Hi Roland,
After a long break, I imported a new route into TS12 the other day. The TransDEM export worked fine. However, I was placing some Trig Station objects in Surveyor (edit: based on Google Earth coordinates) when I noticed that the trig stations tended to be misaligned about 200 m to the northwest of the corresponding points on the UTM tile image (see image https://www.dropbox.com/s/ccdvgphbms4rt ... 7_0000.jpg for example). This misalignment seems to be pretty consistent across the map. The UTM tiles are not misaligned from the DEM baseboard.
Do you know what could cause the misalignment? Did I misconfigure anything in TransDEM before I exported the route? Can I fix it somehow?
Nathan
Surveyor coordinate system misaligned?
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behemoth14
- Posts: 2
- Joined: 28 Jan 2014 09:30
Surveyor coordinate system misaligned?
Last edited by behemoth14 on 29 Jan 2014 04:49, edited 1 time in total.
Re: Surveyor coordinate system misaligned?
Hi Nathan,
the lat/long coordinate system in Trainz Surveyor isn't really suitable for measurement.
It's not easy to understand, but lat/long in Trainz is not lat/long in reality.
Internally, Trainz uses a Cartesian coordinate system, which is based on metres, and is called Trainz World Coordinates, TWC. TransDEM also has a Cartesian metric coordinate system, called UTM (Universal Transverse Mercator), based on an particular geometric model of our planet, which is called a geodetic datum, in this case WGS84 (World Geodetic System 1984). Mapping between UTM/WGS84 and TWC is simple. We just subtract a static offset from UTM and obtain TWC. The offset is always a multiple of 720m, the size of a Trainz baseboard. The offset value itself is not stored inside the route. But it's needed to add more geo data to an existing route. TransDEM uses a trick here. It saves the offset between UTM and TWC indirectly, by placing the Trainz World Origin object on an existing baseboard at a point near the geometric centre of the entire route. World origin means lat/long or geographic coordinates and TransDEM unprojects UTM to geographic for this point.
The lat/long value of the point is correct to the last digit, as seen in the WGS84 system. Because it has only arc min resolution, the world origin object placed by TransDEM needs an additional hint. The hint is that it will always be a the exact geometric centre of a baseboard. That is enough to fill the gap of lat/long accuracy.
So, for TransDEM this is all consistent now. And it has the side effect for Trainz to provide a hint for sun azimuth and elevation, the original intention of the lat/long system in Trainz.
The problem now is, that WGS84 is not the only geodetic datum and UTM only one of many map projections. The geodetic datum describes shape and position of the earth. Simplified, it becomes a sphere but WGS makes in an ellipsoid. And there are many map projections, which transform the surface of our plant onto a plane, our Cartesian coordinates. UTM is the most widespread, but not the simplest.
What we know is that the Trainz mapping between TWC and lat/long is definitely not UTM/WGS84.
The trig station asset surely was created by a user with the best intention, but it simply reads lat/long values for TWC, as provided by the Trainz internal functions.
Trig station reading will only yield correct WGS84 lat/long at or close to the position of the World Origin object. The further away you are from this spot, the greater the discrepancy. And, as always when it comes to geodesy, this error is non-linear.
I can only suggest not to use trig station or similar Trainz assets. They will mislead you.
the lat/long coordinate system in Trainz Surveyor isn't really suitable for measurement.
It's not easy to understand, but lat/long in Trainz is not lat/long in reality.
Internally, Trainz uses a Cartesian coordinate system, which is based on metres, and is called Trainz World Coordinates, TWC. TransDEM also has a Cartesian metric coordinate system, called UTM (Universal Transverse Mercator), based on an particular geometric model of our planet, which is called a geodetic datum, in this case WGS84 (World Geodetic System 1984). Mapping between UTM/WGS84 and TWC is simple. We just subtract a static offset from UTM and obtain TWC. The offset is always a multiple of 720m, the size of a Trainz baseboard. The offset value itself is not stored inside the route. But it's needed to add more geo data to an existing route. TransDEM uses a trick here. It saves the offset between UTM and TWC indirectly, by placing the Trainz World Origin object on an existing baseboard at a point near the geometric centre of the entire route. World origin means lat/long or geographic coordinates and TransDEM unprojects UTM to geographic for this point.
The lat/long value of the point is correct to the last digit, as seen in the WGS84 system. Because it has only arc min resolution, the world origin object placed by TransDEM needs an additional hint. The hint is that it will always be a the exact geometric centre of a baseboard. That is enough to fill the gap of lat/long accuracy.
So, for TransDEM this is all consistent now. And it has the side effect for Trainz to provide a hint for sun azimuth and elevation, the original intention of the lat/long system in Trainz.
The problem now is, that WGS84 is not the only geodetic datum and UTM only one of many map projections. The geodetic datum describes shape and position of the earth. Simplified, it becomes a sphere but WGS makes in an ellipsoid. And there are many map projections, which transform the surface of our plant onto a plane, our Cartesian coordinates. UTM is the most widespread, but not the simplest.
What we know is that the Trainz mapping between TWC and lat/long is definitely not UTM/WGS84.
The trig station asset surely was created by a user with the best intention, but it simply reads lat/long values for TWC, as provided by the Trainz internal functions.
Trig station reading will only yield correct WGS84 lat/long at or close to the position of the World Origin object. The further away you are from this spot, the greater the discrepancy. And, as always when it comes to geodesy, this error is non-linear.
I can only suggest not to use trig station or similar Trainz assets. They will mislead you.
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behemoth14
- Posts: 2
- Joined: 28 Jan 2014 09:30
Re: Surveyor coordinate system misaligned?
Thanks Roland. Looks like the best thing to do then is to match as closely as possible to the UTM tiles.