These can actually be quite a faff to create.
Initially I looked at using actual DEMs (digital elevation models), stitching them together, and the cutting the models into horizontal slices to create these. However, the easily available DEMs of national parks tend to be quite detailed and therefore, very high poly. Additionally I had to stitch together about 12 - 14 of them for the Rocky Mountain park, resulting in a model that was over 9 million polys. I tried using a poly reduction however if I did that with the whole map it ticked away for hours before I gave up.
I then tried loading the DEMs in strips, doing a poly reduction, then stitching the new, lower-poly model together. Quite tedious, but it worked! I then made my horizontal slices, turned them into paths. However the paths were quite messy. It took me almost 3 days to tidy them up and then group them according to their vertical placement. Then, finally, it was ready to extrude!! And it looked crap. Well, not crap, but the large number of slices meant the result was quite detailed and it felt too busy. I almost gave up then, having poured so much time into it. I hadn't really sat down and done any preliminary sketching, and that quite often leads to dead ends for me.
In a last-ditch attempt at saving the idea, I switched back to illustrator. I was lucky that while looking for topo maps of RMNP, I had come across an elevation map split into big sections. I was able to use that as a guide to create more simplified elevation guides. For two of the maps I wasn't able to find anything like that, so I ended up using low poly models of the parks that someone else had created, applied vertical gradient, rendered out the resulting elevation map, then used that to make the linework in Illustrator. And then of course brought it all back into c4d. Round and round and round we go...