[OSM-talk] Limitations of renderers

[Steve Chilton]

I have been following with interest the thread on tagging and rendering,
and would like to make a slight jump to comment on the inherent
limitations for rendering the results.

Whilst I do not wish to stifle the use of a wide-ranging tag-set, and
applaud the attempts by folk to get agreement on track surface types,
variations in access to highways, types of cycle paths, climbing route
grades, types of aerial runways/chairlifts, and deeper and deeper
landuse classifications, it should be remembered that there is a limit
to the human eye's ability to distinguish between similar colours or
linestyles in maps and graphical outputs.

There is already a problem on the mapnik slippy map in distinguishing
between the green tones employed for various landuse types already in
play. Similarly, there is an ever increasing proliferation of linestyles
being employed to try to map the variations in types of "way" in
different situations. For instance, are you aware of the 7 different
styles of purple line used to distinguish admin boundary types on the
mapnik layer?

So, my point is that if you should reach agreement/concensus on
new/deeper tagging, don't automatically think it will be possible to
transfer these new "flavours" to the standard renderers (osmarender or
mapnik). In the last couple of days I have had requests to render to the
mapnik layer natural=cliff and historic=citywalls, and have seen a talk
request for rendering nature reserves a second way to incorporate
loacations that are aprt of wholly water-based (at the moment it is a
green fill with NR overprint). Now there is nothing wrong with any of
those requests, but it just adds further complexity to the range of
linestyles and fills that have to be in place, and be visually
distinguishable, and keyed in the map legend, etc. Some of the examples
listed in para 2 are actually better addressed by specific render cases
(a bike map, piste map, climbing map for example for some). I imagine
these alternate renders will have to become even more abundant. As folk
try to map in depth in their particular field of interest they will need
to develop in parallel appropriate rendering styles/outputs (as has very
successfully happened with cycle interests already).

Anyone wanting to read further on graphical symbology and limitations
should seek out some of the seminal texts that have been produced by
authors such as Bertin, Tufte and Krygier.

Cheers
STEVE

Steve Chilton, Learning Support Fellow
Learning and Technical Support Unit Manager
School of Health and Social Sciences
Middlesex University
phone/fax: 020 8411 5355
email: steve8 at mdx.ac.uk
http://www.mdx.ac.uk/schools/hssc/staff/profiles/technical/chiltons.asp

Chair of the Society of Cartographers: http://www.soc.org.uk/

SoC conference 2008:
http://www.abdn.ac.uk/cartographers08/