I know for sure that my GPS measures grid when the CF is set to 1.000 because I have tested it using 2 county NAD83 published mons that inverse 116303.05' apart and the CF=0.9999746 at the lower point. My GPS grid coords inversed 116302.93' which is pretty hot.
So here is my hypothetical question. Assume that I broke down a section in the high desert for a client that wants his NW QTR of the NE QTR property corners set. I tie into a county published NAD83 mon that is close by and CorpsconV6 says the CF=0.9998383 there. I do everything with GPS in grid with my CF set on 1.000 and his piece calcs out at 1319.787' x 1319.787' square and I set his corners with GPS using my grid network.
My question is if I set up my robot with the CF set on 1.000 will it measure 1320.00' between my new corners (assuming I have no errors)?
Let’s see if we can unravel this a bit. First your GPS is technically of measuring GRID. your GPS is measuring XYZ then a transformation to LAT LONG Height is happening then it’s projecting the LAT LONG Height coordinates to whatever projection and this step is all being done in your data collector. So if you state you want state plane X then it’s projecting it to that. A CF is used to convert ground distance to grid because it takes the Scale Factor x’s the elevation factor or ellipsoid factor. Now if you take some C/F and multiply it by your ground distance it makes it a grid distance. Depending on how many PPM’s and how much acreage it could be different on grid vs ground this all depends on elevation and what type of projection you are on. Lambert or Mercator . All these different projections out there are designed to either maintain shape size distance and or direction. No one projection can do it all.
What software are you running. If your data collector is set up on a projection the. It should scale your ground distance to grid behind the scenes. But without know which software you have I don’t know. In my software I don’t have to type in a c/f when on say state plane. It has everything it needs. If I set up a job on no datum and coordinate system and just use a scale 1.00000000. It will not match the grid distance because at 1.000000 it’s 1:1 on ground no projections have been chosen so no scaling being done.
The answer, assuming no errors, either human or machine, that your TS is reporting ground distances, and that there are no serious elevation changes that would negate using a single combined factor, is yes. And kudos to you for doing that simple empirical test to know exactly what your equipment is measuring. It's a great example of trust but verify.
Your grid measurement is 1319.787' and your combined factor, from a trusted source, is 0.9998383. Using the latter to convert the former to a ground distance gives 1319.787/0.9998383 = 1320.000444.
In this case, you can trust the math to duplicate what's observed. But because errorless measurement is rare, don't declare failure if 1320.00 is not exactly measured.
his piece calcs out at 1319.787' x 1319.787' square
That would be highly unusual.
Some programs have a subroutine that use a scale factor to adjust the coordinates. If you're in a state coordinate system and apply the scale factor it can shift you to ground or whatever other adjustment is needed.
If 1 is set as the scale factor for this sub-routine, then no adjustment is applied. The rational process would be to ignore the application of this sub-routine if you want your coordinates to be on a projection, why even bother adding it into the process?
If you wonder about your total station, which is presumably attached to the same DC with the same projection parameters, then the resulting coordinates when located should measure 1319.787' between them using the TS. If not, then you need to figure out why not, cause you have issues.
You don't need to go to the field to figure out if you're on grid, input a set of lat, long, heights into your job, see if the resulting XYZ is projected to grid, check it against NCAT.
In this case your 1 isn't a combined factor, it isn't a scale factor (unless you consider no scale factor a scale factor), it's simply pointless data entry, totally unnecessary.
Moe, you and I often disagree, and I yield to you in all things surveying. However, 1.000 is a legitimate combined factor, just as 1.001 and 0.999 are. What a 1.000 combined factor says is that point is where grid and ground happen to meet. It's the mathematics of a single-parallel Lambert LDP at ground or the analogous TM LDP at ground. The point of tangency of the projection is at ground level, hence its combined factor is 1.000.
I don't know how surveying software works, but inference tells me that distances are always adjusted from grid to ground via combined factor. If grid distances are desired, then using a combined factor of 1.000 makes no adjustment to the calculated grid distance, so grid distance is reported.
It can be confusing, which is why @landbutcher464mhz empirical test is so admirable. But, as you point out, he needs to do the same test on his total station.
I'll be 80 this month and a long trip for me now is 100 miles, but you're one guy I've always wanted to meet. Keep Montana running!
@bill93 That would be highly unusual.
I agree, more like impossible but this was a hypothetical.
@mightymoe You don't need to go to the field to figure out if you're on grid, input a set of lat, long, heights into your job, see if the resulting XYZ is projected to grid, check it against NCAT.
Well this lat/long entry made me learn a new trick in Fieldgenius. You cannot just punch in the numbers. I had to make a special extended format csv file, import it into a new job that has the same NAD83 settings as my real jobs, then select "convert GNSS points". Amazing, the NAD83 coords and NAVD88 elevations in my DC were the same as those converted using the NOAA Online Converter.
So thank you @mightymoe for helping me get a second confirmation that my GPS settings are giving me proper NAD83 grid coordinates and NAVD88 elevations.
@mathteacher I'll be 80 this month and a long trip for me now is 100 miles, but you're one guy I've always wanted to meet. Keep Montana running!
Ya these 82yo legs do not get me up any rough terrain anymore and I am now in SOCAL doing nice level city jobs except for an occasional high desert job that I can traverse with my old trusty 1996 Jeep. There are 4 of us old farts that meet at Dennys for breakfast on Mondays and you are welcome but wear hightops because the BS gets deeper every time.
