I use the contour plugin (v1.1) within QGIS 2.4 to create contour lines. I have streets with different travel costs. I observe problems for streets with low travel costs if they are surrounded by streets with high travel costs. The low cost street is "piercing" into the low speed area (cf. picture). The points are created as the travel cost markers from Dijkstra algorithm using pgrouting 2.0.5. The Dijkstra algo. calculates correct values for every street.
The problem I observe is now, that due to low speed areas left and right to the highway, the isolines are connecting both areas over the highway (yellow). Based on values, the correct behavior would be the one shown in green.
Can someone recommend me a way of preventing such results?
Before I used GDAL TIN interpolation and contour mapping, which leads to better results if it comes to complex networks. On the other hand it adds transformation errors (rasterizing) and extra steps to the calculations.