simple contour question 3

[newjerseype]

There is a common rule that contour lines cannot branch or split. However if there is a v-shaped valley where the bottom is at elev 10, for example, then the 10 contour will be a straight line and then split once the elevation of the bottom of the valley changes. So is this rule just an arbitrary rule for drawing contours or am I missing something?

 

[LHA]

I work in land development. As such, I've spent dozens of hours per week for the past five years drawing and reviewing contours. I've never heard of the rule you cite. Contours can and do branch, merge and split as needed to accurately reflect the topography...they also end and start up at break points (walls, curbs, cliffs).

The scenario you cited is possible and correct. However, I think if you trace that "merged line" out, you should see that it was two separate contours of the same elevation which came together, then rebranched when their relative elevations dictated.

For proposed conditions, I prefer to end each contour where they enter the flat area, then put several spot elevations (rather than the "merged line") within the flat area. This is not because it is more correct, it just reduces plan clutter; one could just as validly draw the "merged line" through the area.

 

[newjerseype]

lha,

I think you are correct about the "merged" contours. Thanks for the insight. However, you mention "break" points for contours occuring at walls. I do not believe contours break at walls. They continue along the wall or curb. This is just another example of the "merged" contour you mention.

 

[jugglerbri]

Contours cannot split, merge, break, etc. The reason you may see this on plans is due to a misunderstanding of what a contour is. A contour does not dilineate an elevation but rather a change in elevation. For example, a contour with an elevation of 100 does not show where the ground is at an elevation of 100 (although where ever there is a 100 contour, the ground is at an elevation of 100) but rather where the ground transitions from just above a 100 to just below a 100 (or vice versa). If contours showed where the ground is at a certain elevation and you have a large flat area of constant elevation then you would need a very large flat contour, which I've never seen. When you stop thinking of contours as elevation and start thinking of them as change in elevation, they make much more sense. newjerseype, in your example of the valley with a bottom elevation of 10, there would be no contour in the bottom of the valley because there is nothing less than 10 and so there is no transition from something higher than a 10 to something lower than a 10. The way I would show the bottom elevation of the valley would be with spot elevations. If you ever get into computer modeling of surfaces, a drastic change in thinking is needed. The contours do not define the surface but rather reflect the design of the surface. All the engineers in my office (except for me) draw all the contours by hand and have the drafters digitize them. Whereas I design the surface and have the computer display the surface using contours.

I know I've rambled a bit but it's early and I have nothing better to do.

Brian

 

[TerryScan]

jugglerbri,

How would you depict the following?

One has a uniform slope from el. 10 to 100. One constructs a basin in the side of the slope. The basin is part in cut, and part in fill. It has a flat top 6ft wide at an elevation of 50. It slopes to catch at a steeper slope than the original slope.

When the 50 countour hits the top, does it run the back (into the slope) of the pond and the front of the pond does not get a contour (even though it is also 50) but rather spots?

 

[newjerseype]

TerryScan,

I cracked up when I read your post because we had the EXACT same situation in our office last week which prompted a heated debate. And that is what prompted me to create my original post. I don't have an answer for you but you made my day!

njpe

 

[newjerseype]

TerryScan,

Now that I've looked at your hypothetical situation for a bit, based on Brian's definition of a contour, there is no proposed 50 contour since it's flat at 50. There is an existing 50 contour, but no proposed. Am I right Bri?

njpe

 

[LHA]

As someone who does this no fewer times than once a week, you must draw a continuous line from the existing contour out along the outside of the berm AND another at the same elevation on the inside of the berm. Offset the inside the top of berm width. If there is an emergency spillway (and I've never seen one designed without one), you break each line and join them all...you will then have one continuous line when you are done.

Do it any other way, and I will guarantee you no contractor will be able to build it. But that's OK, because no reviewer will be able to approve it.

 

[cvg]

good thing we don't expect contractors to build it from just the contours. That's why we have cross sections. As a reviewer / project manager - I don't expect the full picture from a grading plan. I must see both typical and actual cross sections, elevations, profiles etc.

 

[LHA]

Here in central PA, "We" have contractors that build off survey stakeout. When I used to work construction, I never recall laying out a site by looking at a plan.

Stakeout comes 100% from plan view DTM dumped into the data collectors from our proposed contours. How do your contractors get hot survey points off of profiles? I can't imagine a way to do that which would be more efficient and full-proof than off proposed contours...I guess you could do it though.

