what's chainage in road survey?!!!

[PujanG]

I'm confused!!
Is chainage horizontal distance between IP's or the distance measured along the ground level between IP's???

If it's the horizontal distance btwn IP'S then chainage doesn't represent the actual length of the road.
If it's the distance along the ground then it represents the actual length of road. But, the plot between RL & chainage (Profile leveling) cannot be used to determine actual gradient between IP'S.

Gradient of formation Level inserted using the profile would be steeper in actual field; for distance along ground level is always > hz distance.

So.. can't i obtain gradient between IP's using L-section??

i'm so lost

 

[msquared48]

Chainage is the level horizontal distance between points, or at least it's supposed to be.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[vikram10051]

chainage shows only horizontal distance between two points. it doesnt include the length of longitudinal curves.

 

[PujanG]

i'm still confused.... i'm specially talking about road surveying..

If chainage is the level horizontal distance (leveled as a water surface) between 2 IPS then it solves one of the confusions, but creates another..

Solves: the Gradient plotted in the profile represents true gradient.

Contradicts: If chainage is horizontal distance, not along the ground surface, then i can't use this length to compute earthwork between 2 chainages.

Anyone working on ROAD please help! what is the standard procedure of road surveying & inserting formation level.

 

[Debaser]

This extract from 'Roadwork - Theory and Practice (5th Edition)' shows how a survey plan/chainages relate to a long section.

 

[PujanG]

Thank you Debaser for your efforts.

I looked into the extract; which shows a plan of road & its profile.. But what really bugs me is.. "chainage is length measured following the undulations of ground? or a leveled horizontal distance on air between two points?"

With the information provided in extract, i couldn't really distinguish, whether chainage was measured following ground undulations or a leveled horizontal.

But assuming the length measured along the ground facilitates the computation of earthwork between two chainages, i believe chainage must be measured along ground surface.

On doing so.. i'll have to measure the gradient between chainages manually using abney level & cannot insert formation level into the profile made using RL vs Chainage.
coz rise/sloping distance (aka chainage) doesn't give true gradient.

Guys...anyone who has worked in highway survey pls elaborate how you've done road surveying & designed it's vertical profile..

thanks.

 

[Debaser]

I get you now.

Imagine you're in a plane looking vertically down to some hilly stretch of ground. Project a straight line above the ground, as you say, and mark off intervals, this is chainage, simply the horizontal distance.

If you were to create a section through the ground, the actual distance you'd have to walk between those intervals would vary depending on the vertical change in height. This is NOT chainage.

The difference is taken into account by creating a long section (usually this isn't a natural scale anyway, in the UK the vertical is often 10x the horizontal scale in order to show the relief). The long section is the section created by cutting vertically through the Earth along the line you projected.

10m real-life horizontal distance is 10m on the long section, at natural scale (ie 1:1), and 10m vertical distance is 10m on the section. This allows for gradients to be calculated properly as you alluded.

I'd Google long-sections for further explanation.

 

[PujanG]

OK Debaser, thank you for a quick response.

As you said, chainage is the distance between 2 points on ground as seen from a plane.. ie. Horizontal distance on plan.

If it is so, then i'll have no problem in determining a profile & inserting formation level. Then, my question is:

A. Doesn't chainage represent the length of road?? 1+000 chainage doesn't mean 1km length of road.

B. In common practice, Earthwork between two chainages is computed using (Average of mean area) x (difference of chainage)
if chainage is horizontal (ie. not the true length of road segment) then how can i compute the earthwork??

Debaser eagerly waiting to hear from you thanks.

 

[cherrypicker]

PujanG

There are many methods of calculating earthworks volumes but they all take the same principle.

Area = length x width
Volume = area x height

Given that we use 3D coordinates expressed as X, Y and Z, they are always dimensioned at right angles to each other from an origin.

Therefore as you are working with levels, the area of cross section should be parallel to that plain (ie vertical) and perpendicular to the string line, making the length along the string line horizontal (chainage).

Care should be taken in any case because whichever system is used (Simpsons, Prismoidal etc.) cross sections only sample the model information and will never consider detail between cross sections. Decreasing the interval between sections will increase the level of accuracy but will also significantly increase the level of computation involved.

The main method of calculating volumes that consider detail in both surfaces is complex vertical prisms using TIN models. All information is considered in both surfaces but the overhead is the amount of calculation involved and should really be left to computer software.

Any of the above processes still use Area x the perpendicular distance to calculate volumes.

A chainage is therefore always the distance along the horizontal alignment of a master string and will account for any horizontal curvature whether simple or transitional. The vertical alignment and crossfalls will always be referenced to the horizontal master alignment chainage.

What is you background? Contactor or Client? Take care, this could look like you are just trying to justify maximising a volume calculation for whatever reason...

 

[PujanG]

Thank you cherrypicker .
I understood. I vaguely remember doing survey during 2nd year of engineering studies.
Don't worry for now it's not for calculating earthwork for real project. Yes, i'll take care onwards not to consider slope length.

volume=Length x Perpendicular area

Now no confusion regarding volume, no confusion regarding gradient. everything seems clear.
So, chainage isn't the real length of road.
Everything sorted.
thanks again cherrypicker. by the way, you sound more like a mathematician .
take care.

 

[cherrypicker]

No problem, just a little experience measuring earthworks from working it out with manually drawn cross sections and HP45 calculator (quite a few years ago) to using LSS.

It helps to understand what is going on before blindly accepting a computer printout.

Glad to be of help.