DEM Terrain Quandry

[gisa]

Hi Everyone,


I have what might seem to be a strange/silly question but I'm confused and need some advice.

I created a route using transDEM and am trying to model it after a real life route. Pictures are better than words, so let me explain. First, take a look at this picture:



In the mid upper left you can see brown lines going over the tracks. Note how the tracks cut into the terrain. The brown lines are roads and go over the tracks as an overpass. I can see via google earth and panorama that there is an abutment of earth to help support the bridge.

Up until now, I always tried to lay tracks on top of the DEM terrain. I guess I have several questions I should ask:

1) How is DEM data is collected? Is it the height from space from a satellite that is interpreted as numbers and then mathematically combined to give heights? Because as you can see, if I lay the tracks on top of the terrain, I'd have to raise the land more to compensate reality. That would lead to a nasty gradient that I can't see via panorama.
2) For those who use DEM's, is it normal to have to do some investigative work to know where the track should cut through the terrain and where it shouldn't? This seems logical to me because simply laying the track on top of the terrain would have a killer gradient as I mentioned before.
3) What about other factors such as traffic, trains, snow...does that lead to inaccuracies with the DEM data itself?

Now look at this picture:



If I apply the logic I deduced above and make the tracks cut through the ground, all seems well, but the ground near the overpass is gently sloped and in real life it is not:



So much so that it seems inaccurate. Does this mean I have to adjust that terrain too? There is a shopping mall here:



Which I know is on level terrain so I suppose I'll have to go about adjusting the terrain there manually right?

If so, does this mean that the rest of the data, or that data in DEMs is seriously limited? Hopefully, my questions make sense...

:wave:

Gisa ^^

 

[patchy]

hi gisa,
seems to me you've been around trainz for a long time
DEM is pretty acuate but it's not going to get it right to millimeter
real life railroads do go up and down following terrian
if you think it looks flat it aint
just add splines to make the track undulate a bit and you'll get more realism
word of advice don't try to adjust terrian manually you'll never get it right
it's learning curve i think i've got it pretty right with my dem maps

cheers,
patchy

 

[gisa]

Thanks for the reply Patchy.

You are of course correct in that even a DEM has limitations. It's just that it doesn't make sense in that area based on the pictures I've seen and logic in mind. I suppose I'll have to compromise or something.

For sure, I'm keen on gradients and it's unrealistic to have level track over a consistent basis, but here, it just doesn't add up. Gradient is too high and the track itself, compared to the surrounding scenery is way too high. I'll have to play around with it a bit and think about it.

Again, thanks for the reply. ^^

:wave:

Gisa ^^

 

[JCitron]

Gisa,

I have found too that sometimes the TIGER information is off a bit in relation to the DEM. The colored lines run through hills for example instead of actually following the little cuts and breaks in the terrain, and are actually off by about 1 square or more in some places.

This can be a bit confusing at first, but what I found is if I do the Ctrl+Y and look across the landscape, I can spot where the roads and rail lines should go and compensate for that.

Hope this helps.

John

 

[Jayturf]

I model in mountainous terrain and virtually all of my track has to be hand graded and so forth. Dem data quality really depends on the data source...some have a higher resolution than others. I like to grade the terrain by hand to the track to achieve a more realistic look. check my screenshots out here for a look at what is often done. One of my last posts shows it pretty good. http://forums.auran.com/trainz/showthread.php?t=51395&page=4&highlight=kettle+valley

If you use the built in smooth terrain to track function you get a very wide ROW and it looks bad and really takes a lot of the realism away from the route.

If you can get your hands on grade data for your particular route you will be able to adjust the track exactly as the prototype. This kind of data is hard to come by usually.

good luck

 

[Jerker]

G'day JCitron,

Firstly, John, what does [CTRL]+[Y] do? Secondly, having used TransDEM myself for many years now, I do not believe that gisa's problems are caused by the 'TIGER' data that he hasn't used (the display shown on the terrain in his images is clearly sourced from Topographic maps (the British OS ones, or 'Street Map', perhaps) at a rough guess)...

