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Thread: Best Practice for search coordinate export: elevation setting?

  1. #11
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    Default Re: Best Practice for search coordinate export: elevation setting?

    Quote Originally Posted by remi678731 View Post
    That is your work flow. It doesn't necessarily work for everyone. We do several different types of projects for different reasons. We use assumed coordinates, SPC, CORS, etc depending on the project and requirements. A word of caution with SPC we received a project to stake. The client designed a large wharehouse in a dwg on SPC in grid. The problem they didn't understand the difference between grid and ground.
    >>And it really doesn't matter what the _original_ purpose of the drawing was, inevitably the purpose will morph and expand as the data gets used and reused.

    In doing CAD for 3 decades, the only inviolate rule I've ever found is that.

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    Default Re: Best Practice for search coordinate export: elevation setting?

    Here is an example, a project I just did last week.

    I had to do an Alta Survey. The deed description for the property was not in a State Plan Bearing System. In this case, the survey would remain in a Deed Base Bearing system and would not be moved to SPC.

    Its important to know what the end result of the project is. An Alta survey will never be used for construction purposes as this is a survey strictly for title insurance matters. Therefore its important to make sure that certain elements are maintained when preparing those types of documents.

    A plot plan survey for your house used in a mortgage, does not need to be in SPC.

    These are just some examples of other survey's that are needed to be prepared. I hope that this enlightens you to understanding a bit more of the surveying process and why not all survey's need to be in SPC.

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    Default Re: Best Practice for search coordinate export: elevation setting?

    Quote Originally Posted by Iceberg View Post
    Here is an example, a project I just did last week.

    I had to do an Alta Survey. The deed description for the property was not in a State Plan Bearing System. In this case, the survey would remain in a Deed Base Bearing system and would not be moved to SPC.

    Its important to know what the end result of the project is. An Alta survey will never be used for construction purposes as this is a survey strictly for title insurance matters. Therefore its important to make sure that certain elements are maintained when preparing those types of documents.

    A plot plan survey for your house used in a mortgage, does not need to be in SPC.

    These are just some examples of other survey's that are needed to be prepared. I hope that this enlightens you to understanding a bit more of the surveying process and why not all survey's need to be in SPC.
    I agree, we use various coordinate systems including assumed, and sometimes assumed tied tied to a particular elevation it all depends on the project requirements and use. What I find more useful is communication as what is what then I have the option to make the transformation if needed. Not like last week I got a dwg in SPC for control. Then the design engineer moved everything to CORS for the low distortion projection, the only problem is they didn't tell anyone. And here is my favorite, I got a dwg in feet SPC no problem, except for the two XREFs one in metric and one in inches again no one tells you anything. Just have to figure it out.

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    Default Re: Best Practice for search coordinate export: elevation setting?

    Thanks, guys. This is a bit of a long post, but hopefully provides some food for additional thought. Most of you realize we make a number of assumptions in our work, which may not provide total accuracy (as if anything could ever be), but fulfills the purpose of leading us to an adequate or efficient solution.

    My initial concern had to do with how the C3D software treats a null elevation versus a 0 elevation. The whole question came up because I forgot to change the default elevation of the template before starting a job. Though the mistake was caught quickly, it did set my wheels to turning. So, now I’ve set a default value for where I do 80 percent of my work. I’m still looking for the answer to my original question; however (And if you’ll follow along here, you’ll see this is not a negative criticism.), most of your comments took the thread off in a different direction that made me think about what could be the bigger issue of the effect of elevation on a grid system, whether of the 5000,5000, no datum variety I often use; SPC; or SPC modified, with the grid raised to the elevation of the project. Someone evoked the GIGO postulate, which gave me pause to think about the effect of our assumptions and those times they could come back to bite us. But it was also suggested to avoid using elevations at all, just for coordinate searches. Is that really possible? But if my only purpose for the preliminary coordinates is finding existing control, might the failure to use at least a ballpark elevation as part of my search criteria actually mislead me, because my search locus shifted too much from where it ought to be? In other words, how much apparent horizontal difference would there be on the ground between X, Y, 0.00 and X, Y, 5000.00, where X and Y are equal for both points, if at the same elevation? In most cases, we can probably all agree, if there was only a one-half-foot error from that source, it’s probably OK, just from the standpoint of finding a monument.

