# Contour Lines and Interpolating

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Does anyone have or is there an easy formula for interpolating to find the contour lines I guess is how you would put it. I am doing a Civil Drafting project and it has a grid of 18 horizontal and 12 vertical and they are 50' each square. I have the measurements, but cant seem to figure a formula or figuring them as they go down from 90' on the outer one, and then 80' and then 70' and so on.

Thank you.

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Take the difference in elevation between two points and divide it by the distance. That will give you a rise per foot which can be used to determine the individual contours that fall between the two points.

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I have that part, but I think I need the interpolated distance. cause after it says to use the measure tool and then using a polyline to connect all the points and then PDEDIT with a "FIT" command to make the polyline curve. So i take it there should be only one point between the 50' lines or maybe two points. I don't know.

I think I saw some post from individuals who were at Penn Foster taking Drafting with AutoCAD. This is the Civil Drafting project.

p.s. I am sorry for what are probably easy questions.

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What is the contour interval?

Yes, there have been a few posts from some Penn-Foster students regarding the civil drafting project. Did you read any of them?

You do not have to apologize for asking questions.

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Yes the Contour Interval.

And I looked at past posts from others on Penn Foster, but none on the Civil Project. But I did find things that are useful.

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Shouldn't you be looking at the civil project then? Other projects aren't going to help you at this time.

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I meant for other things, not right now while i am working on this project.

No I haven't.

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No I haven't.

Are you working on the existing site plan for 562 Ocean?

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Yes the existing site plan. Plate 1.

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So your problem is you do not know how to do the interpolation? Answer quickly as I may be leaving the forum soon for the day.

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If you already have done the interpolation and are looking to fit a curve to your polylined contour use the Pedit command, select the polyline when prompted, then pick one of two options: Fit curve or Spline. Try each and see which comes closer to the individual elevations you are working with. Spline might give you the better results.

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Yes the interpolation

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Yes that is where I am stuck, the interpolation

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Sorry for the multiple same answers. Something messed up on my end.

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Sorry for the delayed response. I really did have to leave but I've returned briefly.

The mathematical interpolation of contours goes something like this.

Let's say we have two spot elevations A & B. A = 32.7 and B = 54.0. The distance between A & B = 50 feet.

We want to know where our 40 foot contour would fall between spot elevations A & B.

First obtain the total elevation difference. This is done by subtracting A from B. 54.0 minus 32.7 = 21.3.

Next we want the difference in elevation between our 40 contour interval and the nearest spot elevation which in this case is A or 32.7. That works out to be 7.3.

Now we need to calculate the distance (let's call this "d") we need to go from spot elevation A to our 40 foot contour. That takes the form of:

d/7.3=50/21.3 or d=7.3*50/21.3 = 7.3*2.347 = 17.13 or the distance, in decimal feet, to our 40 foot contour.

Got all that?

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This might be of some interest to you.

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Why would they make you even have to interpolate contours from that many points like that? No one is going to produce a contour map that way in the real world. It is good to know how it can be done, althoguh its real world use is limited and is carried out by computer software rather than tedious manual calculations.

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My guess would be and I could be wrong, but they want you to know all math and aspects of everything and how things could and might have to be done, even by hand without a program. I guess because it is not a full 2-4 year course and you dont really get a degree or nothing from what I understand. Just a cerificate or diploma or something.