[Gman347]

Quote

When railways were laid out in the pre computor days, surely the horizontal distance would NOT have been known. The line was laid out with a team of surveyors, pegging out the line using chains and a set of theodelites. For important projects particularly over difficult terrain a number teams were used and the results compared to get greater accuracy. Due to this method of laying out the line only the distance the line actual covered would have been known, ie in hilly terrain horizontal distance would have had to be calculated.

I assumed that the process would have started with a map, knowing horizontal distances. Interesting stuff. Considering how track might snake around in hilly terrain that explanation certainly makes perfect sense.

Sorry, I didn't ,mean to highjack this thread.

[Lindsayts]

Gman347, on 27 September 2016 - 04:22 AM, said:

I assumed that the process would have started with a map, knowing horizontal distances. Interesting stuff. Considering how track might snake around in hilly terrain that explanation certainly makes perfect sense.

Sorry, I didn't ,mean to highjack this thread.

A couple of points.............

I do not believe the thread was hijacked, the question of the distance as specfied on a railway is definitely something one needs to ask when doing a route, and in fact if the distance was a true horizontal distance or the actual track distance is something a had wondered about. How it was done was expalined to me by an old engineer/surveyor.

On your first point, while the majority of Britain was mapped by at least the 18th century, in places like the America's and Australia, the railways generally proceeded mapping so no distances where known. In hilly terrain often a number of paths were surveyed, all paths being pegged out. If the area has been undisturbed (say a native forest that has not been logged) on a very carefull search one can sometimes still find these pegs for the alternate routes, we (the group I work with) have found some of these.

Imagine what it would have been like surveying a line in mountainous terrain, photographs of these teams in Australia show they were around 20 people involved in each team, everything including any calculations had to be done by hand, even high precision slide rules not being generally availible till around the end of the 19th century.

LIndsay

[WaltN]

Lindsayts, on 27 September 2016 - 01:49 AM, said:

The line was laid out with a team of surveyors, pegging out the line using chains and a set of theodelites.

There are two things wrong with that statement: (1) A theodolite is used for measuring angles (vertical and horizontal planes) not distances, and (2) it is the chain (singular) that is used to measure distance -- horizontal distance.

As a pointed out on Monday, if you do a web search on "survey +grade" you'll find (1) run is measured along the horizontal, and (2) run is measured along the hypotenuse (the path the wheels roll). (The Wikipedia article "Grade (slope)" chooses Interpretation 1 but goes on to say, in railroading, it's Interpretation 2.) Then I said that for typical railroad grades, it's "macht nichts." Goku substantiated that.

I'm going to wax eloquent on this subject, but I need to establish my credibility.

• In 1957, I had a summer job working with a Pennsylvania Highway Department Surveying team. Among other things, we surveyed Interstate 80 in the Stroudsburg area. Well before the end of the summer, I was relieving the transitman and "running the gun."
  
• In the 1970s as an IBM engineer, I augmented an APL implementation of COGO with graphics output.
  
• With Open Rails, I proposed a "route designer" as a tool to accompany a route editor. The proposal went over like the proverbial "lead balloon."

I want to talk about three areas. The first two relate to surveying practice in highway and railway design and surveying. The third relates to an MSTS dirty little secret.

Route Design
Route design largely takes place in an office -- yesteryear and this year. Although CAD has supplanted the drafting table to some degree, it is 2D paper that will be used in the field. Most used are drawings in the horizontal plane. There, the route path is partitioned by stations. In the US, there is a whole-numbered station point every 100 feet. Also, there are construction points: start of curve, end of curve, intersection points, etc. that do not necessarily fall at 100-foot intervals.

The other type of drawing is the vertical profile of the route. Vertical profiles are plots of elevations (heights) of the route at distances along the route path. Given the relatively easy curves associated with highways and railways, the straight-line segments between segments are a reasonable approximation of the route path.

Field Work
Field work, done by surveying teams, is mostly stationing -- erecting stakes with pins at stations -- and leveling -- obtaining an elevation value to enable approximating the terrain.

Tools for stationing are the transit and chain. In 1957, a transit amounted to a small telescope on the top of a tripod. Also, there was a plumb bob centered beneath the scope, such that the scope could be located directly over the center of a pin or nail in a wooden stake marking a station. (As I recall, the head of the nail was 2 or 3 mm.) The base of the scope was levelable, and leveling was an important part of moving the transit from one point to another.

The preponderance of the distance measurements made with the transit were made in the horizontal plane. I only saw one measurement made along a slope all summer long. Trigonometric functions were acquired from a book of tables, and only the Party Chief carried one. Since calculation (multiplication) was long-hand and by the Party Chief, he rarely called for such a measurement. Measurements in the horizontal plane were done with the chain (graduated steel tape marked in feet and hundredthsin the US). One end was held on a stake nail with a plumb bob used at the other end. Occasionally, bobs could be used on both ends, but accuracy suffered.