    So I went off into a corner to ponder and analyze this a bit and maybe come up with some simple examples to demonstrate certain effects elevation may introduce into our work. With respect to point geometry, I’ve known having a point’s elevation at least in the ball park of a true elevation was useful, but was less certain of the effect of no elevation. Now that I have the lint cleaned out of my navel about that, it seemed some of these results are worth sharing. For some it is a review of what we know. The bottom line is the answer would vary from job to job and depends on how much area is covered by the project and maybe even more, how far one is from the origin of the coordinate system, and of course, the absolute elevation of the project. So SPCs being at one extreme where an effect could be determined, when I might be out 3 million feet or more on one of the axes, elevation matters at least as much on as using the correct foot per the coordinate system definition. But could it be a lot more?

    In the old days, by which I mean the 20th Century, those of us practicing then might remember being taught the Earth is flat and is at the center of the universe, or at least to pretend it is. For probably 99% of survey work many of us do, living by that assumption worked just fine for every task; even when shooting a solar. Nearly all of our measurement errors (I am talking about errors, not mistakes.) were larger than any systematic corrections we could make due to the effect of the Earth’s curvature, or we learned techniques to “eliminate” those effects, provided we were aware of certain constraints, one would never be able to measure the difference. That was a good thing, because the formulae for dealing with those items were complex and had to be done with every observation in the days when every calc had to be written down before the PC mostly replaced the HP41 as the office computer.

    So in the world of plane surveying, we were taught, for example, as long as a triangle on the ground was less than 75 square miles (For comparison, a triangle with a base and altitude of just over 12 miles each would fill the bill.), the theoretical sum of its interior angles will still be 180d00’00”. And when leveling, if you your FS and BS lengths were reasonably balanced on your level loops, open or closed, you would avoid what could amount to substantial systematic errors from the curvature of the Earth, which no amount of re-measurement could eliminate, if that mandate was not followed. Above all, we knew and still know all of our measurements are estimates to some degree, to which we apply only the degree of refinement necessary to accomplish the task at hand; i.e., don’t waste time and the client’s money measuring to the hundredth, when results only need to be to the nearest foot.

    Now the considerations in the previous paragraph are still valid today, but due to the laws of physics and the measurement system we've created, GPS could not exist in the realm of plane surveying, since it is by definition a global system. I believe in the term “Geospatial, ” spatial intends to bring to mind the integrated effect of observing the Earth and points under, over, and above its surface in a purely three-dimensional, geometric (Euclidean) space, at least within the roughly 8,000-mile cube that contains the Earth, rather than interstellar space. As a result, we had to become geodesists to a degree; at least until we properly set up the job, after which software handles all of the geodesy and it mostly recedes into the background again. But it is probably a good idea to look under the hood once in a while to make sure everything is still running smoothly. That is what is going on here.

    Seeing Iceman is located in New Jersey, where Mr. Google says the mean elevation is 250 feet, he may never experience the effect of elevation to the degree it can affect us in Colorado with a mean elevation of some 6,800 feet, with our state’s terrain ranging between 3,350 and 14,400+ feet. Personally, eighty percent of my survey own projects are in the neighborhood of 5,000 feet elevation and cover no more than a quarter-section in extent. (The actual subject parcel might not extend near that much but its control does.) Most of the rest of my work is carried on between elevations of 6,000 and 10, 000 feet. Also, I get the occasional project in the mountains with as much as a 2,000-foot elevation delta from the highest to lowest surveyed point. Such jobs create excellent text book cases for consider establishing a ground datum, especially for retracement of an ancient survey, which, due to those flat earth principles, yielded ground values by default or when your coordinate system may live on after your initial work is done in a construction project, say a road or a pipeline is planned. Also, it becomes essential, if a mixture of GPS and conventional survey observations are both used for measurement or layout.

