Is Transdem or DEM in general inaccurate for ground distance?

Chris750

Member
Putting in mile markers on my Polar Bear express route (TS12) that travels between Cochrane Ontario and Moosonee Ontario, a track mile distance of 186 miles in real life. I am about half way done putting in my mile posts and I am already at the 140 mile marker. To get my distance I laid out straight track, laid down a ruler distance of 5280 feet, put down cars for that distance and transferred the cars to my route and did that 5 times to get a distance of 5 miles, the distance between my posts.

So is DEM wrong, the ruler wrong, or am I wrong?
 
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I have constructed consists with carriages etc which I save in 1 furlong lengths, ie 1, 2 ,3 etc to 8. 1 mile is 8 furlong. Use the 4 furlong for half a mile which is how the tracks are marked in the UK. I place the consist on the track at the zero point and a 1/2 mile marker at the other end. This is track distance not map distance.
As far as I am aware
- DEMS are real scale however the are three distances between points, as the crow flies and as the ruler measures, in the case of the ruler I believe that this follows the contours and of course the track distance..
- Map overlays are also inserted by TranzDEM on top of the DEM's and are also to scale.
- Finally are you building the route in imperial units or metric units, default I believe is metric.
- If your 5280 feet is in fact 5280 meter then you are about 3.28 mile between what you thought are mile markers.
Hope this makes sense.
 
Your best bet, Chris would be to use these here:

<kuid2:97008:60821:2> Measuring Wagon Imperial 52'8 Green
<kuid2:97008:60822:2> Measuring Wagon Imperial 52'8 Pink
<kuid2:97008:60823:2> Measuring Wagon Imperial 52'8 Red
<kuid2:97008:60824:2> Measuring Wagon Imperial 52'8 Yellow
<kuid2:97008:60811:2> Measuring Wagon Metric 25m Green
<kuid2:97008:60812:2> Measuring Wagon Metric 25m Orange
<kuid2:97008:60814:2> Measuring Wagon Metric 25m Red
<kuid2:97008:60813:2> Measuring Wagon Metric 25m Yellow

You use these to build a train exactly 5,280 feet long, or 1km long if you want. :)

Straight as the crow flies is a shorter distance between two points because the route doesn't have to go around things.

John
 
This all assumes that the track in your route follows the prototype track exactly, no bulges or shortcuts. And how accurate are the proto mile posts anyway?
 
like i mentioned in another thread a few days ago, prototype mile markers are rarely ever exactly 5280 feet on the mark. they can vary hundreds of feet in either direction.
 
Is Transdem or DEM in general inaccurate for ground distance?

The mathematical formulas in TransDEM fall in two categories: exact ones and approximations. Map projections, for instance, are always exact, while geodetic datum transformations are an approximation. TransDEM uses the 7 parameter Helmert transformation there which is as accurate as you can get with a general purpose solution. Then there is distortion, of course, since our planet is not flat, but the resulting terrain coordinate system is. TransDEM employs UTM here, which is a cylindrical projection and as long as you stay in the native zone you should be more than fine. Even if you map the adjacent zone into the current one and move to the far end, distortion should still be hard to detect.

Internally, TransDEM rounds down floating point computations to 1 mm accuracy and, by default, displays coordinates with 1 m accuracy on the user interface.

The rest is up to geo the data sources and possibly to their pre-processing outside TransDEM. So, when you use a 25m DEM, your accuracy is around 25m (a bit less than that, for the math purists). One can make big mistakes, though, particularly when georeferencing manually. However, as long as you work with pre-georeferenced stuff you should be rather safe.

In the old days, when people used MicroDEM and HOG, you could easily miss the DEM projection, which is often Plate Carrée (actually a pseudo-projection) and would end up with a huge distortion of 25% or even 33% further up north, which is a bit like watching a 4:3 TV program stretched to a 16:9 screen.

A good indicator for ground distances are UTM grid coordinates. They are in meters (as with almost every other map coordinate system, including national coordinate systems in countries which stick to imperial units elsewhere). By default, TransDEM will transfer the 1000m grid lines to the ground textures painting them in purple. Check these grid lines, compare with the Trainz 10 m ground raster and with the Trainz 720 m baseboard raster. Then check with a different geo data source, like Google Earth. Display the UTM grid there and compare with the TransDEM values. They should match. TransDEM would not work if they didn't.
 
Thanks for the replies

1. I am using imperial it is 5280 feet, not meters.
2. I don't understand what the difference would be between using a car 52.8' long or measuring out as I did using a car already in game, my consist is only slightly (very slight) over 5280 which means the distance should be less if anything. I will check my distance again to make sure
3. Straight distance in RW is 156 miles, track mileage is 186, this is from Ontario Northland itself.
4. The real world markers would have to be out by over 100% to equal the DEM/TS12 route.
5. When I measure the distance in Transdem using the route laid on the DEM it is 186 miles, so why the difference when placed into TS12?


***OK, rechecked my consists and each section is exactly 5280 feet, 5 of them for a 5 mile consist, so it isn't the ruler or measuring stick.***
^^ Tried it using measuring wagon and it also works out correctly^^
 
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Last time I looked, T:ANE UI outputs both time and distance data. Why can't you simply use that (assuming we're talking T:ANE here)?
 
Your best bet, Chris would be to use these here:

<kuid2:97008:60821:2> Measuring Wagon Imperial 52'8 Green
<kuid2:97008:60822:2> Measuring Wagon Imperial 52'8 Pink
<kuid2:97008:60823:2> Measuring Wagon Imperial 52'8 Red
<kuid2:97008:60824:2> Measuring Wagon Imperial 52'8 Yellow
...
You use these to build a train exactly 5,280 feet long, ...

John
Is the math(s) correct here? If the 8 refers to inches, then the train would surely be 5266 feet long. (8 inches X 100 = 800 inches / 12 = 66.6 feet). If it's 0.8 of a foot then fine.

R3
 
Well I'll be damned! As stated I was at the half way mark of the maps and at mile marker 140, well just finished marking the route AND.....it was 186.6 miles long! Must have been an illusion or something, very strange, although the northern half did have less curves but more maps. Anyway, thanks all. Now signalling my route and then finishing painting, then I want to try and create sessions, I hate routes that are just that, a bunch of track and nothing else including the one I paid for, very disappointed with that.
 
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