As a reviewer / project manager, I review all that other stuff too. But a plan view top of berm shown without contours inside and outside would be nearly impossible to stake. You would have to set the one elevation you had a contour for, then measure with a tape or chain or something the horizontal offset. But then you would need three chainings per point...wow, that must take forever. Next time you do a stakeout, try just taking contours off plan view, it goes much faster.

 

[cvg]

In 20 years, I have never released a digital file to a contractor for use in a stake out. Most (almost all) of my work has been for public agencies, and the only contract document that is official is the stamped and signed mylar and any prints made from it. Digital files are for archival / reference only.

The "hot" survey points are generated by the contractor's engineer or survey party chief in the office or in the field by whatever method deemed suitable. If the surveyor wants to digitize contours or generate staking information by scaling the contour plan, that's his call.

 

[LHA]

My 20 years in the construction trades have been vastly different than cvg's (I've only been an engineer for 8 of them).

My company regularly releases digital files to contractors, and so do my competitors. This makes sense, since they paid us a lot of money to make them. Most often (in the private sector) the engineer and surveyor are one in the same. And both work for the developer and contractor, who is, most often, one in the same.

I agree, the official contract document is the sealed mylar recorded at the courthouse. I disagree wholeheartedly that ...""hot" survey points are generated...by whatever method deemed suitable." It has been my experience that: since the late 90's en masse (but beginning in the mid-90's), in the mid-Atlantic and New England regions of the USA, digital files are the sole source of private sector land development stakeout. Perhaps the public sector still digitizes contours by hand, or scales points off plans and triangulates all day long to get survey datum; perhaps they do not. I can't speak to how governmental agencies input data.

In either case, however, DTM is generated soley from plan view data; profiles are used to check logic and fine-tune cut/fills. The more contours (which spot elevations do assist) given, the hotter the cut/fill stake is going to be, in my opinion. Cross sections, elevations and profiles do not come into play, in my experience, until much later.

 

[jugglerbri]

Terry,

That is exactly how I would design it. Here's my description of the contours as we follow a line that slices through the pond. As we are coming down from the existing 100 contour, we reach a point where the slope steepens from existing in order to fit the pond, say an elevation of 60. We then continue on down to the bottom of the pond which is, say, an elevation of 45, crossing the 50 contour in the process. Since there is no elevation less than a 45, there is no 45 contour in the bottom of the pond. Now we start going back up the other side of the pond to an elevation of 50. Again, since there is no elevation greater that a 50, there is no 50 contour. We then span the 6' berm and begin back down to catch back into existing grade. The proposed 50 contour will travel around the hill side of the pond but not the berm side.

Let's now look at a little bit of a different situation, the top of the berm is at an elevation of 50.01. This time we'll start at the bottom of the hill. Begining at the existing 10 contour we move up to an elevation where we begin filling for the berm. We then continue up to the top of the berm (50.01). Since the top of the berm is higher than a 50, there is a 50 contour. We span the 6' wide berm and then begin back down into the pond, crossing another 50 contour to the bottom of the pond (which is now at 45.01 (same design, just shifted up slightly). We then begin back up the slope to catch back into the existing ground. In this situation, there are two 50 contours, the existing contour ties into the proposed contour that runs around the front of the berm and the independent contours runs around the inside of the pond. I'll do a quick grading of this using LDT when I get to work and if anyone would like to see the results, I'ld be glad to e-mail them to you. Just send your e-mail adress to jugglerbri at hotmail dot com.

Brian

 

[LHA]

It sounds like jugglebri has also done this several hundred times. I agree with his procedure verbatim. This is how I create low points (and high points, but in reverse) everyday. I could not have described the process better.

 

[jugglerbri]

Just to add a little to this conversation, here is probably the most important rule of using contours:

Every contour has no end and no begining and is one continuous line.

This may seem wrong at first but it's not. You may say, "Well, what about at the edge of the page?" The fact is, you are only showing a portion of the contour. "What about at building at retaining walls?" You don't show the portion of the contour that runs along the face of the retaining wall or wall of the building. If you look at a USGS topo map, you will never see a contour that splits, joins another contour, or stops or starts in the middle of the page.

Brian

 

[TerryScan]

I probably should keep this to myself, but I do what juglebri mentions in his 0.01' scenario, but without actually assigning the 0.01. Yes, I say the top is 50, but draw it like it is 50.01. I say the bottom is 45 but draw it like it is 44.99. I will do my pennance in the next life.