indeed, if you expect grid to match ground. But why would anyone have that expectation given that the combined factor exists to match grid to ground? Back when calculating point by point combined factors was laborious and error prone, a single combined factor made sense. But today, not so much.
When Trimble works its magic, the original data are replaced by the recomputed data. How does that help?
We have the capability to use state plane as it was designed to be used without intensive labor. Why wouldn't we do that instead of having umpteen coordinate systems scattered over umpteen projects?
I have run across many discussion threads about grid vs. ground, and I didn't intend for this topic to turn into another one. Great input but not addressing my primary question.
In summary, I performed a boundary survey using GNSS, adjusted using least squares and achieving acceptable relative positional accuracy. After the boundary was complete, I needed to set a new corner marker in an existing 800 foot line, and the point needing to be set was 1000 feet from any vehicular access, meaning that it would be much less trouble to stake the corner marker using GNSS than to carry in the conventional equipment, verify the existing corner markers at each end of the line in question, and traverse to set the new corner. In the past I would have had a nearby (adjusted) traverse loop from which I could set the new marker. Essentially I am asking, if you need to stake out a critical point as accurately as you can using a GNSS system (base/rover), do you have a particular method to accurately place the point (in my case a new corner marker in an existing line), if the data is at ground, not grid?
Am I wrong in thinking that there are essentially two ways to attack this:
1. Scale the point to be staked to grid, and stake the point using a grid-to-ground correction in the field.
2. Use the ground coordinate for the point, but do NOT use grid-to-ground correction in the field (i.e. the data collector treats the coordinate as a grid coordinate).
These methods assume there is at least a small necessary correction to scale from grid to ground. If the grid factor is approaching unity and the distance from the scale point to the staked point is not too crazy then there is probably more error hammering the iron into the ground than there is with an ignored or improperly used grid factor. On the other hand, if the grid factor is somewhere in the range of 0.9998, then a 1000-foot distance from scale point to staked point will have a difference between grid distance and ground distance of 0.2 feet or so. If I'm trying to place an iron in an existing line, what do I do to compensate for this large difference, to ensure that the iron is truly in the existing line to the best of my ability?
I'm just throwing this out there as a discussion point. Unless someone screams otherwise, I will likely use one of the methods I mentioned above. As with other tasks where I have begun using GNSS in place of conventional equipment, I intend to experiment with this to determine that these methods will or will not produce repeatable results that are satisfactory. But if someone has already walked through this exercise, I'd appreciate the feedback.
Bud
when you calculated the new corners to be set. What did you compute them in. I as it based off the grid or ground version of your survey. That’s my first question. I don’t know much about the Carlson. So I am not much help I. Exactly how it works and thinks. How we do this is. We always start in state plane. GNSS robot etc. once the he control as been evaluated I have to scale to ground here. Least squares all is done. Boundary is resolved in ground. When we return I use that ground based system call it my project datum / coordinate system. This is now used in stead of state plane for setting new corners or missing corners. This no matter what tool. GNSS or conventional. Ow I have the ability to switch back and forth between grid and ground in office or field.
As far as staking out a point the corner with GNSS rtk. I set the corner like you state. Reset rtk re observe go set the others same again re observe. I go back. At a different time. The time gap is more important than the day. 8 am today and the next 30 days you are only 4 minutes difference in the constellation. Ow I like to kick a leg or move my base. This also helps prove your base was plumb centering error. On my 2nd observation to points. Just like traversing and all. If the base has never moved or broken down and re set up. If it was not over the point you can still repeat to that point u set disregarding any multi path etc. so you can be precisely wrong. Sometimes I will set a temp nail or nails 10 ft or so from the corner and observe those in canopy along with the rod. I tape them in and inverse with his checks my multi path. I will often set up a 2nd base before hiking in and switch to it so no gap I. Time really but I have nails outside the area and a second centering error check. On the set rods.
The math answer is that they are equivalent, so it doesn't matter which you choose. However, your original question seemed to be more about how your equipment works rather than math. I can't offer specific help there, but after more than 50 years of messing with computer input, programming and output, i would look at what the data collector is expecting as input.
In your case, it seems to be grid coordinates, so I would offer as a first thought that entering a ground coordinate does indeed apply the grid to ground adjustment twice.
Can you tell whether the coordinates that the GPS used match your calculated ground coordinates? If they do, then you're ok, otherwise, enter grid into the data collector.
