Tracing of roads and buildings can be a difficult task if the satellite imagery is blurry or the resolution is not good enough to map. But with some interpretation techniques it can be made less difficult. This post covers my experiences with the mapping of buildings & roads and the techniques I used to do it.

Road Tracing

I tried to map some roads in Tiruchengode. The road mapping in Tiruchengode was a great experience overall. Before I started mapping in Tiruchengode most of the primary and secondary roads are already mapped, so it was easy for me to start with the tertiary and residential roads. The road map of Tiruchengode looked like this before I started mapping

Before mapping new roads, I aligned the existing roads and properly tagged the roads. After that I mapped all the roads in and around the city. The road map of Tiruchengode looks totally different compared to before

The major difficulty I faced in this task is in interpreting the roads. Most of the residential roads in this region are between highly dense buildings, so the imagery couldn’t not pick up the reflections from road due to the shadows of the building. In order to overcome this I tried to use shadows and the building association as the proxies. For example :point_down:

The above image show two different ways of interpreting a road:

1) On the left you can see a perfect reflection from a road because it is broad enough to be recorded by the satellite at the given resolution.

2) But that is not the case with the road in the middle of the image, where you can only see some shadows of the buildings because the road is not broad enough. In this case, the alignment of the buildings and the shadow can be used as a proxy (i.e.) it can be easily interpreted that all the building boundaries on the left and right of the shadow is aligned roughly parallel and straight, this can be possible if there is road between the buildings.

The road map in this region can be further improved by adding all the road names and improving the alignment of the residential roads.

Building Tracing

After mapping roads I moved on to map buildings in Thanjavur city. While mapping the building foot prints the resolution of the image played a crucial role, since the resolution of the imagery is good enough to see the buildings but not enough to map the exact boundaries of them. In this region the buildings are built very close to each other and very dense, due to this the extent of them is not clear. So, I mapped only those buildings which I am sure of its foot print.

Before I started mapping in this region only the important buildings like monuments, government offices and tourist attractions were mapped. The image below gives an idea about the state of building foot prints in the region after mapping

So before mapping new buildings I mapped all the building inside a Government Office complex and provided the proper tags to add more detail to the map and then I moved on to adding buildings. I used the Terracer plugin to map the buildings with common shared boundaries.

Apart from resolution, the alignment of road near the buildings have to be corrected in order to improve the accuracy of mapping. This took some time to get used to because of the filters that were active.

There are a lot of buildings to map, this cannot be done without the help of the local community. This map can be improved by adding all the addresses to the buildings and all the associated information.