[OSM-talk] rendering some large maps, e.g. whole world

D Tucny d at tucny.com
Sat Apr 18 09:34:06 BST 2009


2009/4/18 Torsten Mohr <tmohr at s.netic.de>

> Hello,
>
> i installed PostgreSQL / PostGIS and use a modified version of this
> script to render maps:
>
> svn.openstreetmap.org/applications/render/mapnik/generate_image.py
>
> I've just created two maps of Germany, one is 12000 x 16000, the other
> is 3000 x 4000.  I then get two different 'levels of detail', as expected.
>
> I'd like to print a map of Germany as a poster.  My understanding is that
> osm.xml is configured to create maps that look fine on a screen.
> But the Pixels per Inch on a computer monitor are different compared to
> the PPI on a printed poster.
> So e.g. text size, symbol size and others may not look optimal when
> printed.
>
> Has anybody got any experience with changing osm.xml to create maps
> that look fine when printed (readable text, ...)?
>
> Or do i worry too much and printing PNGs looks just all right?
>

A standard/typical print resolution is 300dpi, whereas a standard/typical
screen resolution would be 96 dpi... So, if you had an image, viewed at
native resolution, on a screen that took up an area 30cm by 30cm and looked
good, to print it at it's native resolution the image would be 10cm by 10cm
(approximately)... Your 12000 x 16000 image would be good for printing about
1m by 1.3m at 300dpi, any smaller and you would loose some of that
resolution, any bigger and the image would start to appear pixelated... Your
3000 x 4000 image would be good for up to 25cm by 34cm at 300dpi...

If you wanted to print an A0 poster, with no margins, you have a print area
of 118.9cm by 84.1cm, so, an optimum resolution for your image would be
14043 x 9933 if printing at 300dpi... If you view this image at 33% scale on
your computer, you'll get a rough idea of the size of features when printed
(though you may want to work out the real DPI of your screen if you want it
to be more accurate, e.g. the screen I'm using now is 37.6cm wide with a
horizontal resolution of 1280 pixels, or 86dpi, so I'd get more accurate
feel of size by using a scale of 29% when viewing the image on screen)...

The maps are typically rendered for viewing on a screen, so, text and
symbols may appear too small when printed, you would likely need to adjust
their size for print, but, how much you adjust it can be a taste thing...

I haven't done it with a map myself, but others have and hopefully they will
be able to contribute here with their experiences...

2.
> When rendering the whole world with coordinates like this:
>
> # unused:   ll = (4.5, 46, 16, 56) # Germany
>    ll = (-180.0, -90.0, 180.0, 90.0) # World
>
> Then i just get an empty file (just background).
>
> Is this related to the scale denominators in osm.xml?
>

It likely is a combination of the scale and potentially a lack of the coast
shapefiles... The lowest level of detail just has the coast line rendered
from the shapefiles...


>
> Can anybody give me a hint on what values to use when printing
> the whole planet?
>

To give accurate values, people would need to know what size image you will
be looking to create, so, would need to know the size and resolution you
will be printing at...

Hope that helps...

d
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