots. Not
only is the image viewed on the screen this way it is stored
in the database this way as well. In the most primitive form
a bitmap on the screen is an exact image of the bitmap in
the file on the disk. One form of this type is simply called
a bitmap or bmp file after it file extension. This displays
rapidly on the screen but takes a huge amount of storage on
the disk. Since a map usually consists of some lines on a
large background area it would be really nice to compress
the disk copy to save some space. One method, supported transparently
to bmp by Microsoft is called rle. Another really popular
method is called gif. The design of gif can provide excellent
compression particularly for maps. Unfortunately the design
of gif is patented and many companies won't pay the royalties
so they use a different, possibly less efficient scheme. One
such scheme is jpg. This format is excellent for pictures
of people and scenery but less suited for maps, since it can
be a lossy compression scheme that tends to blur lines just
a bit. To overcome this limitation the png format has been
developed. Another scheme is tif or tiff. Tiff is basically
as inefficient in disk use as bmp files but there is a version
of compressed tiff that helps the image size significantly.
Of course, a compressed image must be uncompressed before
we can view it so computer power is needed to avoid really
slow viewing of compressed images. Another format you will
hear about is drg, digital raster graphics. These are nothing
more that tiff files with some calibration data and is used
for USGS maps. There are other image formats as well but you
are not as likely to see them used to store maps.
Bitmaps, also known as raster images,
can certainly be used as maps. You can use a drawing of a
map or you could even use an actual photograph as a map. You
could even draw your own map from scratch using a bitmap editor.
But note that it was you that interpreted the data as being
like a map. To the computer it is treated like a picture.
There may be roads indicated on the map, or buildings, or
water, or any number of other things but the computer simply
knows about the image color of individual pixels and nothing
about how you are interpreting it. This is one of the big
limitations in using bitmaps as maps.
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