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    Default Re: Best Practice for search coordinate export: elevation setting?

    Layer 0 & 3/4 post continued:

    What I do know in regard to working on larger tracts with long boundaries at this elevation is the difference between grid and ground becomes both significant and apparent. A mile-long line (which I believe might be more common here in CO, the land of the PLSS, than NJ) measured at a ground elevation of 5,000 feet gets about 1.25 feet shorter, when projected fully to the SPC grid, considering the sea-level correction alone. Even a 500-foot distance, being 0.13 shorter at grid, crosses the threshold of a measureable difference. (As you may recall, because the model SPC zone is supposed to have, by design, a mapping error between the curved reference surface and the flat grid cutting it not to exceed 1:10,000, a limit which is controlled by limiting the extent of a zone in either the east-west (Mercator) or north-south (Lambert) direction, the SPC grid factor therefore ranges between 0.9999 and 1.0001 to achieve that, and varies between those values, depending on where one is in the zone. In my state, once the sea-level and grid factors are established for a project and multiplied together, the result is published and reported as the “combined factor.” The combined factor, unless you work in an area located below sea level, will generate numbers in a similar range but slightly smaller. By the way, while I’m not necessarily advocating SPC be used on every project, though if any available control, including CORS and any of the VRS type networks, has a good Lat, Long associated with it or an established SPC position, it is very easy to apply to a project, it was merely included it in my analysis just to weigh the effects of big coordinate values on the issue to see whether GIGO would have large or negligible effect in every instance. Probably because software lets us get away with it, I have seen that some practitioners use SPC, without having even a basic understanding of the general features of the system, which are actually quite simple to comprehend, it seemed best to spend time rehashing some of its basics for the benefit of anyone reading this, who wants to know more.

    Actually with the large coordinate values intrinsic to SPCs, somewhat surprisingly, it actually matters a whole lot more in absolute terms at the elevations I do my work than I would have ever thought, even though I’ve been working with SPCs for over 30 years and unexpectedly large but dramatically less nearer to sea-level. Consider a certain line in some Lambert conformal zone extended approximately along one of that zone’s standard parallels (For those of you more used to working in transverse Mercator zones, this location was purposely selected to set the scale factor to 1.0000 and consider only the effect of elevation.) Using the sea-level, or reference surface, reduction formula, R / (R + E), where R is the radius of the Earth at the project latitude (Here, the classic value of 20,906,000 feet is sufficient for purposes of this demonstration.), and E is the average elevation of the line. Extending a line 3,000,000 feet due east from the Y axis (0,Y) at the elevation of 5,000 feet gives you a grid-value X coordinate of not 3,000,000, but more than 700 feet less! For the sake of comparison, let’s change the elevation to 100 feet and repeat the calculation and we find the coordinate changes only about 15 feet from ground to grid, a dramatic difference. So the conclusion for working with SPC is that reasonably correct elevations are important regardless of where you are.

    For the local, assumed coordinate system, the elevation is inconsequential, when working near sea level, but at elevation, does double that self-imposed half-foot limit mentioned earlier, when the job extends over a one-mile range. For a modified (ground) system, note the term R in the formula above becomes (R + H); where H is the job elevation, and the term E is replaced by (E – H). So the difference in elevation between the project elevation and a point on the ground would be a fairly small value, elevations could be completely ignored in a local system. the elevation doesn’t matter in the sense all the points could be set to the elevation established for the project, and could easily vary from the true elevation of the point being sought by a hundred feet without generating significant error; however, if working up here where I am, the points preliminary elevation should not be set to zero, which is part of the answer to the original question.

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    Default Re: Best Practice for search coordinate export: elevation setting?

    Super helpful post!!

    That sounds like a great article for Augi World.

    FWIW, I mostly do work in FL, and is a site has three ft of elevation change across it, that's due to the stormwater ponds.... ZERO elevation is a no-no around here, since that happens to be a real value that we work with.

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