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[udig-devel] Re: How to get the display properties
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Ian wrote:
> But as a WMS I just send the client an image, the client gets no
> choice as to how it renders that image.
OK. I see the problem now. I didn't consider WMS in this context, since I'm
plainly not very familiar with it, especially with the specification.
Dave wrote:
> Summary - for unknown display sizes use "0.28mm × 0.28mm" as your pixel
> area. Near the bottom of page 27.
WMS Specification 1.3 says (pp. 20/21):
7.2.4.6.9 Scale denominators [...]
"Because maps from a Web Map Service may be viewed on arbitrary displays rather
than on paper, the values used are actually the scale denominators relative to a
common display pixel size. The intent of scale denominators is not the
translation between actual and standard scales always be completely
accurate."
[...]
For the purposes of this International Standard, the common pixel size is
defined to be 0.28mm × 0.28mm (millimetres). Because arbitrary clients can
request maps from a server, the true pixel size of the final rendering device
is unknown to the server.
-----END OF CITATION
Again, I'm not familiar with WMS, but if there obviously is no way to send the
display dpi value together with the map request to the server then I can only
conclude that WMS-generated maps are NOT suitable for precision mapping as used
by cartographers and public offices.
Using just a "standard" value of 0.28mm for pixel size is just good enough for
some road mapping services and tourist maps on websites, perhaps. But I like it
precise. If I order a 1:50000 map for display I don't want to receive one that
is 1:48344 version on my display.
BTW.: For printing this seems not to be big a problem. All current image
formates contain a dpi value stored in their metadata. Image processing
programs usually take this into account when printing. So, in contrast to the
display case, the WMS server can determine with what size a map should get
printed in order to have the required scale and the correctly sized features.
So if the WMS server assumes 0.28mm pixel size (~90 dpi) because it doesn't know
better, it renders the image accordingly and simply tags the image to have a
resolution of 90 dpi. This image should print just fine on every printer.
Generally formulated: Images are a really bad means of transportation for maps
if precision in scale and styling matters and if the map is to be displayed,
not printed.
Matthias Basler
c9bama@xxxxxxxxxxx
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