G'day gisa,

I must remind you (assuming that you have used the freely available SRTM DEM data for your terrain heights) that this data is of 30 arcsec resolution. This means that there is but one item of height datum for approximately every 90 square meters - everything in between these 'spot heights' is 'interpolated' accordingly. My personal advice to you would be to retain the existing track gradient (this is always the overriding factor, in my opinion). If this is 'level' (or relatively so at this location), I would then use the smooth tool to lower the terrain along the sections that are forced under the terrain, which will create a small cutting from about where the track appears to 'disappear' under the terrain in your first image, to about the same distance on the other side of the road bridge. Given that the road that crosses the railway line will need to gain height to do so, there WILL be a cutting here (even if this is only as wide as the abutment of the bridge - it's still a 'cutting'). If this is the case, then I would manually 'level' the rest of the terrain to match the correct surrounding requirements. Always bear in mind that despite what you might think, terrain is NEVER as flat or as level as it looks...

Jerker {}

 

[gisa]

Gisa,

I have found too that sometimes the TIGER information is off a bit in relation to the DEM. The colored lines run through hills for example instead of actually following the little cuts and breaks in the terrain, and are actually off by about 1 square or more in some places.

This can be a bit confusing at first, but what I found is if I do the Ctrl+Y and look across the landscape, I can spot where the roads and rail lines should go and compensate for that.

Hope this helps.

John

Hi John,

Sure it does help. I imported google earth tiles which have demonstrated the limitations/inaccuracies of track placement. This is a Canadian DEM, so I can import vectors of limited accuracy (still useful though, especially for other types like watercourses, powerlines, etc). The DEM data I obtained is supposed to be of high quality if I recall but I guess there are limitations when factoring in urban settings, cuts, fills, and what not. It's okay: I'll just cut through the area and re-level the nearby terrain as per Jerker's suggestion.

I model in mountainous terrain and virtually all of my track has to be hand graded and so forth. Dem data quality really depends on the data source...some have a higher resolution than others. I like to grade the terrain by hand to the track to achieve a more realistic look. check my screenshots out here for a look at what is often done. One of my last posts shows it pretty good. http://forums.auran.com/trainz/showthread.php?t=51395&page=4&highlight=kettle+valley

If you use the built in smooth terrain to track function you get a very wide ROW and it looks bad and really takes a lot of the realism away from the route.

If you can get your hands on grade data for your particular route you will be able to adjust the track exactly as the prototype. This kind of data is hard to come by usually.

good luck

Hi Jayturf,

I would love to get my hands on this kind of data but it's pretty much impossible. The route I'm working on isn't really mountainous thankfully but yeah, I'll have to do some adjusting once I start using the smooth spline tool.

BTW, nice route. Looks VERY scenic!

G'day JCitron,

Firstly, John, what does [CTRL]+[Y] do? Secondly, having used TransDEM myself for many years now, I do not believe that gisa's problems are caused by the 'TIGER' data that he hasn't used (the display shown on the terrain in his images is clearly sourced from Topographic maps (the British OS ones, or 'Street Map', perhaps) at a rough guess)...

G'day gisa,

I must remind you (assuming that you have used the freely available SRTM DEM data for your terrain heights) that this data is of 30 arcsec resolution. This means that there is but one item of height datum for approximately every 90 square meters - everything in between these 'spot heights' is 'interpolated' accordingly. My personal advice to you would be to retain the existing track gradient (this is always the overriding factor, in my opinion). If this is 'level' (or relatively so at this location), I would then use the smooth tool to lower the terrain along the sections that are forced under the terrain, which will create a small cutting from about where the track appears to 'disappear' under the terrain in your first image, to about the same distance on the other side of the road bridge. Given that the road that crosses the railway line will need to gain height to do so, there WILL be a cutting here (even if this is only as wide as the abutment of the bridge - it's still a 'cutting'). If this is the case, then I would manually 'level' the rest of the terrain to match the correct surrounding requirements. Always bear in mind that despite what you might think, terrain is NEVER as flat or as level as it looks...

Jerker {}

Hi Jerker,


Yup, I got the topographic data from the Canadian government's website.

Yeah, terrain does change and what is odd is that there are sections in this route where the terrain is relatively level for a good portion which makes me wonder...but those sections aren't common or plentiful so I have some faith in the data I have.

Unfortunately, there are limitations with placing objects, rolling them, and if they are even capable of the latter. This commuter route I'm emulating/constructing has many passenger stations and that makes it hard to change a gradient through the station.

So far so good though! ^^

I appreciate all your thoughts and input everyone.

:wave:

Gisa ^^