> If the measurement from string to ground at point C is close to or more than 0.5' you would take a shot there. That would take care of that segment. If I'm doing a 1' topo, I would take that shot if the ground was 0.3' or more from the "stringline".
Okay, I got it. Phew!!! Thank so much again!
Think Triangles!
Lots of good advice. I would only add that I agree that looking up the National standards for the creation of surfaces is highly illuminating.
The biggest obstacle, in my opinion, to understanding the creation of a DTM is an inability to grasp the concept of thinking in triangles.
Think in triangles, and you will be able to visualize the DTM as you create it. You will even realize that perfectly precise spot shots can create a very inaccurate Digital Terrain Model(DTM). And well placed, but perhaps less precise measurements can create very accurate DTM's at times.
Publish the surface, with spot shots, and let us see what you did. That would produce more specific feedback.
I tend to map all curves in what I like to call, 3 round bursts. That is to say that I get a beginning, point in the middle, and one at the end. If you have a transitioning curve in say a driveway or a pond even, the end of the first curve is the beginning of the second curve so I still use the "3 round burst" method. It sounds cool and causes me to think in odd numbers and it works VERY well.
For ANY curve, if you are worried about the size of the curve or whether or not you can see small transitions, then of course use more. For me with Carlson, it draws curves through three points (hence the 3 round burst method) and looks like a slick nickle when it's done right. For it to really look right though, the ends of the curve need to be pretty close and the curve consistent.
If I had a waterway that I was mapping, say like a pond, I would probably get a shot every 25' or as needed. At some point you're gonna get on the damn and unless it's real x,y,&z topo, one shot on each end, regardless of distance (unless the dam isn't uniform) should suffice.
When in doubt, get more shots.
The often mentioned "Standards":
United States National Map Accuracy Standards
With a view to the utmost economy and expedition in producing maps which fulfill not only the broad needs for standard or principal maps, but also the reasonable particular needs of individual agencies, standards of accuracy for published maps are defined as follows:
1. Horizontal accuracy. For maps on publication scales larger than 1:20,000, not more than 10 percent of the points tested shall be in error by more than 1/30 inch, measured on the publication scale; for maps on publication scales of 1:20,000 or smaller, 1/50 inch. These limits of accuracy shall apply in all cases to positions of well-defined points only. Well-defined points are those that are easily visible or recoverable on the ground, such as the following: monuments or markers, such as bench marks, property boundary monuments; intersections of roads, railroads, etc.; corners of large buildings or structures (or center points of small buildings); etc. In general what is well defined will be determined by what is plottable on the scale of the map within 1/100 inch. Thus while the intersection of two road or property lines meeting at right angles would come within a sensible interpretation, identification of the intersection of such lines meeting at an acute angle would obviously not be practicable within 1/100 inch. Similarly, features not identifiable upon the ground within close limits are not to be considered as test points within the limits quoted, even though their positions may be scaled closely upon the map. In this class would come timber lines, soil boundaries, etc.
2. Vertical accuracy, as applied to contour maps on all publication scales, shall be such that not more than 10 percent of the elevations tested shall be in error more than one-half the contour interval. In checking elevations taken from the map, the apparent vertical error may be decreased by assuming a horizontal displacement within the permissible horizontal error for a map of that scale.
3. The accuracy of any map may be tested by comparing the positions of points whose locations or elevations are shown upon it with corresponding positions as determined by surveys of a higher accuracy. Tests shall be made by the producing agency, which shall also determine which of its maps are to be tested, and the extent of the testing.
4. Published maps meeting these accuracy requirements shall note this fact on their legends, as follows: “This map complies with National Map accuracy Standards.”
5. Published maps whose errors exceed those aforestated shall omit from their legends all mention of standard accuracy.
6. When a published map is a considerable enlargement of a map drawing (manuscript) or of a published map, that fact shall be stated in the legend. For example, “This map is an enlargement of a 1:20,000-scale map drawing,” or “This map is an enlargement of a 1:24,000-scale published map.”
