The combined scale factors are derived from the elevation (ellipsoid height) and the grid scale factor.
Multiply them together and you arrive at a combined scale factor. In this case from posters above at your location it's 0.99994655. I don't typically look beyond the 6th place for scale factors so 0.999947 or 53PPM because the 6th place is 1PPM.
Since the rough radius of the earth is 20,906,000', twenty-one feet of elevation change creates 1ppm scale factor change. This is also true as you survey with a total station.
It's not noticeable normally: if you shoot a 1000' distance to a point 110' above you, then move to that point and shoot back expect 5ppm difference in the two shots. This has nothing to do with State Plane, it happens in the course of typical TS surveying using any flavor of control coordinates.
You need to travel back in time to the T2/distance meter traverses up and down a mountain to really experience this effect. Yet it's always there. This is why to change from grid to ground we always pick a site scale factor to simulate ground, we understand it's not really ground but a surface close to ground with one scale factor for the entire area. I try to make it so the shifted surface stays within 10PPM anywhere on site. But that depends on the geometry of the area.
10PPM equates to .10' in 10,000', 53PPM is .53' in 10,000'.
All this is a way to look at your error budget for an area, is that acceptable or not. For small property surveys, you should be good with .005' in 100'.
@mightymoe I thought it was 8 or 9 places one should round to.
That's what we do for DOT, if our DAF is 1.000235, we show it as 1.000235000. That makes them happy. That relates to 235PPM or 2.35' in 10,000'. For us anything after that 6th place is meaningless.
I checked recently and the new given Scale Grid Factor for the Central Meridian of a local 2022 zone will be 1.00025 (5 places). The designers have figured out that using that scale factor instead of the old one of 0.9999375 (7 places) will push 93% of the population to living within 75PPM of surface distances using the new coordinate system instead of 0.2% with the old system. A welcome change.
@mightymoe yeah when they decided to do the new datum and projections they weighted the projection at the surface based on the population. This also means that the planes are also tilted in places. If I understand it correctly they also have incorporated the deflection of the vertical within the projection. Although not many on the land survey side probably ever worried about considering that on a daily basis. Michael Dennis truly I believe did a great job in all of this. Esp for you folks where the projection is so far away from the surface. Here it truly is not a big deal. I do still think we live in a time where the NGS should supply the datum and at this point each state should have the knowledge to create LDP’s in a sensible way throughout the states that would meet the needs. But maybe in the future. Some states have grasped this idea which I believe is great. It keeps math simple for surveying and yet can hold the distortion and convergence angles to smaller values based at the county or even city levels and still have ties to the NSRS so still could make GIS folks happy. Even say larger projects like highway or railroad surveys simply design a good LDP for that project and everyone is on the same page. Although we could be like some other countries and just reduce everything to the ellipsoid but that would affect surface area unless one used a scale to still produce acreage at the surface which is not hard. Then we could just move to Xyz coords and have fun.
That scale factor is highly suspect. The scale factor calculated above is 0.99994655 and the SF for NGS AK7138 about a mile away is 0.99994645. The Elevation Factor for AK7138 is 1.00000101.
Now, that's significant because the Scale Factor that your software uses is what NGS calls the Combined Factor. That's the product of the NGS Scale Factor and the Elevation Factor. Note that some software uses the reciprocal of the Combined Factor. but it's still based on the Combined Factor.
Your machine's Scale Factor should be greater than 0.99994655, or greater than 1 if it's a reciprocal, but yours is showing to be less than either of those. My guess is that if you use data calculated with that, you're going to have some serious differences compared to your total station.
There also seems to be some confusion as to whether the scale factor is 1.00000000 or something else. If it's 1, you're on Grid (State Plane), if it's something else, you're trying to be on ground.
Give us some help, guys!
@mightymoe this is scale factor here not combined factor. Under local site settings you can get the reciprocal of the combined factor or in points spreadsheet under project settings for each point you can see scale factor elevation factor and combined all listed out. I use this a lot on sites with lots of relief to try and decide the best c/f for a project to be on ground and check it. Not the best but is a way to mitigate the distortion for design purposes. I know ever-time I update the coordinates system manager I grab ran points around the state and check it vs NCAT because I have seen them way off and I would revert back until they fixed it.
