So…ich hab mal ein wenig den Gedanken freien Lauf gelassen.
Wenn man davon ausgeht, dass Kurve=Kreisbogen, ist es mit dem Datentyp curve recht simpel.
<?xml version='1.0' encoding='UTF-8'?>
<osm version='0.6'>
<node id='1' lat='...' lon='...' />
<node id='2' lat='...' lon='...' />
<node id='3' lat='...' lon='...' />
<curve id='1' >
<nd ref='3' /> //Startpunkt
<nd ref='2' /> //Hilfspunkt
<nd ref='1' /> //Endpunkt
</curve>
</osm>
Also egtl. ein wie ein Way nur mit genau 3 Nodes. Als kleine Demo hab ich es mal für einen Straßenabschnitt fix umgesetzt (trunk auf Mapnik-Primary, damit man es besser sieht):
Damit josm mit den Daten klar kommt, hab ich den Datentyp bei way belassen, zum Nachvollziehen aber curve=yes getaggt und den Hilfspunkt mit curve=middle getaggt. Beides ist mit dem Datentyp curve (siehe oben) nicht nötig.
<?xml version='1.0' encoding='UTF-8'?>
<osm version='0.6' upload='true' generator='JOSM'>
<node id='-58' action='modify' visible='true' lat='50.97624267299224' lon='11.284868318566938' />
<node id='-56' action='modify' visible='true' lat='50.977472509120204' lon='11.28845722585104' />
<node id='-54' action='modify' visible='true' lat='50.98111183334305' lon='11.291720590660256' />
<node id='-52' action='modify' visible='true' lat='50.98141676376676' lon='11.291657070177354' />
<node id='-50' action='modify' visible='true' lat='50.98317132450159' lon='11.292062013255869' />
<node id='-48' action='modify' visible='true' lat='50.98435599040213' lon='11.292284334946036' />
<node id='-46' action='modify' visible='true' lat='50.98523072430183' lon='11.292284334946036' />
<node id='-44' action='modify' visible='true' lat='50.987679891534434' lon='11.291180666555572' />
<node id='-42' action='modify' visible='true' lat='50.989524179113495' lon='11.289394152973882' />
<node id='-40' action='modify' visible='true' lat='50.99463182341778' lon='11.291188606615933' />
<node id='-38' action='modify' visible='true' lat='50.99615101423253' lon='11.294801334081122' />
<node id='-36' action='modify' visible='true' lat='50.998042099052995' lon='11.29783956551705' />
<node id='-34' action='modify' visible='true' lat='50.99984412350129' lon='11.30159003182825' />
<node id='-32' action='modify' visible='true' lat='51.00016918712704' lon='11.30342034921964' />
<node id='-30' action='modify' visible='true' lat='51.000416516620994' lon='11.304262519798504' />
<node id='-28' action='modify' visible='true' lat='51.00187926695615' lon='11.309214482802213' />
<node id='-26' action='modify' visible='true' lat='51.00249403231801' lon='11.31265053876397' />
<node id='-24' action='modify' visible='true' lat='51.00130689188247' lon='11.30663182636037'>
<tag k='curve' v='middle' />
</node>
<node id='-22' action='modify' visible='true' lat='51.00026433469981' lon='11.303872231459675'>
<tag k='curve' v='middle' />
</node>
<node id='-20' action='modify' visible='true' lat='50.999219988940716' lon='11.29937815729419'>
<tag k='curve' v='middle' />
</node>
<node id='-18' action='modify' visible='true' lat='50.99705127241943' lon='11.296710297012199'>
<tag k='curve' v='middle' />
</node>
<node id='-16' action='modify' visible='true' lat='50.992198776887136' lon='11.288524094777891'>
<tag k='curve' v='middle' />
</node>
<node id='-14' action='modify' visible='true' lat='50.98665101830255' lon='11.292033601458362'>
<tag k='curve' v='middle' />
</node>
<node id='-12' action='modify' visible='true' lat='50.9836819453887' lon='11.292176522544896'>
<tag k='curve' v='middle' />
</node>
<node id='-10' action='modify' visible='true' lat='50.98237229395303' lon='11.29166835868166'>
<tag k='curve' v='middle' />
</node>
<node id='-8' action='modify' visible='true' lat='50.97900302087013' lon='11.29141427675004'>
<tag k='curve' v='middle' />
</node>
<way id='-90' action='modify' visible='true'>
<nd ref='-58' />
<nd ref='-56' />
<tag k='highway' v='primary' />
</way>
<way id='-88' action='modify' visible='true'>
<nd ref='-30' />
<nd ref='-24' />
<nd ref='-28' />
<tag k='curve' v='yes' />
<tag k='highway' v='primary' />
</way>
<way id='-86' action='modify' visible='true'>
<nd ref='-28' />
<nd ref='-26' />
<tag k='highway' v='primary' />
</way>
<way id='-84' action='modify' visible='true'>
<nd ref='-32' />
<nd ref='-22' />
<nd ref='-30' />
<tag k='curve' v='yes' />
<tag k='highway' v='primary' />
</way>
<way id='-82' action='modify' visible='true'>
<nd ref='-34' />
<nd ref='-32' />
<tag k='highway' v='primary' />
</way>
<way id='-80' action='modify' visible='true'>
<nd ref='-36' />
<nd ref='-20' />
<nd ref='-34' />
<tag k='curve' v='yes' />
<tag k='highway' v='primary' />
</way>
<way id='-78' action='modify' visible='true'>
<nd ref='-38' />
<nd ref='-18' />
<nd ref='-36' />
<tag k='curve' v='yes' />
<tag k='highway' v='primary' />
</way>
<way id='-76' action='modify' visible='true'>
<nd ref='-42' />
<nd ref='-16' />
<nd ref='-40' />
<tag k='curve' v='yes' />
<tag k='highway' v='primary' />
</way>
<way id='-74' action='modify' visible='true'>
<nd ref='-40' />
<nd ref='-38' />
<tag k='highway' v='primary' />
</way>
<way id='-72' action='modify' visible='true'>
<nd ref='-46' />
<nd ref='-14' />
<nd ref='-44' />
<tag k='curve' v='yes' />
<tag k='highway' v='primary' />
</way>
<way id='-70' action='modify' visible='true'>
<nd ref='-44' />
<nd ref='-42' />
<tag k='highway' v='primary' />
</way>
<way id='-68' action='modify' visible='true'>
<nd ref='-50' />
<nd ref='-12' />
<nd ref='-48' />
<tag k='curve' v='yes' />
<tag k='highway' v='primary' />
</way>
<way id='-66' action='modify' visible='true'>
<nd ref='-48' />
<nd ref='-46' />
<tag k='highway' v='primary' />
</way>
<way id='-64' action='modify' visible='true'>
<nd ref='-52' />
<nd ref='-10' />
<nd ref='-50' />
<tag k='curve' v='yes' />
<tag k='highway' v='primary' />
</way>
<way id='-62' action='modify' visible='true'>
<nd ref='-54' />
<nd ref='-52' />
<tag k='highway' v='primary' />
</way>
<way id='-60' action='modify' visible='true'>
<nd ref='-56' />
<nd ref='-8' />
<nd ref='-54' />
<tag k='curve' v='yes' />
<tag k='highway' v='primary' />
</way>
</osm>
Evtl. hat ja jemand auch an dem Thema Interesse und die Kenntnisse einen kleinen Renderer zu schreiben, der das umsetzen würde, oder hat bessere/andere Vorschläge zu dem Thema.