7. To facilitate ready interchange and use of basic information for map construction among all Federal mapmaking agencies, manuscript maps and published maps, wherever economically feasible and consistent with the uses to which the map is to be put, shall conform to latitude and longitude boundaries, being 15 minutes of latitude and longitude, or 7.5 minutes, or 3-3/4 minutes in size.
Issued June 10, 194l U.S. BUREAU OF THE BUDGET Revised April 26, 1943 Revised June 17, 1947
I just had a quick thought on this. It might not hurt to experiment a bit and see for yourself the difference. Do everything you think you need to do, plus some overkill. Maybe if you can code the shots that you think are overkill a certain way to easily identify them, and also possibly a second level of shots. Edit out the "overkill shots (by a search for your coding in the raw file). See how much change it makes for the same contour interval. Try coding some of the other shots so as to make it what you would think would be "underkill", and seeing if that would have made much difference by not shooting those.
It's good in my mind to go through some experimentation to get a good feel for the extent of work you need to do. I agree with the general consensus that too many shots are better than not enough. Think about the time it takes to take an extra shot, vs. having to come back out, resetup and try to figure out where you need to supplement your earlier survey.
Think Triangles!
> Lots of good advice. I would only add that I agree that looking up the National standards for the creation of surfaces is highly illuminating.
>
> The biggest obstacle, in my opinion, to understanding the creation of a DTM is an inability to grasp the concept of thinking in triangles.
>
> Think in triangles, and you will be able to visualize the DTM as you create it. You will even realize that perfectly precise spot shots can create a very inaccurate Digital Terrain Model(DTM). And well placed, but perhaps less precise measurements can create very accurate DTM's at times.
>
> Publish the surface, with spot shots, and let us see what you did. That would produce more specific feedback.
Aloha, lots of good advice indeed. I love this forum for this very reason.
I have to admit visualizing triangle is very new to me. I hope you can share more of your thoughts on this. It would be greatly appreciated!
We have about over 30 ponds. Different sizes. Some of them less than 15' diameter. I will post sample file of my initial work for further guidance. That was an excellent idea. Thanks
> The often mentioned "Standards":
>
>
> United States National Map Accuracy Standards
>
>
> Issued June 10, 194l U.S. BUREAU OF THE BUDGET Revised April 26, 1943 Revised June 17, 1947
Aloha, Dent: Thanks!
> I tend to map all curves in what I like to call, 3 round bursts. That is to say that I get a beginning, point in the middle, and one at the end. If you have a transitioning curve in say a driveway or a pond even, the end of the first curve is the beginning of the second curve so I still use the "3 round burst" method. It sounds cool and causes me to think in odd numbers and it works VERY well.
>
> For ANY curve, if you are worried about the size of the curve or whether or not you can see small transitions, then of course use more. For me with Carlson, it draws curves through three points (hence the 3 round burst method) and looks like a slick nickle when it's done right. For it to really look right though, the ends of the curve need to be pretty close and the curve consistent.
>
> If I had a waterway that I was mapping, say like a pond, I would probably get a shot every 25' or as needed. At some point you're gonna get on the damn and unless it's real x,y,&z topo, one shot on each end, regardless of distance (unless the dam isn't uniform) should suffice.
>
> When in doubt, get more shots.
Aloha, Kris:
As always, thank you for the step by step procedure. Many of the ponds are pretty small, so I may need to take very close shots. I mean four to five feet a part maybe...
Photos of the various ponds
Aloha, posting few photos of the monastery ground with ponds. The ponds in the first four images are former sugarcane irrigation ditches that was repurposed for landscaping
Water from the ponds forming waterfall right before running into the Wailua River. My favorite spot--when there are no mosquitos--winter time.
These is the largest pond. About 250 diameter I think. It is a natural pond. The river makes 90º turn here. We don't know how deep the pond is. It is very, very deep. Some say it could be an extinct lava tube!
The view of the pond and river from my office 🙂
Photos of the various ponds
You have an amazing view! I wouldn't mind that work environment!
