Why do I get bad signal / traces in city centers / near big metal buildings?
For a GPS to work and achieve some accuracy it needs to receive at least three satellite signals. Often when in city centers there are many tall buildings that can block these signals and stop the GPS from being able to work out where it is. There may also be multi-path effects from the material making up the buildings around you, whereby signals bounce off them so the receiver actually thinks it is somewhere in the buildings around. Another factor that can affect this is the number and position of satellites that can be seen at the time of logging; trying the route on another day or a different time of day may give better results.