New maps: lake generalization and polar views


I updated the demonstration map with generalized lakes as well as views in polar stereographic projection in north and south.  Also included are some improvements in relief rendering and in the map display in the browser.

Like the coastline the lakes are based on Openstreetmap data.  Since – unlike the coastlines – the Openstreetmap lake data is not commonly processed as a whole quality is much more variable.  In particular there are no uniform rules what features are tagged as lake, river or reservoir.  With lakes and coastlines now present in the map the rivers are of course seriously missing.

Processing the Openstreetmap data in polar projection leads to additional problems.  There is the implicit (although not formally documented) assumption that data structures in Openstreetmap (in particular ways and multipolygon relations) are to be interpreted in web mercator projection.  This can be boiled down to a simple example: take two neighboring nodes (points) in a way (linestring).  The connection between those two nodes is assumed to go through the midpoint in web mercator coordinates.  This however is not usually the midpoint in other coordinate systems.    How visible this effect can be depends on the map projection used.  In plate caree (EPSG:4326) line segments directly in north-south or east-west direction have the same midpoints as in web mercator projection so this issue only shows up with longer diagonal line segments.  In polar stereographic projection only lines in north-south direction are not affected. This leads to serious problems when there is a larger distance between nodes as can be seen in my arctic map.

I also updated the generalized coastline data available for download.

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