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D'Jewel

Penn Foster Civil Drafting Project

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D'Jewel

Good Afternoon, Everyone.

 

I'm working on the Civil Drafting Project and I don't understand how to interpolate where the contour intervals cross the gridlines.🤨  I have drawn my grid with all of the values included, but I'm not understanding how to calculate the values. Could someone please assist me. I'm at a standstill.

 

I would really appreciate it and be forever thankful! 🙂

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ammobake

Are you doing this in AutoCAD, Civil3d, or mapping software?

It sounds like you're doing this in AutoCAD.

 

when you say you have a grid with all the values included do you mean you have labeled points in the drawing with varying elevations and you don't know how to interpolate where the contours should be?

 

If you are trying to draw a traditional topographic map a good starting point is to download the map associated with the location from the USGS website for this location.  You can specify the location, quadrant township, etc.. and download the topo of that area for free from the USGS website.  We usually use those PDF's for site maps.

 

https://www.usgs.gov/core-science-systems/national-geospatial-program/topographic-maps

 

If you need to model the existing topography in AutoCAD, you can then import that map to model space, scale it to 100% scale, and trace out the existing contours with closed polylines.

If you also need to model the topography in 3D,  for each polyline that loops around, you would close each polyline and then specify the elevation in the properties window using the map for reference.  Some people use splines for this step.

 

Once you have the needed contours drawn and elevated at the correct height at full scale, you can then start to model in 3D by means of either a rulesurf command that will interpolate between each closed polyline, or a loft command that will do the same thing basically just as a "lofted surface".

 

There used to be a google earth add-on for autocad that was free and it would import the topography as a polyface mesh and overlay the imagery over that as a texture file.

I'm sure it's still available somewhere in the autodesk app store but don't know for sure.

But it just depends on how accurate you need/want to be.  I just don't know if the google earth add-on has an option to add topo data via major/minor contours.

 

Civil3D would be ideal for this because it will do all the interpolating for you and create a "surface" from which you can base your alignment(s).

 

At my current employer it's not much of an issue because our survey department compiles all the data, creates the surface, etc.. So when the designers get the go-ahead we have a base map and a surface to use on the project for doing the alignments in civil3d.

 

ChriS

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ReMark

The project is being done in AutoCAD as part of the Penn-Foster AutoCAD course.  This is the second of two civil projects.

 

The OP needs help interpolating the contours.

 

The grid was used to the plot elevations.

 

Instructions for interpolating between elevations have been posted in one or more of the corresponding project threads.  I know because I have posted at least one set of instructions myself.  At the moment I cannot recall the particular thread.  If and when I have a chance I will try to find it.

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ReMark

Perhaps this explanation might help.

 

Figure out the difference in elevation between the lowest and highest spot elevation.

 

Divide this by the distance between the two spot elevations. This will give you the percentage of slope.

 

Calculate the difference in elevation between the new spot elevation you wish to plot and the lowest spot elevation.

 

Divide this by the percent slope previously calculated. This will result in the distance from the lowest elevation to your new spot elevation.

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ReMark

Let's try this example.

 

You have two spot elevations. We'll call them the high spot and the low spot. The high spot = 54.0 and the low spot = 32.7. What is the difference in elevation between the two?

 

Subtract the low spot of 32.7 from the high spot of 54.0 which equals 21.3.

 

What is the distance between the high spot and the low spot? On our drawing we can either measure it using the Distance command or read it directly if the spot elevations are plotted on our grid which uses a square pattern of 50x50. Just for the sake of argument lets say the spot elevations fall on the grid and our distance then equals 50.

 

Divide the difference in elevation (21.3) by the distance (50) and what do you get? The answer is 0.426 which is the percent of grade.

 

With me so far? I hope so.

 

Now let's say we want to know, and plot, the 40 foot contour. 40 minus 32.7 (our low spot) = 7.3.

 

Divide 7.3 by the percent of grade of 0.426 and what do you get? The answer is 17.136 which we can round off to 17.14. This is the distance FROM the low spot you have to measure out.

 

Draw a circle with a radius of 17.14 using as your basepoint the location (on the grid) of your low spot as it's center. Where the circle crosses the grid between the low and high spot elevations is where your 40 foot contour will be located. Place some sort of line or block or whatever you want to use at this point (preferably something you can 'snap' to later on).

 

That's it basically in a nutshell.

 

Personally I like to always work from the lowest elevation towards the highest elevation although it can be done the opposite way. No matter which way you use the new elevation (40) should be located in the same spot between the two known elevations you started with.

 

Now, it's your turn. Go. Do. Now.

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D'Jewel

Thanks ReMark. I'm going to go and try this right now and let you know how it goes. This makes way more sense than the directions in the assignment. I'm visual and your directions help me to better "see" what I need to do.

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D'Jewel

Quick question ( I think I know, but when in doubt, check it out): where does the 40 foot contour come from?