Think Triangles!
yswami,
I would recommend you go through an exercise of plotting out the contour lines at least once. This would entail plotting out the points where you took your shots and scaling/interpolating the even contours in between them. (Maybe you've done this before). You could plot out the points on a paper with a printer to scale if you like, or plot them out on some kind of grid paper by hand. when you draw lines between adjacent points, you get a whole series of triangles. Scale between two points, figure out the the distance to the even foot-contour interval, by comparing the change in elevation between points to the change in elevation from one of the points to the even contour between them. Use that relative difference to determine how far to scale to the plot a point on the next even foot contour interval. Don't scale across "break lines".
Anyway, think in terms of those triangles of where you would like to interpolate between. I believe that is the "thinking in terms of triangles" that was being referred to above. If you can visualize how the software will create the triangles and how it will determine the contour lines between the points, that is enough....but the hand-drafting of some contours might give you some good hands-on ability to visualize it as well.
By the way, thank you for sharing the pictures below. What a beautiful area you live in.
> Aloha, Kris:
> As always, thank you for the step by step procedure. Many of the ponds are pretty small, so I may need to take very close shots. I mean four to five feet a part maybe...
That brings up another set of issues. You can have TOO MANY shots. We run into this when you couple a large as-built with a topographic (true x,y&z with contour lines) survey. They always seem to have the issues at curb cuts, curb returns, steps, et cetera where you need a bunch of shots sometimes within two feet of one another for mapping purposes, but you don't need them that close for the topo. Sometimes, you will find the contours don't match the ground. Breaklines become EXTREMELY important in this environment.
However, for raw land topo, shouldn't be a problem as you probably will not have that many hard drastic changes. Keep in mind that your water level isn't static and will fluctuate over time. I may be inclined to get into the water 10 feet or so and run a topo line around there as well to show the water shed into the pond also. If you really want to get with it, cross section the entire pond. That truly sucks.
Also, around any "soft soils" such as mud or sand, and the topo boot has been mentioned, but if elevations are truly critical, then watch that topo boot carefully. They will sink even with a larger displacement at the bottom of the pole.
When you work around the ponds, make sure you have a buddy with you. Getting stuck in mud or worse requires two people, even if one can only call for help.
It is great to read of your enjoyment while surveying and mapping. I love to see the connection between precise measurements and works of art.
Don't forget the value of a photograph.
A high resolution aerial image is not only useful for survey planning but can also help you spot missing shot data. With experience, you will see where you are taking too many shots or not enough. The imagery might help you "connect the dots".
Have fun and be sure to post lots of examples of your work.
Kevin Brown, RLS, Georgia & Tennessee
Atlanta area
Photos of the various ponds
> You have an amazing view! I wouldn't mind that work environment!
We are very fortunate!
Think Triangles!
> yswami,
> I would recommend you go through an exercise of plotting out the contour lines at least once. This would entail plotting out the points where you took your shots and scaling/interpolating the even contours in between them. (Maybe you've done this before). You could plot out the points on a paper with a printer to scale if you like, or plot them out on some kind of grid paper by hand. when you draw lines between adjacent points, you get a whole series of triangles. Scale between two points, figure out the the distance to the even foot-contour interval, by comparing the change in elevation between points to the change in elevation from one of the points to the even contour between them. Use that relative difference to determine how far to scale to the plot a point on the next even foot contour interval. Don't scale across "break lines".
>
> Anyway, think in terms of those triangles of where you would like to interpolate between. I believe that is the "thinking in terms of triangles" that was being referred to above. If you can visualize how the software will create the triangles and how it will determine the contour lines between the points, that is enough....but the hand-drafting of some contours might give you some good hands-on ability to visualize it as well.
>
> By the way, thank you for sharing the pictures below. What a beautiful area you live in.
Aloha, Tom:
Thank you for taking the time to explain these to me. No I haven't done this by hand before. As you well aware I am eager but very new to this. I didn't realize that many details are involved in creating topo! I only realized this after Stacy point this out.
I recently purchased book by Barry Kavanagh and Tom Mastin "Surveying Principles and Applications." I begin to study the book little by little. Which only brought up a lot of questions about surveying in my mind. I am planning to post one at time so that I can assimilate all the knowledge generously shared by all of you here. I am attaching couple pages from this book. I think these pages demonstrate what you wrote above. Could you please confirm?
