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MAP DISPLAY FORMATS
by Dale DePriest - All rights reserved.
(...continued)
The big advantage of raster image
maps is that you can generally get maps from anywhere in the
world if you are willing to scan them in yourself. You also
have full control of the detail. You can even edit the map
yourself with a bitmap editing program to add roads or annotation.
The biggest negative is that this is really only a picture.
There is no underlying intelligence available from these maps.
While raster based programs may offer a zooming feature this
is generally accomplished simply by replacing the current
map with another map having more or less detail. Some programs
zoom in by using pixel replication which gets pretty ugly
after a few zooms since the pixels start looking like blocks.
Zooming out leaves out or merges pixels together in some fashion.
This can be successful but text will soon be unreadable and
roads may disappear entirely. In addition maps can generally
be scrolled to cover a larger area than would be available
on the screen display. At some point you will hit the edge
of the map and you will either have to switch to a different
map or somehow stitch two maps together so that it seems like
one to the viewer. Some programs will automatically switch
maps as you move and will show the available map with the
most detail.
Generally speaking the raster map
programs often do not come with maps. Exceptions include Delorme
that generates its maps on the fly for the palm from the pc
database, the drg viewing program that accompanies the drg
cdroms available from USGS, some specialized topo, marine
and aviation chart programs, and gpss a program that features
extensive worldwide bitmap maps. Most programs will have a
sample map or two that may be useful for some people but you
are expected to supply your own maps. This can be done by
capturing images from the available mapping programs on a
pc, by scanning in paper maps, or by downloading maps from
the web. Any web based map generation program can supply the
maps for these programs. Some programs such as oziexplorer
can also read drg maps directly from a cdrom.
Once you have an image you may need
to save it in a form that is recognized by the mapping program
you intend to use. Usually photo and graphic editing programs
can be used to view the image, crop it as required, and resave
it in the image type you need. Luckily the image formats are
pretty standard and this is typically an easy job. For computers
like the palm that does not have a standard image format for
maps you will need a custom program to convert your map to
a form that is recognized by the program you intend to use.
This image converter is usually supplied as part of the software.
However it is not enough just to have
a picture you will also need to calibrate it before it can
be used with a gps. Because a flat map must be projected from
a curved earth there are distortions in the image itself.
In addition it may not be oriented exactly north/south due
to the projection used or perhaps to errors when you scanned
it in. The minimum calibration is to supply 2 points at opposite
corners of the image. The program then assumes that the map
is oriented correctly to north and is linear. That is to say:
any pixel location can be interpreted directly in terms of
the grid system it was calibrated with. In this way a gps
position can be translated from coordinates to pixel positions
and indicated on the map. If the program will accept 3 points
then it can use the third point to compensate for a map that
isn't oriented exactly north/south. Additional calibration
points can permit a program to adjust for non-linear maps
that vary in a prescribed way over the surface. For small
maps 2 points is enough but for larger areas you may be restricted
to using a map that has a projection system that can be supported
with only 2 points. Drg maps come with calibration information,
for others you will have to figure it out for yourself. You
may have a grid drawn on the map that you can use or you may
have to place some calibration points on the paper map before
you scan it. You may even be able to use objects drawn on
the map that have known locations. However you get them, they
need to be accurate. The maps usefulness depends on the accuracy
of this calibration.
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