Preliminary Summary
When railroads (real or virtual) are built with sectional track or rail of specific length it is only natural to use the length as the run. To do otherwise would require extracting a square root.

An MSTS Dirty Little Secret
A bunch of years ago when I first joined the Open Rails Development Team, I discovered that MSTS track sections are rigid bodies. That's an important realization for curves. Elevate a curve section at one degree say, and the outbound end of the curve is no longer level; it is only level at the pivot end. Of course curved sections are generally pretty short (short meaning limited angular span). So the tip is very slight.

However, dynamic track is different; you can make an angular span up to 90 degrees. Worse, dynamic track can have five subsections (straight, curve, straight, curve, straight), and the whole business is treated as a rigid section. Now (this will blow your mind), perform an experiment: Layout a single dynamic track section with subsections 0, R<90, 0, R<90, 0. That is, the straight sections are of zero length, and the two curved section are of radius R (whatever you want to make it) and 90 degrees in span. (Both curves have the same specification.) Finally, elevate the section at, say, 3 degrees. Question: If the height at the start of the section is H, what is the elevation at the end of the section? Answer: H. How can that be? Answer: The section (all subsections) is a rigid body. So what is the problem? We are led to believe the grade is constant (3 degrees) all throughout the path. It isn't. (The path does not follow the helical curve we expected.)

Open Rails, with its objective of matching MSTS behavior does just that. But, any new route editor should consider, in the future, handling curved sections the RIGHT way in addition to the MSTS way. (It would take an option setting, and that is the domain of Open Rails. It is important that the traveler and the geometry have the same behavior.)

Please forgive the length of this post.

[longiron]

WaltN, on 28 September 2016 - 06:53 AM, said:

An MSTS Dirty Little Secret
A bunch of years ago when I first joined the Open Rails Development Team, I discovered that MSTS track sections are rigid bodies. That's an important realization for curves. Elevate a curve section at one degree say, and the outbound end of the curve is no longer level; it is only level at the pivot end. Of course curved sections are generally pretty short (short meaning limited angular span). So the tip is very slight.

this phenomena is readily apparent when you try to lay 3T curves on an rapidly rising (or declining) gradient. I've experienced it first hand in the route I'm building. Snap to the inside track of the three, and the train experiences a 'bump' on the middle and outside paths while in inner is smooth. Snap to the middle track and the train experiences 'bumps' on either of the outside paths. If you get really close to the track joint you can easily see the mismatch. Lay each of the 3T individually, and you have to compensate the gradient to get individual sections to seamlessly snap to the successive 3T section. Total PIA.

chris

[WaltN]

I guess I don't know what "3T" curves are. But, if you can SEE the discontinuity, then it's in the geometry. The phenomenon I was referring to caused trains to gradually leave the geometry vertically throughout the curve section because the traveler was following a path different from the geometry. When the section end was reached, the train would slam back in contact with the rails. Now, both the dynamic track curve and traveler use the same code.

[longiron]

WaltN, on 28 September 2016 - 12:01 PM, said:

I guess I don't know what "3T" curves are. But, if you can SEE the discontinuity, then it's in the geometry.

A picture is worth a thousand words. When you snap using the inside rail as attach point, then the other two tracks don't line up properly on gradients

 TrackIssue.jpg (188.3K)
Number of downloads: 6

[Goku]

You can't fix this. It is property of predefined shapes. In that case better use different shape for every track.

[Jovet]

WaltN, on 28 September 2016 - 12:01 PM, said:

I guess I don't know what "3T" curves are.

3T = Three tracks

The more tracks on a curved piece of track, the more evident the slope at the far end of the curve when the track is not level. You can easily demonstrate this yourself, by holding your arm and hand horizontally (on a table) out in front of you. Pivot your wrist to point your hand across your body. Then bend your elbow to raise your forearm and hand. Eventually you'll notice your pinky is much higher than your thumb, but they were both even before you bent your elbow.

[WaltN]

Thanks, all three of you, for the clarification of "3T." Sometimes, my denseness astounds me. Maybe it's old age.

Goku, on 28 September 2016 - 05:59 PM, said:

You can't fix this. It is property of predefined shapes. In that case better use different shape for every track.

But it is fixable with procedural (a la "dynamic") track. My initial OR approach (perhaps five years ago) was to generate a helical form for the centerline path of a curve on a grade, only to discover that it was incompatible with the rigid form of MSTS curved sections. But, now with a new start on a route editor, it's doable with the addition of one parameter, a flag that distinguishes rigid versus helical forms. Of course, it takes a responding change on the OR side -- to add the helical behavior. It's been done once; it can be done again.

Walt

[Goku]

Yes, but your solution requires to get rid of these 3T, 4T etc shapes anyway. And these shapes are problematic :)