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PostPosted: 08 Feb 2017 19:54 
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Joined: 08 Feb 2017 00:46
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Hello All,

I am new here, and intereted in getting to use TransDEM for Trainz, however i wish to combine DEM data with maps that i have .jpeg files of.

So far when looking at tutorials online and on the forum, everything seems to be pointing to web based maps, where TransDEM will then automatically extract high resolution "screenshots" of these maps, and lay them over the top of the DEM data automatically (or will extract Vector data, useful for rails etc).

What i Wish to do is to aquire DEM data, "cut it down" to the size i need, and then overlay a 1:1 representation of the map files that i have (due to there being no accesible web based version of the maps that appear to be compatible with TransDEM)

So really a Yes / No question, is it possible to use your own file and allign that over DEM data, and export it to Trainz?

Many thanks,
Richard.


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PostPosted: 08 Feb 2017 22:18 
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Joined: 05 Jan 2011 16:45
Posts: 1463
It's not a simple yes/no question, I am afraid.

When TransDEM first came out more than 10 years ago, manual geo-referencing was the only approach. In TransDEM that's an affine transformation with three points (plus one for verification). If the map is an undistorted scan and the coordinate system is one of those known to TransDEM, it will work quite well. In the years that followed, more and more online map resources sprang to life, including historic maps. Now it's normally much faster, to acquire the data from those, than to follow the manual procedure. But 3+1 point geo-referencing is still there and for a number of map sources still the only way to work with them.

What kind of coordinate grid does your map carry? What scale is it?

TransDEM supports quite a variety of map grids, but not every single one on this planet and particularity not all the historic grids in use since sustainable surveying started in the 19th century. In case it's not a known grid to TransDEM, it still may work, with the help of other sources, but that may depend on scale and map size.

Last summer I had a request for WW II British military maps of former Yugoslavia, based on a Greek coordinate system. It didn't work. I could not find reliable resources for the geodetic parameters, and the other approach, doing it from scratch, defining my own Helmert transformation by running a regression algorithm, came to no usable result either. The error was still in the 100m range, for a typical 1:50,000 sheet, pretty useless.


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PostPosted: 08 Feb 2017 22:34 
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Joined: 08 Feb 2017 00:46
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Its good to know that it was built on "the old way" and its simply modern software advances have made the process almost obsolete. I think i understand how the 3+1 system may work? and the maps are pretty straight on and well preserved

The map in question is a WW1 British trench map, 1:20,000 scale with grids at 1000 yards. (im trying to be a bit screteive, because i know many people utterly clawing to get there hands on a WW1 WDLR route, and I am the sort of person who may just give up half way through!)

a link to the viewable web version - http://maps.nls.uk/view/101464963

I think this route also benefits from the are is pretty flat in the first place. I played around with some microDEM and HOG, and nearly all of the map was so flat, its almost like I never applied any DEM data at all! its almost like the countour lines were where someone was doodling on the map, and forgot which way they had the pen up!

so thankfully utter 1:1 accuracy is not as required as say a railway in a gorge etc.

also as a side note, congratulations on 1000 posts


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PostPosted: 09 Feb 2017 21:02 
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Joined: 05 Jan 2011 16:45
Posts: 1463
Interesting, a metric grid on a British map in WW I. Under French influence? Regrettably, I don't have any background information here. As I said, I investigated on the WW II maps for Yugoslavia but never looked into WW I military cartography until now. But I have read Andrew Martin's crime novel "The Somme Stations" which tells us quite a bit about the Trench Railways. (The German counterpart was called "Heeresfeldbahn"). And I also had a glimpse or two at those maps on the NLS website.

Unfortunately, the map grid is unknown to TransDEM. But as the area in question is a rural one, with limited change to the watercourses and infrastructure over the last 100 years, I tried to use a modern map as a reference. This will always increase the error, so we better do it with smaller clippings. I downloaded a PDF screenshot of the SW corner of your map, about 3 x 2 km, and tried to georeference that, using Open Topo Map as the reference. (Open Topo Map is one of the more appealing styles for Open Street Map rendering.)

That's the result:

Image

Not that bad, I'd say.

RJC0235 wrote:
also as a side note, congratulations on 1000 posts
Thank you. It probably comes with the "job". When running a product support forum, you are more or less expected to write a significant share of the posts. 8-)


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