[OSM-talk] SVG tiles

[elvin ibbotson]

poco.org.uk

I have developed a mobile-phone Java app (called 'mom') to navigate  
OSM maps and save GPX tracks (amongst other things) which will soon  
be out there for people to download. It uses mapnick PNG tiles at 5  
of OSM's scales (3, 6, 9, 12 & 15) which look nice but are quite big  
files to download to a phone (typically 12kB-15kB for scale 15) so  
take a significant amount of time and eat into a user's data  
allowance to fetch.

I considered using the compact binary downloads aimed at mobile apps,  
but this is raw data and the graphics limitations of mobile Java mean  
the maps drawn from it would not look very pretty.

I am fairly ignorant of OSM data structures and back-room software  
but I understand SVG is used in producing bitmap tiles. As I  
understand it, the idea of SVG is not only to give nice, scalable,  
graphics, but to do so using smaller file/download sizes than  
bitmaps. Many/most of the newest mobile phones are able to draw SVG  
graphics in Java, as are browsers, and desktop Java will soon include  
SVG graphics, so it looks to me like the way forward. If tiles were  
available as SVG I am sure it would be relatively easy to substitute  
them for bitmap tiles in slippy maps or apps like mine. Not only  
would downloads be faster but a smaller range of scales would be  
needed, with the same data set and appearance being used for a range  
of scales and scaling of the SVG image used for intermediate (or  
infinitely adjustable) scales.

I had been intending to get round to mailing this list enquiring if  
SVG downloads were possible/available when the Export tab appeared.  
My initial delight** was slightly diminished when I exported a map in  
two formats - SVG and PNG - and found the SVG version was 340kB while  
the PNG file was 132kB. A glance at the SVG data suggests that text  
is actually drawn (sometimes more than once (for background then  
again for the text itself) using long, elaborate paths and shape  
definitions of every character at every orientation and size, rather  
than just using the SVG text element!!! I suspect it also  
incorporates bitmap images as icons rather than using SVG  
definitions. I think SVG is the way forward, but not if the file  
sizes end up almost three times bigger than bitmaps!

I suggest that, if has not already been done and is hidden somewhere  
I haven't looked, that a server should be dedicated to scalable map  
tiles using a compact and efficient implementation of SVG coding.

**Hats off to those involved, by the way <     :-)

elvin.ibbotson at poco.org.uk





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