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Grid vs Ground Coordinates
I broke in the new studio with the topic of Grid Vs. Ground coordinates. Quite possibly the most import subject I’ve covered on The 3rd Dimension and all too often overlooked by newer surveyors. Check it out…
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22 Replies
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Any “ground coordinates” I’ve ever dealt with are grid.
Using the euphemism grid for State Plane or State Coordinates keeps the issue going more than anything.
I’m not criticizing your excellent video because that’s how it’s presented now.
I never heard the term used for a defined coordinate system until recently and never by my colleagues locally, they all seem to understand the terms.
There are an infinite number of grid coordinates for any L,L position.
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There are an infinite number of grid coordinates for any L,L position.
Then doesn’t it make total sense to collectively refer to those coordinates as “grid”?
I went through school when grid and ground were the common terms and I never thought, and still don’t think, grid automatically means state plane.
The complaints about “ground” make a bit more sense to me, but overall I find the nitpicking to be ridiculous.
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The names for State Coordinates are codified in at least the states I work in. If you submit a coordinate list or a plat to DOT and call it “Grid Coordinates” or even “State Plane Coordinates” you will get slapped hard by the reviewer. The wrong terms for these systems is a big problem.
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Excellent work! Here are some comments, perhaps nit-picky in some cases. No significant criticism intended.
In calculating an elevation factor, the actual radius of the ellipsoid at the point under consideration is used in precise work. The radius of the ellipsoid changes from equator to pole. While the distortion introduced by using the average radius, and there are several different ways to calculate even that simple concept, might be small the goal should be to eliminate all sources of distortion.
Using the term “reciprocal” instead of “inverse” adds clarity. Inverse also refers to finding the distance between two points while reciprocal has only one mathematical meaning.
Simple average of scale factors at the end of a line is ok if the line isn’t too long. For longer lines, the scale factor of the midpoint should be found and the Simpson’s Rule formula given in Stem’s publication 5 should be used. Again, eliminate all sources of distortion.
In scaling about a point, a point with the coordinates and elevation used to compute the overall combined factor does not have to physically exist. The coordinates of the point and its elevation can be assigned from the results of independent calculations. When this is done, looking for that point on the ground is futile.
We should realize that a state plane projection is a life-sized map depicted as a Cartesian coordinate system, or grid. For example, the NC state road map is a table-top version of the NC state plane projection scaled to an average of one inch to 11 miles. Years ago, I read an answer to the question, “Where is the state plane?” The reply was, “You’re standing on it.”
Although the terms “Elevation Scale Factor” and “Combined Scale Factor” are so imbedded that nothing will ever root them out, neither term appears on an NGS Data Sheet. The Elevation Factor cannot be calculated independent of outside measurements; hence it is not a mathematical scale factor. Because the Combined Factor is a combination of a scale factor and a non-scale factor, it is not a scale factor either.
As an aside, since the Elevation Factor is not a scale factor, the Simpson’s Rule formula should not be applied to it or the Combined Factor.
Your video is an exceptionally good presentation and I’m a crotchety old man. Please take my comments for what they’re worth to you and know that I mean no offense.
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Not if I had been doing the reviewing.
It is with some hesitancy that I contribute to this thread because I have observed this discussion many times leads into greater confusion on the part of the reader. Most of us realize that the term ground in the context it is used is a grid plane located near the surface of the earth in or near the project site. Somewhere in my foggy memory I seem to recall we once referred to this type of coordinate system as a grid ground system. That didn’t help understanding much so we then started using the term modified state plane coordinate system. Our state law states Distance measurements shall refer to the horizontal plane. When a tape is used this requires use of a plumb bob to measure on the plane perpendicular to gravity at the location of the plumb bob projected to the higher end of the tape along the plumb line. When a total station is used it requires reducing the observed slope distance between ground points to a grid distance on a plane perpendicular to gravity at the location of the total station. When GNSS antennas are used the measurements from the satellites to the antennas are reduced to some grid plane through the magic of geodesy. The point is all measurements are projected to a grid plane of some kind designed to be useful at the location of the measurement. A ground coordinate system in the truest sense might be a combination of an infinite number of planes perpendicular to gravity at every point in the system. Since such a system is impossible to deal with we are forced to invent “the horizontal plane” by whatever means are a standard of practice. As a result all distances are uncertain to some degree even when measurements can be repeated 100 times out of 100.
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I hear you, but when I first turned in a set of drawings with State Plane after the new Statute was passed I got back a redlined copy with the correct terminology and a reference to the state statute mandating the correct terminology for the coordinate system. It even names the correct way to refer to both NAD 27 & 83. Last year Montana passed a new statute that includes the imaginary NATRF2022 system. I think they got ahead of themselves for that one.
