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...
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.
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.
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.
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.
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.
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.
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!
Thanks for all your work putting these together. I really appreciate them.
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.
