Here is a picture of the defense wall around Grindavik and the potential location. Construction has started today. Tha map below shows elevations around Grindavik and the Eruption and fissure that formed on Dec 18th. An eruption along that red line is still considered most likely although it can happen further south as well. A potential lava flow from there can flow into Grindavik, that is why they have started to build a defense wall to divert lava around the town and into the sea. You can see elevations on the right. Grindavik lies at sea level and lava will flow downhill towards Grindavik should the eruption happen either long enough or close enough to Grindavik. It is more likely that the next Eruption will be closer to Grindavik according to Scientists. 