Careful reading of these statutes really doesn’t leave you with any choice, these systems shall be referred to as the Statute defines them. If you use them as a basis for coordinates or for bearings they shall be referenced as mandated. We since always use the terminology. It’s picky, an overstep as far as I’m concerned, but it does give a path to keep these systems under control.
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Thank you all for the comments so far, some great replies here & points I can take and use to continuing improving for the next video!
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Thanks for all your work putting these together. I really appreciate them.
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I like that. I don’t believe there is no such thing as ground coordinates. Only way my brain can state them is if we measured all along the slope of the ground at all times.
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Very well written. I could only add that so many make the mistake of when pulling the combined factor from a NGS data sheet that NGS always publishes ground to grid yet we all keep stating grid to ground. The combined factor is for scaling the ground distance to the grid distance.
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Very very nicely written. However attached is a snippet of a NGS data sheet elev factor scale factor combined factor. I do wish they would reword the elev factor as it is in NAD83 at least ellipsoid. NAD27 elev at MSI
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@bc-surveyor I liked your video just had a chance to watch it. A couple things. One the 1:10000 was not for grid and ground distance. That was for the state plane as we say to be 1;10000 to geodetic aka ellipsoid distance. It was documented that 1:10000 could be achieved if all other corrections were mad. Aka compute the CSF and apply that to the ground distance measured. Curvature and refraction. The corrected zenith angle before reducing the slope distance to a horizontal distance.
The 2nd thing I would add is bring in the topic of mapping scale as I have witnessed people confusing this with the scale factor itself. Not the same.
3rd NORTH
True north aka hardly no one uses this
Geodetic north nad83 non grid or nad27 non grid
Grid north when the geodetic north is the only place and all others are parallel which causes the deflection as we move east or west from the central meridian
Astro nominal north some assume that geodetic and true are all the same they are not
Of course magnetic north
And assumed north
This is where just like you stated when scaling coordinates META DATA is important That’s what tells us about the data I see nad83 only for a north arrow I know when it is on a plat it more than likely means grid north but meta data would make no doubts aka nad83 (2011) my state coordinate system and zone META DATA is important.
I grabbed a plat today magnetic north might I assume the surveyor actually sighted a line and recorded that and which date was it done or did he choose a bearing on a plat pre that stated mag north and a date . For the most part we care about being relative but that magnetic north could be the last hope if all but one monument was destroyed . One could use that and bring it all the way to today’s geodetic bearings or azimuth to grid within the tolerance of the models historically and the precision and care of how it was taken .
I honestly wish we would stop scaleing coordinates yes it works but scaling at one point creates a project coordinates system not a ground. Again even if we threw away the GNSS and went back to assuming 10000 5000 we are still working on a grid system or a plane. Just not a projection. We are actually working on multiple planes based on gravity and we must assume gravity is the same at every set up which when we are at the base of a mountain and go around it gravity is pulling us to center mass not the same direction. This is also proved from running geodetic levels as the direction of the line we run in can change yes ever so slightly our differences in heights.
I do enjoy videos and truly wish I had you ability to communicate so clearly and concisely. From and old man that is unedumacated. lol Great work. Keep teaching you are doing a great job.
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This reply was modified 6 days, 17 hours ago by OleManRiver. Reason: Added north sorry on the meds for bee stings so not all myself
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This reply was modified 6 days, 17 hours ago by
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Great subject and thanks for another well crafted video. Here in North Carolina, the birthplace of the State Plane Coordinate System, NC Grid is shorthand for the NCSPCS. My assumption is that when a name gets too long, such as the North Carolina State Plane Coordinate System North American Datum of 1983 Version 2011 Epoch 2010.00, it’s going to get a nickname. Fortunately for us, NC Grid is mentioned in our GNSS and metadata regs (attached).
Naming conventions aside, I’d like to see more states require that at least one boundary corner be tied to their state’s SPCS. While I have sympathy for older surveyors not wanting to invest in GNSS, I think the needs of the public, the profession, and the younger generation of surveyors outweigh the imagined hardships. I can understand how a PLS working in a state that hasn’t had said requirement for over a decade might think it a burden or might imagine coordinates somehow replacing found physical monumentation. Well, fear not. I’m unaware of any PLSs that have reset an obliterated corner from a NC-Grid coordinate callout, but I suspect it would be a better option than proration if they did. We’ve been slapping GNSS coordinates on plats in NC for many years and it’s made it easier to find irons and allowed us to provide better services to our clients. It even helps the GIS folks out at the tax department.