Thanks for the the invite. We're roughly 2400 miles apart, so at 200 miles a day (long trip for me is 100 miles one-way, 200 total), that's 12 days travel time.
Keep working, doing otherwise is to quit.
In a situation where the TS ground distance didn't match the GPS grid distance, one assumes that all equipment had been properly calibrated, and the recorded GPS positions had good residuals, right? I'm not trained in fast static, but would a 5-minute session be helpful? Can a position be adjusted in the field?
Generally speaking, ground distance and grid distance will not match regardless of the source of either. It will happen, though, when a rigorously-computed combined factor for the line between two points is very, very close to 1.
That's not highly unusual. When either the scale factor or the elevation factor is greater than one, and the other is less than one, the combined factor can equal 1 or be very, very close to 1.
It means that grid = ground in this area.
Mathteacher, you'll need to find that section of land where 1.00000000 is the State Plane combined scale factor across the entire section such as Landbutcher related. I'll wait.
Might not be able to do that in the High California Desert or Montana, although Montana's single-zone system might create the right combination somewhere.
But the question is about the use of a combined factor of 1 in survey software calculations. The existence of a point with that characteristic is immaterial.
If survey software always multiplies grid distances by a combined factor, and the desired output is a grid distance, then that combined factor has to be 1. In that case, the entry of a CF of one is necessary.
Again, I'm inferring the software's calculation routine from the input required for a desired output.
And again, the existence of such a point is immaterial, although there might well be one in the vicinity of Victorville, CA.
That's an interesting challenge. I hope that you will permit me to substitute NC for CA. I'm more familiar with my home state.
My starting point is latitude 36d 10m north, the northern standard parallel for the NC State Plane System. The scale factor for all points at this latitude is 1.
The second step is to find elevation factors very near to 1. Those are typically found east of Raleigh. In NGS Radial Search, enter N361000 and W0773000 with a radius of 1.1 miles.
AI5318 and AI5317 are returned. For the former, the CF = 1.00000137 and for the latter, it's 1.00000257. For distances in this area, grid and ground are going to be virtually indistinguishable.
Now, to be absolutely certain, a bit of math would have to be done, but I suspect that a combined factor equal to 1 would work quite well over a square mile centered at either point.
Note that a scale factor of 0.9999 coupled with an elevation factor of 1.00010001 produces a combined factor of 1.00000000. Perhaps finding sections that can be measured using CF = 1 aren't all that rare and Landbutcher can find one in the High Desert.
Looking at surveying models and calculations from different perspectives is sometimes surprising and always educational.
In western states there will be combined and grid scale factors of 1. The grid scale factors of 1 will be pushed east or west of the Central Meridian in TM zones, and near the north south edges of Lambert zones. The combined factors will probably be only after passing the zone edges, although, in some irregular shaped zones grid factor 1 might be found within the edges of the zone, but those areas with a 1 scale factor will be very small areas, not sectional sized.
The NAD83 Montana zone was designed so there wouldn't be a grid factor of 1 or larger inside the big rectangle of Montana along the south line with Wyoming and the north line with Canada. The area dipping south along the border with Idaho will have a grid factor of 1, of course this means the combined factor of 1 will have to wait until farther into Wyoming or Canada. Along coastal areas the 1 combined and grid factors should be fairly common near zone edges, but again, very limited in area.
However, computer programs I'm familiar with don't further scale grid coordinates unless that option is applied. In Access and TBC the option is available as Local Site Setting. When the Local Site Setting is selected it's shown as defaults with no lat, long, 0 for false N and E, and no ground coordinates, the scale factor is shown as 1 but that's because it's the place holder. Since there is no position yet then there is no scale applied. Once you click into the option you need to select where you want to scale from, what scale factor is to be applied and then accept it. Then there is a scale factor applied to the State Plane coordinates. However, using 1 on top of the State Plane numbers is an irrational process, why even bother, it doesn't do anything and you've wasted time doing it.
It's a similar process in AutoCad. You need to apply the scale factor and position to scale from. In AutoCad if you wish to lift the surface up using the scale factor, it's necessary to apply the inverse of the Trimble local site scale. If you use 1.0002 to increase the distances in Trimble, .99980004 will be needed in AutoCad.
I do know about my old programs from the 1980's and Trimble from the 1990s and forward, FieldGenius isn't one I've dealt with, if it's applying 1 on top of the projection it must be doing it to try and stop users from doing something wrong.
Indeed. Just a cursory look at Montana State Plane coordinates near the Northern and Southern Borders shows that the elevation factors are too low to be brought up to a combined factor of 1 by the highest of scale factors. That's an amazing state that you live in.
In the California case, NGS mark AI4447 has a combined factor of 0.99999710 which makes grid and ground distances virtually equal near that point. It's in a town or suburb somewhat close to mountains, so 1 as a section combined factor may not hold throughout. I suspect thought that moving west a bit would produce a section where a combined factor of 1 would work fine for the whole section.
Just a great exercise in grid, ground, and US geography.
Here's a cut and paste from a ChatGPT session about Access and combined factors. Corrections and comments are solicited and welcome!
@mathteacher Yes I would like to see your example 1000 ft baseline.