Mostly because SP was developed for huge mapping projects, not construction, are you arguing that .8' in 1000' is somehow an acceptable error? For the Telluride calculation it is 5/8" per 100', that is pathetic. And there isn't a reason to do it. Airport projects have tight tolerances. Being on "grid" 2 miles below the workers feet is not rational. There is not an advantage to grid distances unless they are within an error budget, otherwise all the vendors making steel, girders, building bridges, pipelines have to retool each project to meet some shrink, just so it's "easy" for button pushers?
Stay on surface, it's simple. No reason to shrink projects to some imaginary data surface.
There is no error if you calculate grid distance and then adjust it by the combined factors at each end. That's what the airport example shows.
I won't argue that LDPs or single factor coordinate adjustments are somehow wrong or insufficient, they're not. But I do wonder why with computers taking over the drudgery and eliminating human calculation errors we don't use state plane as it was designed to be used.
There's no need for a state-wide collection of mini state plane systems to obtain accurate surveys. Even states like California and Texas could use a single state plane system. Back somewhere in the not-too-distant past, we used the NC state plane system to compute a distance in Montana with no problem save for initially omitting the t-T correction.
If you follow the rules, you get the right answer.
No error? Map projections are inherently full of error, that's why Antarctica looks so huge on most world projections. It's the same thing for State Coordinate projections but on a smaller scale. Sure, we can project the Geographic numbers to a repeatable XY coordinate, but that doesn't mean it's without error. It only means that the rules defining the error are being observed. If you measure with a 100' tape along the ground better than the GPS can because of the projection, then that's an issue. I can assuredly measure along the Telluride runway with my 300' tape over 1000' closer than .5'.
We just matched a 7111-foot measured runway at 9000 feet elevation with a state plane calculation. We were able to do that because the combined factor adjusts grid distances precisely to ground distances.
What it doesn't do is match the path of a line on the surface. The surface line is curved, the equal distance state plane line is straight. And that's true for LDP lines and single scale factor lines. If any of these get too long, the coordinates of the midpoint of the ground line will be significantly off from those of the midpoint of the plane line.
By the way, one of the primary reasons we use a single combined factor is that CAD is a simple-minded plane geometry system that can't deal with multiple scales in a single drawing. So, CAD can't duplicate something that surveyors of yore did routinely. They could stand ambiguity, CAD can't.
Delving into plane projections is always a fascinating pursuit.
@mathteacher One reason might be errors can be expensive. If you make an error in scale or an error of US feet vs International feet when your coordinate values are in the millions 1,333,444.55 3,123,456.78 it is much more likely to cause a problem. Problems are expensive. When someone makes the same error when the coordinates are 33,444.55 12,345.67, sometimes it is not even noticeable. So risk management might be one reason.
Yes. The simple geometry does reduce risk with acceptable error margins. Also, CAD is a productivity driver. In the old days, a project combined factor improved productivity by reducing the labor required. CAD is broader, but requires single combined factor for a different reason.
You really, really want surveys to be in state plane. I don't know of any engineering or surveying company wanting to survey deep in the earth instead of on the surface. The Telluride example is two miles underground, a projection can easily be calculated to me miles above ground, heck you can push it so far up that the 7111 ft. distance becomes 14,000 ft. The math works, so why not do that.
When you calculate a single combined factor, you're still using state plane all of those feet underground. If you inverse between two points on any of your projects using state plane coordinates, and then multiply the result by your adjustment factor, you'll get the exact same answer that you get by inversing between your ground coordinates for those points.
The only caveat is what Trimble's magic may have done with rotation and least squares adjustments. Even then, you're still using state plane coordinates as your geodetic basis.
Try it and see. It should take only a few minutes and it's very educational.
Yep, you're exactly correct, however that means that each distance needs to be recalculated, by globally multiplying the coordinates that never needs to happen. Imagine a contractor laying out a 200'x100' building, telling the crews that each measurements has to be increased or decreased by a factor cause the building was designed using state plane. It's not an imaginary issue. The most recent one I had was a switch for a railroad. They insisted it be done in state plane, then complained about the results of doing that. They complained over and over, but I couldn't help them resolve their issues. They didn't like my plat showing the ROW being 199.84 feet wide. I asked them if they wanted to take a tape measure and measure 200.00' between ROW monuments, they said yes. They didn't want to measure 200.16' which would have been the surface distance for a 200.00' state plane designed number. The state told me my areas were all wrong, they weren't, they were surface areas, not SP areas. All those issues vanish if you simply multiply the coordinates.