 

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D'Jewel

Another quick question: how does it work when the spot elevations are both within 10? Meaning I have two spot elevations that are 93.8 and 92.2. In the example you gave, there is a 40 foot contour that is between the two numbers 54.0 and 32.7. Between 93.8 and 92.2 there is no rounded contour. How is that calculated? I hope that makes sense.

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D'Jewel

Also, if the spot elevations are something like 113.3 and 144.9, what contour do I use: 130 perhaps? 

 

i have used the instructions you gave me above and it makes it so much easier to complete. Please forgive my questions. I know very little about Civil Drafting (only what the course explains, which did not include much about calculating the contour intervals in this project, I'm afraid).

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D'Jewel

Also, if the spot elevations are something like 113.3 and 144.9, what contour do I use: 130 perhaps? Or, are you using the contour closest to the low spot? I guess that would make more sense. This still leaves the question about numbers within the same contour, such as 93.8 and 92.2. I have attached a screenshot of what I already have done. I'm going to remove the circles when I finish.

 

I have used the instructions you gave me above and it makes it so much easier to complete. Please forgive my questions. I know very little about Civil Drafting (only what the course explains, which did not include much about calculating the contour intervals in this project, I'm afraid).

Civil Screenshot.gif

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ReMark
1 hour ago, D'Jewel said:

Another quick question: how does it work when the spot elevations are both within 10? Meaning I have two spot elevations that are 93.8 and 92.2. In the example you gave, there is a 40 foot contour that is between the two numbers 54.0 and 32.7. Between 93.8 and 92.2 there is no rounded contour. How is that calculated? I hope that makes sense.

I beg to differ with you.  The contour between 93.8 and 92.2 is 93.0.  The project uses one foot contour intervals.

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ReMark
1 hour ago, D'Jewel said:

Also, if the spot elevations are something like 113.3 and 144.9, what contour do I use: 130 perhaps? 

 

i have used the instructions you gave me above and it makes it so much easier to complete. Please forgive my questions. I know very little about Civil Drafting (only what the course explains, which did not include much about calculating the contour intervals in this project, I'm afraid).

Contour levels that fall between 113.3 and 144.9 would start with 114 then proceed in one foot intervals all the way up to 144.  If the contour level is 5 then the contour levels would start with 115 and proceed in five foot intervals all the way up to 140.  And if the contour level was 10 then the contour levels would start with 120 then proceed to 130 and finally 140.  The course instructions denote which contour level should be used.

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D'Jewel

I understand for the most part. I went back and looked again at the directions, and it says that the "company will conduct this survey at 10-foot contour intervals". Thank you for that info. That being said, could I use 1-foot intervals for the numbers like 93.8 and 92.2, since both fall within the 10-foot interval, or would that mess up the project?

 

Again, thanks for everything. The information you've already given me has helped me past a stopping point. I've been working on it ever since I received your advice. I've done all of the ones that follow the example you gave me earlier. I don't mean to be special, I just want to understand. 😊

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ReMark

There is no 10 foot interval between 93.8 and 92.2 therefore there would be no contour.  Leave it alone.  Sorry I mislead you regarding the contour interval (10 foot not 1 foot).  Good work.

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D'Jewel

You're fine. I've been working on these projects intensely, and I guess I just missed the information in the booklet. I appreciate you pointing that out. I'm glad I didn't try to do anything on that part prior to hearing from you. Now I can move forward a bit.

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Nate.straightline

I have been working through this same project and read every thread available to me on here which has been very helpful and is honestly the only reason i have made it this far. This appears to be the most recent thread so hopefully its still active and someone can help me understand plotting section A. Other threads have helped me figure out the Driveway profile but I am still at a loss as to how i should plot the section on the grid. From examples i have created the initial grid and added the distances along the side and Stationing along the bottom. If anyone can help me understand where to go from here i would really appreciate it. 

Edited by Nate.straightline
errors

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ReMark

Plot the elevation on the grid at its respective stationing.  Do this for all the contours that are crossed.  When done connect the dots. 

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Nate.straightline

so that is what i did for the center line profile that runs the length of the road i am confused how i can plot points at respective stationing for a cross section when my stations run along the center line not the section. do i have to add stations to my cross section line? civil drafting really is not my thing so I apologize if i am asking stupid questions this one is just hard to get my head around.  

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ReMark

Select a section of the road that is relatively straight and draw a line perpendicular to the centerline of the road that passes through all contours to the left and right of the centerline.  You measure from the center point of the road to the contours in both directions to get your distance.  Then you plot the elevation of the contour relative to that distance to create your cross-section.  Think of the road like a "Twinkie" that you have sliced in half.  Pick up one half of the Twinkie and look at it on end.  That's your cross-section.

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BIGAL

Another way is to add a Point at each intersection to the perp line and contour, draw the points on a new layer. Open properties click on point make the Z value to the contour value, use layiso to display the points only, join them together. Make sure you have snapz set to 0.

 

You can flip this sequence of lines to vertical using UCS and rotate3d 

Edited by BIGAL

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