building a map using DEM generated terrain

martinvk

since 10 Aug 2002
When building a route with terrain elevations generated by DEM data, it is rare that there are any truly flat areas. From my understanding of how DEM data is made, the tops of trees, buildings and other objects influence the data so what might look like hills are actually trees, buildings, etc.

So when building a station or railyard, can we assume the terrain should be relatively flat, and thus any bumps and other elevations are not the ground itself but objects on the ground? At least in Trainz, the turnouts are very sensitive to elevation differences between their various parts.

Do you use the terrain panel and flatten the part of the baseboard where the tracks are placed? Do you use the smallest radius to only affect the area directly under the tracks or level the whole general area? Do you use the average or the lowest point as a baseline elevation?
 
You are correct to assume that bumps etc can be tree tops and buildings. This also means that the elevations of a heavily forested area will be higher in the DEM than in the real world.

I proved this to myself when I created a DEM that included Vancouver BC and noticed the bumpiness in downtown Vancouver where I knew the terrain was flat but had many high-rise buildings.

As far as the rest of the question, it's a matter of individual taste and whether you are creating an exact prototype or using TransDEM to create realistic terrain for a non-prototypical route, which is what I usually do. So I do a lot of levelling when laying track for the reasons you mentioned.

Either way, Google Maps or Google Earth can be very useful in the route building process.
 
Depends on the source of the DEM, some do ignore buildings and trees, for example in the UK you can get Lidar with buildings or without.

The DEM I used for my Ffestiniog Route which was done with Microdem and Hog was a tree line DEM, easy to check by measuring a few heights and comparing to maps Google Earth and such and consequently I had to do a lot of manual adjustment.
Best is Lidar if it's available as you get embankments and such current L&B WIP is Transdem using Lidar, far better and minimal hand carving required.
 
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