Many highway surveys (I avoid railroads like the plague) specify SP coordinates and ground distances. As a surveying layman, that seems like a good approach.
Points are reproducible and findable by coordinates and distances are reproducible by ground measurement. Labels on drawings show SP coordinates and ground distances.
Perhaps that's what your RR guys were after. I have no idea how the resulting CAD drawing would be done or what it would show.
Inversing between coordinate pairs does not compute the distances shown without combined factor application, but that's the compromise that preserves coordinate basis system-wide.
Which is better? It depends on the user.
Many highway surveys (I avoid railroads like the plague) specify SP coordinates and ground distances. As a surveying layman, that seems like a good approach.
I've never heard of that, SP coordinates and ground distances published for a DOT project, that would be a painful experience of pointless calculations. All plan sets I've ever seen use surface coordinates and distances. Those plans go back into at least the 1970's, they've been doing it for 50 years that way.
As far as the railroad, they have a huge inventory of ROW and I don't believe they've worked in that area for decades so they didn't understand the implications of NAD83 State Plane there, but they wanted to keep it all seamlessly on their newer computer GIS system. Which makes perfect sense to me. I offered to do the platting on the surface and give them a reduced drawing 16000 feet below ground on the SP surface but they wanted everything three miles below ground from start to finish.
The railroad project quashed because of a dispute about the switch construction.
The railroad people were really good to work with, but the SP was a new version, and I believe it's quite different than other areas they deal with.
Yep, I spoke without thinking. The airport data above uses SP coordinates and ground distances, but I don't know of any DOTs that do.
There are 3 math elements involved: grid distance, ground distance and combined factor. If you're given 2, you can solve for the third. For example, given SP coordinates and ground distance, the combined factor is the inversed grid distance divided by the given ground distance.
Obviously, modified coordinates solve most problems and are the best current method except for a wider area LDP. But just as DOS is still the heart of Windows, SP is the heart of ground coordinates.
On the other hand, explain what the coordinate system note on this one means: https://www.scdot.org/content/dam/scdot-legacy/business/pdf/surveys/survey_control_data_sheet/Example%20Survey%20Control%20Data%20Sheet.pdf?v=2
It won't download for me. I will also add that there are some local projects that were Grid, not many that I've run into, both coordinates and distances are in grid. That does happen, not a big deal for me, I'm used to SP grid.
The attempt for the new projection is to put 95% of the state's population within a grid system that has less than 75PPM of error. I talked with DOT if this will negate Project Adjustment Factors, I thought they might have a heart attack right there. Short answer is no way will they stop modifying the projection.
Yeah, me neither. Here's a screen shot. Are the coordinates state plane or modified state plane?
That is just a brutal Datum Description. Is it State Plane or just developed based on State Plane? CSF applied to ground distances in order to what? Get grid distances? Is he trying to tell me I need to apply the CSF to get ground distances? Look how easy and convenient using State Plane coordinates can be. No way any of this is legitimate. Theo may have a telescope but a theodolite could not have written that statement.
Friends don't let friends modify State Plane coordinates without truncating them.
I found the explanation here on page 57. The coordinates are State Plane and if you want a distance, you have to calculate a combined factor for each end of the line. That's a piece of cake with GPS and it is an example of classical use of State Plane on a DOT project.
Of course, this just says that it is allowed. Whether it's ever used or not is unknown to me, but it's the first time I've ever seen such a thing.
2023 SCDOT Preconstruction Survey Manual.pdf
The statement leaves out too much.
It would be better to reduce it to one clear sentence:
"Coordinates, bearings and distances shown hereon are on the South Carolina State Plane Coordinate System, NAD 1983(2011), North/South Zone (Metric/US Survey/International Feet)."
Or even.
That would be sufficient. The second sentence adds confusion. If this information is all on the SP grid then it's a needless statement, if not, then the metadata statement needs much more information.
Not saying which zone the project is located is a huge no-no, particularly near the common line of the North/South Zones.
Not telling which measurement standard used is also a no-no.
I tried to blow up the notes, but it was too blurry and I couldn't tell if any of that information cleared up the statement.