Question:

We are cyclists and are planning a circuit from Kars to Van via Iran including around Urmia lake. The paper maps show a crossing between Khoy and Van. Google maps show no crossing but it could be that the Turkish side is better maintained. Other sources suggest there is just a train crossing. Searches on this site are also not conclusive.

Answer:

There are two borders opened to foreigners between Iran and Turkey. The first is the Gürbulak–Bazargan border near Dogubayazit in Turkey and Sahabat in Iran, this is the busiest. The second border crossing between Van and Khoy is called Esendere–Sero and is further south. The road turns into a dirt road up in the mountain.
It is better to make sure it is open if you're planning to cross during the winter. The road thourgh Dogubayazit is a beautiful region with the stunning view of the Mount Ararat. The road going through Esendere-Sero is even more scenic.
You can find Sero border on Google map at 37°43'24"N 44°38'26"E.

Check the cheapest Iran tours.