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This old surveyor was including grid coordinates 24 years ago to aid in recovery.
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This old surveyor was including grid coordinates 24 years ago to aid in recovery.
Never used to do anything with SPC. Purposely kept off of it.
Moved to a new firm, and this place is all SPC. They have records 30+ years old with SPC, and the break downs of sections, etc. It is very helpful.
-All thoughts my own, except my typos and when I am wrong.-
This reply was modified 5 days, 17 hours ago by dmyhill.
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This reply was modified 5 days, 17 hours ago by
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It’s interesting reviewing the statutes for this nomenclature; it looks like North Carolina recently updated the naming convention. I don’t quite understand why 102.1.1 was repealed but they updated the statute effective 7-10-23. The statute states that the new name is to be the “North Carolina Coordinate System of 2022”. It further defines 2022 with a location of the Lambert system and scale factor. However, later in the statute (102-1.2.) it states that for coordinates used in description or identification of surface area or location within this State the coordinates shall be identified as “NATRF2022” or “NAD83” or “NAD27”.
I find the statute confusing.
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Now, I’m no absolute defender of either NC statutes or procedures, but this one passes my personal muster.
Note that in prior state plane systems, Lambert projections were defined by a central parallel and two secant parallels, the north and south standard parallels. Changing the definition to a central parallel and scale factor is an improvement.
The new definition allows system designers to define the central parallel simply (without excessive decimal places) and also to set the accuracy (1:10000, for example) at the central parallel.
Under the old definition, north and south standard parallels were determined simply and the latitude of the central parallel was calculated. If you refer to Stem’s publication 5, you will see the unwieldy result for the central parallel in NC.
The same reference will show that the resulting scale factor at the central parallel does not meet the 1:10000 criterion.
As to projection names vs coordinate descriptions, NAD doesn’t correspond to rhe current name for the NC State Plane Corrdinate System, either. I suspect thar similar divergences appear in other states also.
In any event, Lambert projections can be defined by either defining standard parallels and computing the central parallel or by defining the central parallel, assigning a scale factor to it while ignoring the resulting standard parallels.
If you look at the Lambert zones in states like Iowa that have many LDPs, you will find that those zones are defined by central parallel and assigned scale factor. As an aside, all Lambert projections, including the current NC state plane projection, can be defined by a central parallel and its scale factor.
It’s all just applied mathematics, and that’s the easy part. The hard part is what surveyors do; use the resulting projections to produce a valuable product.
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I’m only concerned about the naming convention. Are they wanting it to be named “North Carolina Coordinate System 2022” or the “North Carolina Coordinate System NATRF2022” or “NATRF2022”. The first name seems to be the sweet spot, the second seems to be redundant, and the third doesn’t give enough information.
The Montana code is more precise as it mandates the new name to be the “Montana plane coordinate system”. The older systems are the “Montana coordinate system NAD83” and the “Montana coordinate system NAD27 (zone north, central or south)”. I’m assuming from the new name 2022 is only going to have one zone like NAD83. Not a fan!!!
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Overall, this is another great video. I would make a few comments.
First, you can almost always be certain that the metadata will be lost as soon as engineers or their ilk get their hands on it. I can not tell you how many times I seen plans based on my design surveys included in plan sets with none of the metadata attached.
Second, maybe I misunderstood what you were saying but you can most certainly derive a project combined scale factor from multiple locations within a project. This often provides a solution minimizing distortion over the entire project area. In a smaller area with minimal vertical relief a single point might work but not so much for larger projects.
It is best to scale about 0,0,0 and then truncate the resulting coordinate to ensure the resulting values are not confused with the grid values. See my first comment for reasoning. I have always disagreed with, the rational with scaling about a particular point to allow for inclusion of some data on an actual projection. I contend that you could just as well translate the data to fit the project coordinates, the distortions will be the same and when your values are released into the wild they will not be confused for something they are not. It makes even less sense now with CAD systems that let you specify how your project coordinates are transformed from projected grid values.
Remember, friends don’t let friends scale grid coordinates without truncating.
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I agree with John that we may assume that any metadata we offer in our submissions will be jettisoned by the time final plans are generated.
And while I see his point about scaling about 0,0 and truncating I also understand why people scale about a point central to the project and don’t truncate. Doing so leaves a survey that will fit to GIS planning level stuff that designers rely on so heavily while having true ground distances – as long as the project area is contained within a few square miles. Which is most of them. It’s a PITA, but that is just our cross to bear. Low Distortion Projections address this scaling problem very nicely.
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This reply was modified 4 days, 18 hours ago by Norman_Oklahoma.
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This reply was modified 4 days, 18 hours ago by
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