There also seems to be some confusion as to whether the scale factor is 1.00000000 or something else. If it's 1, you're on Grid (State Plane), if it's something else, you're trying to be on ground.
Give us some help, guys!
I'm not the guy for this one, but there are some DC programs that will "set" your survey to "ground" using GPS equipment with no reference to a projection. In Trimble you can pick "No projection, No datum" as an option. Presumably this places a 10,000, 10,000 type survey on the ground. I can't say that's really true cause early on I decided not to do that. I couldn't puzzle out the advantage beyond an easy button to hit to give me the feels.
Also, programs will treat scale factors differently and you have to understand what they are doing to your coordinate systems. Using 1.000000 can place your coordinates on ground (I think), but you are correct, a 1 scale factor also means in other programs that you're on the state plane grid.
So, a local site scale in Trimble of 1 means grid, increase the number and that will "lift" the surface to the ground. While the same effect in AutoCad will need the combined factor to be less than one to lift the surface.
With GPS equipment we are always working with geodetic coordinates, making them grid will need scales and rotational calculations. There are many ways to do it, so it's important to understand each program you're working with. I do believe setting 1 as a scale factor in some of them will put your coordinates on the ground,,,,,,,,,somehow.
For me it's:
State Plane projections-OK under certain circumstances
State Plane lifted to ground-Most of our work using the vast network DOT has developed
LDP-Useful for many reasons, but limited for sharing
Calibrations-Only if I really, really, really have to, maybe once a decade
No projection, No datum-Never
I remember using no projection no datum once or twice in the past and I'm trying to remember why we did that. I know on one of the projects we had crews from 2 different offices on site to knock out a boundary topo in 1 day. Hmm, yeah I'm drawing a blank, but I remember the explanation at the time making sense.
When I worked in a Trimble shop I was told scale factor of 1 = ground.
I whole heartedly agree as long a you note the reference system and realization used. I can not tell you how many surveys I've seen with a the basis of bears being 'GPS' with no mention of datum or projection.
@mightymoe Another use for no projection no datum in starting 99% of the time is when you are performing a site calibration in Trimble. That’s the best way to start a site calibration. Especially if you don’t know if the coordinates look like state plane but have been scaled to the ground. So no projection no datum allows for a free fit to the control itself without re projecting from a state plane job set up. You can when you choose state plane go ahead and do state plane by keyed in factor which is the grid to ground you want or use the here position and roll. Your scale origin and ellipsoid height will gather everything else. But I have seen crews do one in one part of the state and go to another job at the other end and wonder why they can’t check to a point which is caused by them using the last settings and scaling from very far away. lol. Been a lot of midnight calls back in my support days answering that one.
When I worked in a Trimble shop I was told scale factor of 1 = ground.
I was taught that SF or CF = 1 meant that the software or equipment was not applying any adjustment to the data being recorded. The TS would be recording distances as measured which of course would be ground for wherever they were working. The GPS would be recording in grid for wherever they were working. I keep my TS (aka Robot) data in a certain point number group and layer and GPS data in a different point number group and layer. Any scaling adjustments to the data are done in the office and then a new job file is created for any continued work.
While a rarity anymore, I do use the 'None' (no datum/projection) coordinate system when working in an assumed coordinate system utilizing terrestrial instruments.
At least within the Leica world, if you specify a TS factor of 1 means you're working on the surface and both grid and ground distances are equal.
With today's instruments, there is no reason not to utilize the proper coordinate system, and appropriate combined factor, with a TS. Leica has multiple options for setting a scale factor. You enter a scale factor or ppm manually; you can let it calculate the combined factor based on the current coordinate system and station coordinate; or you can enter your own projection variable and let it calculate based on the station coordinates. I'm sure all of the major collection software's are similar.
It's been years since I dug deeply into this, but if memory serves me right, no projection, no datum told some software to create a Transverse Mercator projection with "here" as the origin. The 10000, 10000, or something selected by the user were the coordinates assigned to this origin. The one I dug into was Leica; they were the most public with what their stuff did.
