It has been done from building footprint associated with their height and and then blended together by vector software.
- Find the city data of building footprint associated with their height. From this data, use a GIS Software to create different shade of one color for each rank of height.
- With vector software, use the effect "Accented Edges" to create a blend effect.
- Then convert this map in picture and thanks to vector software, trace the picture with multiple scans and color option. This will create several layers of different color.
- Add for each layers a stroke to create a topographic design map.