Polar Regions Rendering Issues
The normal Mapnik rendering style used for the main openstreetmap.org map as well as many other map styles are optimized for temperate regions of Europe and North America. This page is meant to collect some of the issues of these rendering styles with respect to the higher latitude (i.e. polar) regions and suggestion how to improve on these.
Variable map scale
The web mercator projection used in nearly all web maps has the important property of a map scale that varies with latitude. This means a pixel in the map equals a different distance in reality depending on where on earth the pixel is. The following table shows the scale ratios and resulting zoom level differences at various latitudes relative to London
|latitude||region example||scale ratio||zoom level|
|30°||India, Mexico, China, south end of Japan, South Africa, Middle East||0.73||-0.45|
|45°||France, northern US, southern New Zealand, Chile, north end of Japan, south parts of Canada||0.89||-0.17|
|51°||London, Brussels, Berlin, Amsterdam, Warsaw||1||0|
|60°||Sweden, southern Alaska||1.25||0.32|
|70.5°||northern Norway, Baffin Island||1.89||0.91|
|75.5°||Devon Island, Novaya Zemlya, Antarctic coast||2.51||1.32|
|82.8°||Northern Greenland, Ellesmere island, Antarctic interior||5.02||2.33|
|85.05°||Edge of web mercator map||7.29||2.87|
This means that for example at 75.5° latitude the map has the same scale at z=15 that it has at z=17 when at the equator. The map rendering style does not usually take this into account.
The possibility to use different map projections is discussed separately on Antarctica/Creating a map.
Specific features and tagging
Polar regions differ from other parts of the earth in various ways that show up in the data:
- Human settlements are fewer and smaller. The northmost settlements that regularly show up at z=6 on the main map are probably Murmansk and Tromsø. Barrow and Alert on the other hand have been tagged for the renderer.
- While in temperate regions landuse=forest and natural=wood tend to be the dominating landcover tags, there are no larger trees north of about 70 degrees north and south of about 55 degrees south. Instead the dominating landcovers are various types of tundra and bare ground as well as glaciers. Of these only natural=sand and natural=glacier are currently rendered by the default style (glaciers starting with z=6, sand with z=10). Sand is not particularly common as a surface type since it is not produced by glacial deposits.
Specific tagging has been necessary to map the Antarctic ice features as it is described in Antarctica/Tagging. This concerns primarily glacial features, in particular the mapping of ice shelves and the inverse mapping of the inland ice (i.e. mapping the ice free areas rather than the ice itself). None of this is currently properly shown in the map.
Suggestions for rendering style changes
Here is a list of changes in the rendering style that would be important for proper display of the polar region (which in turn is important to encourage mapping of these regions). It is split into several parts based on priority.
high priority changes
These changes are most important, without them there is no adequate display of the polar regions, especially in the south.
- make ice (natural=glacier) the default for all land south of 60° south. All land south of this limit that is not covered by other surface types is ice.
- show natural=scree and natural=bare_rock areas, best starting at a fairly low zoom level.
- show ice shelves (natural=glacier + glacier:type=shelf) in a style distinct from normal glaciers
lower priority changes
These changes would be important to encourage differentiated mapping of the polar regions and to discourage tagging for the renderer.
- differenciated rendering of glaciers based on glacier:type=*. glacier:type=shelf and glacier:type=ice_cap could be shown starting at a fairly low zoom level without negatively affecting the result in other regions.
- starting display of various features at a lower zoom level. Some of these would cause problems outside polar regions but can maybe be restricted based on latitude:
- labels for place=island (currently starting at z=12)
- human settlement labels (smaller ones at lower zoom levels)
- roads (smaller ones at lower zoom levels), special display of winter ice roads (surface=ice)
- natural=peak (currently starting at z=11)
- labels for natural=peak (currently starting at z=13)
- labels for natural=bay and natural=cape
- natural=water, waterway=*, natural=glacier
- show areas natural=fell and/or natural=tundra. This is currently not widely used but would discourage use of other tags that often do not fit like (natural=grassland and natural=heath)