Even without the elevation factor.........
We have to be clear with what we mean by "grid factors." I haven't looked at central Montana, but I suspect that the Scale Factors will fall between 0.9999 and 1.0001. Combined factors obviously won't and that's Loyal's point. You can't substitute unadjusted grid distances for ground distances when your points are significantly above the ellipsoid.
North Carolina, though not as high as Wyoming or Montana, does have significant relief from mountains to coast. Mount Mitchell's elevation is more than 6600 feet and parts of the coastal area have negative elevations. An acceptable state plane grid, that is, a grid covering the entire state, cannot be defined for NC if it is to provide 1:10,000 accuracy with the ground. And that's true for Wyoming and Montana as well.
But, if the area covered is limited, then an acceptable plane grid, spelled LDP, that provides high accuracy between grid and ground can be defined. And that's what Loyal and so many other people do.
jpb, post: 375918, member: 9284 wrote: http://www.ngs.noaa.gov/OPUS/getDatasheet.jsp?PID=BBBS28&style=modern
Got to love the convergence angle for that one
MathTeacher, post: 375923, member: 7674 wrote: We have to be clear with what we mean by "grid factors." I haven't looked at central Montana, but I suspect that the Scale Factors will fall between 0.9999 and 1.0001.
Math Teacher:
Here's the [Lambert] Projection Parameters (constants) for Montana NAD83 2500 SPC:
North Standard Parallel 49å¡00'
South Standard Parallel 45å¡00'
Origin Latitude 44å¡15'
Central Meridian 109å¡30'
False Northing 0 meters
False Easting 600,000 meters
Only a very small portion of Montana (extreme SW) falls outside of the Standard Parallels.
However, the "Central Parallel" has an effective scale factor (ko) of .999 392 636 277, which works out to about 1:1645 ellipsoid to grid. Basically 608 ppm, and folks freak about UTM zones @ .9996 (400 ppm).
Loyal
There is an IMPORTANT piece of the overall puzzle[ment] here, that seems to fall through the cracks.
When the USC&GS created (invented) the State Plane Coordinate Systems back in the mid 1930s, they ASSUMED that the local yokel surveyor would know enough to REDUCE his distances to "sea level" before calculating his coordinates This was NOT that big a deal, even without a fancy computer.
By doing so, the +/- 1:10,000 (or so) threshold would be met. If one needed (or wanted) to be more precise, there were State specific Tables published to generate State Plane 'k' (scale factors) for a given SPC Coordinate. Problem solved.
Loyal
Loyal, thanks for cutting to the chase and correcting me with a clear example from Montana. The scale factor on the central parallel is quite obviously less than 0.9999, so the Montana state plane does not meet the 1:10,000 criterion along that parallel.
Looking further in NGS Manual 5, Montana had 3 zones, 2501, 2502, and 2503 under SPCS 27. A note on page 5 says cryptically that these were "...eliminated in favor of a single State zone MT No. 2500." Similar notes appear for Nebraska, South Carolina, and Puerto Rico and Virgin Islands.
It would be interesting to know why Montana opted for a single zone with substantial error and decided to put the maximum error along the central parallel. In North Carolina, the single-zone design affected a relatively small area to a relatively small degree. That's obviously not the case in Montana.
Again, as you pointed out, the error is overcome by proper computations. The sloppiness that can be tolerated in some other states leads to disaster in Montana.
The idea that the 1:10000 refers to the grid factor may well be something that the designers were interested in TRYING to hold for much of the state plane systems but it clearly wasn't used as a line they wouldn't cross.
It was always my understanding that 1:10000 was the lower limit of the actual ACCURACY of the state plane network.
You can expect to at least close your traverse between NGS monuments that well or better. I don't mean just measuring distances and turning angles, I mean doing all the calculations to reduce your distance measurements to "sea level" and then calculating your coordinates in the state plane system. State Plane was not intended to be used as a ground system.
And it was not all that accurate even when reduced to "sea level". The monuments were triangulated off base lines that were measured with tapes..........
It's an amazing system, but it became clear when accurate measuring devices became available that it needed upgrading. Still, to have those monuments in the field and for them to be accurate to 1:10000 and better is a wonderful resource,,,,,,,,,,,how do you think all those quad sheets and photos were made.
When the big solar flare hits and wipes out all the GNSS sats. we will all be glad those monuments are still out thereB-)
Shawn Billings, post: 375719, member: 6521 wrote: Since there are places in Texas where the combined factor is acceptably near enough to 1 that grid and ground measurements are functionally equivalent, clearly LDP are a fools errand. Carry on.
And there are places in Texas where the CSF plays hell with acreage. LDP would be awesome there. 🙂
Gotta be honest here, I love the grid. Too easy not to be on it. However, most of my projects have an ortho of under 500' and some are as much as 800' Not many and it's mainly the town of Rusk.
I am blessed to have most of my projects VERY near the zone line between the Central and North-Central Zone, so my scale is very close to 1 to begin with and then factoring in the elevation factor makes it less than a tenth per mile.
One project we have is stacked in the center of a zone at about 700 feet. That one gives me the blues to a certain degree, but it's so easy to scale up and down with deeds and their calls vs. found corners, I really don't mind. The corner is back in the right spot (hopefully). The data reported has all of the meta-data so anyone can go up with it to surface. I leave all my projects (now) on the grid and report grid acreage. For most of my projects (95%) the grid vs. surface acreages are the same to three decimal places so why fret.
If I lived in the Davis mountains or somewhere with a lot of terrain relief, I might have a different outlook or do the same field work, but report surface distances. Who knows?!?! However, I'm not leaving the piney woods anytime soon and the grid is just too damn easy to hook into these days. Hopefully, in 80 years, the next group griping about the old timers will remember that some of us tried to add extra value to the data for ease in retracing later. But I doubt it. They'll probably note that we HAD to use static gear and connect via computers to achieve a piddly 0.02'!?!?!?
That's probably true. I look at scale, elevation, and combined factors as indicators of the amount of error that would be present if no adjustment were made to convert between grid and ground. If the computed combined factor of a line is 0.9999, then using an unadjusted ground measurement as a grid distance would still produce 1:10,0000 accuracy. It's sloppy practice, but it works in a lot of places.
It's easy to see how Montana surveyors would prefer PLSS or UTM to state plane.
MathTeacher, post: 375964, member: 7674 wrote: That's probably true. I look at scale, elevation, and combined factors as indicators of the amount of error that would be present if no adjustment were made to convert between grid and ground. If the computed combined factor of a line is 0.9999, then using an unadjusted ground measurement as a grid distance would still produce 1:10,0000 accuracy. It's sloppy practice, but it works in a lot of places.
It's easy to see how Montana surveyors would prefer PLSS or UTM to state plane.
UTM,,,,,,,nooooooo,,,,,it's awful too
For Montana the grid to ground isn't a big deal, it's the rotation that is the main issue.
The grid to ground is easily deal with, but that convergence is extreme, particularly at the east and west boundaries.
Looking at plats in Montana State Plane make you tilt your head to the left or right depending which side of the state you are in.
I've never heard of anyone working in state plane using surface distances, if you are using state plane then use grid distances. Otherwise nothing will work.
