Using a point on the rod. It’s above my pay grade to consider the sod removal. Will tape a sod sample to the field notes. We have to shoot a 25’ overlap past the right-of-way line. When I questioned why I was told it’s necessary to create contour lines.
But surely there will be match points somewhere in this that need to be known more accurately.
Not 100% sure what you mean by match points. There is a paved road running through the project. Do you mean matching a sod elevation with the nearest pavement elevation?
Do you mean matching a sod elevation with the nearest pavement elevation?
Yeah, that or maybe if there's existing sidewalk at each end of where the new sidewalk will be and you're just filling in the sidewalk in between then you'll need good elevations on the existing stuff to match the new to it.
whatever your shown contour interval is, 90% of random points selected must fall within the 1/2 contour interval range at that point vs a "real elevation" on the ground at that point.
This is confusing me somewhat. Basically it sounds like all points need to be within half a contour interval of the next closest (elevationwise) point.
The random points are points not used in the DTM creation. By definition, any of the points used in the DTM will fit the DTM perfectly. You need to have additional confidence points for use in QA/QC.
The National Map Accuracy Standards written for photogrammetrists in the days of the Kelsh stereo plotter at best. The stereo plotter operator would actually trace the contours by keeping the dots together. Discrete points were just spot elevations not used for the creation of contours. Unfortunately, I spent a bit of time mapping Bismark, ND on with a stereo plotter while in photogrammetry purgatory before being called up the show.
If you don't have a topo foot, consider taping a cap to the bottom of the point when taking ground shots. That should stop the point from harpooning into the sod. If you are really concerned, you can subtract the thickness of the top of the cap.
When shooting existing topo for design, I think the best approach is to understand how a modeled TIN and breaklines define your existing surfaces, so if your proposed sidewalk (are you shooting for design or an shooting existing sidewalk?) is changinging grade quickly, going up over hills or sagging in depressions you'll want to space your shots more closely at those more dramatic changes in grade
We’re shooting for design. Took an abundance of shots in an area with a mitered end section. Got top and bottom of bank too.
Running topo in the field with an understanding of how the models are built is an art form to some degree. The more complex a site we always had a goto party chief to do the job.
whatever your shown contour interval is, 90% of random points selected must fall within the 1/2 contour interval range at that point vs a "real elevation" on the ground at that point.
This is confusing me somewhat. Basically it sounds like all points need to be within half a contour interval of the next closest (elevationwise) point.
I'm not sure I can describe it adequately but my understanding is that, yes, most or all (90%) random points selected off a topo map, when shot in the field using the same basis of elevation, must shoot to +/- half the contour interval.
EG, on a map I pick a point at the corner of a 90 degree bend in a sidewalk. I calculate that elevation by whatever means (interpolation). I go out and shoot that point. The shot elevation must fall within 1/2 a contour interval.
So if I'm taking the calced point from a detailed existing topo map showing 1' intervals, the shot elevation must fall within 0.50' of the calculated point. Same map, same point, but 5' contour intervals - same point must shoot within 2.50' vertically of the calculated point. It's a resolution tolerance.
I'm not sure I can describe it adequately but my understanding is that, yes, most or all (90%) random points selected off a topo map, when shot in the field using the same basis of elevation, must shoot to +/- half the contour interval.
Yeah, it makes sense as a way of ensuring you end up with consistent data for any given site but boy the government sure does find the most unintuitive way possible to word things. 😆
This is confusing me somewhat. Basically it sounds like all points need to be within half a contour interval of the next closest (elevationwise) point.
I normally wouldn't let someone go out and shoot topo who I'm not positive understands what they're trying to accomplish. It looks like its all dirt, rock, trees, asphalt and grass so "tolerance is not as critical as, say, a control survey or a K2 mortgage survey". True, but knowing what you're trying to get and how not getting it right can dramatically affect design and save or cost a lot of money and headache (as well as time in the field) is important.
About point spacing, the vertical difference between points isn't as important as identifying grade changes. For a retaining wall you'd shoot top and bottom of wall. Those shots might be just inches or less apart and may be much more separate vertically than your contour interval, but there's a consistent drop between them. You'd have to create those breaklines (top and bottom) to get your topo right along it.
Conversely, on a parking lot that's sloping consistently (and perfectly) in one direction you might only take 4 shots, 2 at the top and 2 at the bottom, that are all 200' apart but 8' difference vertically. Both those scenarios would give you an accurate TIN that would keep your mapped grade accurate at any resolution (0.5' or 25' contour intervals).
Understanding this resolution when you're out shooting topo is important, though, not so much in determining some maximum point collection distance, but to save time in the field not collecting more than necessary. ie, if you'll be creating 2' contours and that "perfect" 200x200 parking lot is actually only 3% grade in the upper third and makes up for the difference in the lower 2/3, you don't care, it won't significantly affect your contour shape or the drainage pattern. On a putting green topo, though, you're going to want to catch all those variations, so points would have to be closely spaced, unless of course the green was designed at some perfectly continuous grade in the area you're getting.