Well what’s being suggested in the OP is called gray boxing where the intent is to establish a source chain from a host application to Unreal4 and replace blocks via the iteration process and using a map as a blueprint for a level is easy to set up once you have the relative scaling set.
How I would set the relative scale I would need to know the dimension of a single landmark like a water fountain which I then would create a proxy object the same size as the true world measurement (let’s say it’s 10 meters across).
In Unreal 4 world space I would create a material and use the map texture as the default defused color.
To lay the map out I would then add a planner object equal in size as to the defused texture in resolution converted to metric units. Since metric converts perfectly to 10 base units the planer object would be 1024X1024 cm=ue4 units which would be relative to the same size as the applied texture/material if it was 1024X1024.
To convert the asset to real world scale we have a proxy of a know fixed size of 10 meters across so you would just have to align the object as to the world location of the water fountain and uniform scale the planer object up until it matches the volume of space occupied on the planer object and tada your map is set to true world scale inside of Unreal 4.
After that I did a series on map conversion from idtech3 to Unreal 4 as to scene management using the iteration process as well as level streaming.
5 episodes in the series and should be enough info to clone a true world scale version of Seattle 