Stacy was very generous to explain few things to me over the phone. That helped immensely! I was thinking deeply about David's thought on "Think Triangles." I think combination of Stacy's imaginary string line and thinking in terms of triangle, I think I got the idea. I was visualizing having an elastic triangle and if I can place the on a surface of the ground and stretch it so that it lies flat or less than 0.3' that will give me my first TIN. Then repeat by forming another triangle which will share two common points but never crossing the previous imaginary line. Now I am thinking triangles!:party:
I don't think I can ever pay back the generous mentoring I am getting here. I will hold all of you in prayers always! :angel:
> > Aloha, Kris:
> > As always, thank you for the step by step procedure. Many of the ponds are pretty small, so I may need to take very close shots. I mean four to five feet a part maybe...
>
> That brings up another set of issues. You can have TOO MANY shots. We run into this when you couple a large as-built with a topographic (true x,y&z with contour lines) survey. They always seem to have the issues at curb cuts, curb returns, steps, et cetera where you need a bunch of shots sometimes within two feet of one another for mapping purposes, but you don't need them that close for the topo. Sometimes, you will find the contours don't match the ground. Breaklines become EXTREMELY important in this environment.
>
> However, for raw land topo, shouldn't be a problem as you probably will not have that many hard drastic changes. Keep in mind that your water level isn't static and will fluctuate over time. I may be inclined to get into the water 10 feet or so and run a topo line around there as well to show the water shed into the pond also. If you really want to get with it, cross section the entire pond. That truly sucks.
>
> Also, around any "soft soils" such as mud or sand, and the topo boot has been mentioned, but if elevations are truly critical, then watch that topo boot carefully. They will sink even with a larger displacement at the bottom of the pole.
>
> When you work around the ponds, make sure you have a buddy with you. Getting stuck in mud or worse requires two people, even if one can only call for help.
Aloha, Kris:
Thank you all these pointers particularly on safety!
I didn't even know the purpose and the importance of the Breaklines until yesterday. I realize how crucial it is now.
Some of the area around the ponds are pretty marshy. Certain area will act like quicksand!! We got our Skidsteer stuck once…oh boy! Had to bring in our neighbor's large backhoe to pull it out.
Your crew are very fortunate to have a boss like you! You are very thoughtful on every aspect of the job!:good:
> It is great to read of your enjoyment while surveying and mapping. I love to see the connection between precise measurements and works of art.
>
> Don't forget the value of a photograph.
> A high resolution aerial image is not only useful for survey planning but can also help you spot missing shot data. With experience, you will see where you are taking too many shots or not enough. The imagery might help you "connect the dots".
>
> Have fun and be sure to post lots of examples of your work.
>
> Kevin Brown, RLS, Georgia & Tennessee
> Atlanta area
Aloha, Kevin:
Thank you for sharing your thoughts! I will post example of my work here soon. Please chime in and give your insights.
For what it is worth, (aren't we surveyors, sittin in our chairs a bit pedantic?) What happens to your water's edge, if the water goes UP 1/2 a foot? or DOWN 1/2 a foot? Where the grade is not steep, this moves your shots a long ways.
So, now, what kind of accuracy do you need?
Jus thinking....
N
>
> Your crew are very fortunate to have a boss like you! You are very thoughtful on every aspect of the job!:good:
I feel very confident that on some days, they do not hold that to be the truth. LOL! 🙂 They do learn though. 🙂
> For what it is worth, (aren't we surveyors, sittin in our chairs a bit pedantic?) What happens to your water's edge, if the water goes UP 1/2 a foot? or DOWN 1/2 a foot? Where the grade is not steep, this moves your shots a long ways.
>
> So, now, what kind of accuracy do you need?
>
> Jus thinking....
>
> N
Aloha, Nate:
I haven't really thought about water edge. There is no accuracy requirement; however, I want to perform the job with highest accuracy that is possible with given equipment.
What are you thinking? Any thoughts?