It's likely that the software assigned a combined factor of 1 to the whole thing, making the coordinates ground coordinates.
Either way, no projection, no datum does not mean some random, no geodetic basis of assigning coordinates. But, as you point out, it's a sub-optimal method. It's just an isolated, standalone LDP that may or may not be transformable into something more general.
It's been years since I dug deeply into this, but if memory serves me right, no projection, no datum told some software to create a Transverse Mercator projection with "here" as the origin. The 10000, 10000, or something selected by the user were the coordinates assigned to this origin. The one I dug into was Leica; they were the most public with what their stuff did.
It's likely that the software assigned a combined factor of 1 to the whole thing, making the coordinates ground coordinates.
Either way, no projection, no datum does not mean some random, no geodetic basis of assigning coordinates. But, as you point out, it's a sub-optimal method. It's just an isolated, standalone LDP that may or may not be transformable into something more general.
I do believe you're correct, basically an LDP with the Central Meridian passing through the set-up point. I prefer to set-up my own if that's what the task is. Pick my own origin point using a Lat., Long. pair near the site. Put it into the data collector before leaving the office and all the projections are fixed for the project. I will use an even number Lat, Long. Of course with a TM projection the Long is the most important for the site, get it close to "true north" and still use a number that isn't 4 decimals of a second. Something like N30d30'00" and W90d25'10", placing the CM along that line of longitude instead of the set-up point which might be at N30d33'15.7578" and W90d25'08.3324". It's much easier to use the former number than the latter. Plus, you will want to see what other jobs might come up nearby and adjust the origin their direction.
With today's instruments, there is no reason not to utilize the proper coordinate system, and appropriate combined factor, with a TS.
Yeah, I've seen the option to set a scale factor with total station work but I've never been asked to do it or even seen anyone else do it. For whatever reason people don't seem to trust the scale factor the software generates in the field and insists on doing all of that processing in the office, so we're stuck screwing around with GPS data on grid and total station data on ground until the office has had a chance to chew on it.
The keys to everything involving application of hardware and software successfully, for me anyway are:
1. Know precisely and unambiguously what you want to accomplish, and
2. Know precisely and unambiguously how to make your hardware and software accomplish it.
Here's a link to a Trimble help page on ground coordinates. It's better now than it was a few years ago, but the wording still gets a bit tortured as the writer tries to keep track of scale factor, elevation factor, and combined factor.
I'm not a surveyor, but as a retired teacher, I know how much confusion can be introduced when you start bending language away from its original usage.
https://help.fieldsystems.trimble.com/trimble-access/latest/en/coordinate-system-ground.htm
@field-dog , hang in there. The grid is flat, the ellipsoid is curved, and the surface is bumpy. The first two are mathematician's imaginations gone wild, the third one is real. The software helps you calculate in the imaginary world where everything is perfect and translate the results to the real world where nothing is perfect and nothing can be calculated.
I don't think Leica creates a coordinate system when none is specified. When no coordinate system is chosen, only local grid values are created via total station. All geodetic values are null. If I hand enter a geodetic value in the project, no local grid values is created and the point will not display on the map. I've never used the no coordinate system option for a project where GNSS is involved, but int that case I would assume that only ECEF values are created.
With today's instruments, there is no reason not to utilize the proper coordinate system, and appropriate combined factor, with a TS.
Yeah, I've seen the option to set a scale factor with total station work but I've never been asked to do it or even seen anyone else do it. For whatever reason people don't seem to trust the scale factor the software generates in the field and insists on doing all of that processing in the office, so we're stuck screwing around with GPS data on grid and total station data on ground until the office has had a chance to chew on it.
I'm not sure what there is not to trust about it. Computing the combined factor is a simple math formula once you know the projection and location. (Okay, to more precisely determine the CF you should do so from the midpoint of the observation but I'm not sure any commercial software does that.)
To be honest, in most cases I would say it is a lack of knowledge that leads to that kind of thinking and verges on practicing outside one's expertise.