> Exactly. And the difference between using a 3 in the seventh place instead of a 1 is 0.2 parts per million, or 1 part in 5 million. I've never known the exact value of a whit of difference, but I'll bet that 1 part in 5 million qualifies as less than one.
It's worth pointing out that in the example I posted, the values of CSF vary in a range of 11ppm. Choosing the middle of that range means that all ground distances computed using that value of CSF would be expected to have scale errors of less than 5.5ppm which for a project 1km x 1km in extent means less than 6mm across the project.
> StarNet handles projections in the field? My mistake, I thought it was office software.
StarNet is, indeed, office software. Your post in no way specified that you were talking exclusively about field software. In the recent past I've used:
- Microstation Field Genius
- Trimble Access & Survey Controller
- Lieca Viva
- Spectra Precision Survey Pro
All field software that readily handled custom projections. I am sorry that your experience has varied.
> StarNet handles custom projections readily. Kent has all the software he needs to do the job.
But he remains puzzled by why any professional land surveyor who wasn't a handmaiden to some engineers would want to abandon the SPCS which works perfectly well at a much, much larger scale than just 160 acres.
actually my point was that once setup in the GPS software, LDP's are handled with ease by the various softwares and there is no need to setup the projection in your total station software, this makes working with GPS and conventional instruments on the same job easier than working with SP
> LDP does not stand for "No Distortion Projection". Using an LDP zone does not preclude providing a CSF for further refinement.
So, you apply a CSF to your Custom Projection grid distances to make them even approximately as accurate as the surface distances I'll derive by ... applying a CSF to the SPCS grid distances? And, aside from feeding CAD files to clueless engineers, this is hot stuff why?
> Even with and LDP Zone the size of several counties you would still achieve significant improvement in scale factors and convergence angles.
I'll bite. Who really cares about whether the convergence angle is 0-05-45 or 2-05-45 as long as the value is known? I've never, ever been even slighly inconvenienced by dealing with even the relatively large values of convergences (greater than 2-00-00) that are encountered in the West Texas end of the SPCS. That's a non-issue.
As for minimizing the change in the projection scale factor, as I think I mentioned, that scale factor changed by about 1.2ppm across the project, so it is insignificant. On most real world projects, the change in the projection scale factor gets flooded by changes to the height component scale factor anyway.
how do you mix GPS and conventional data in the field with your method? do you work on grid and scale your conventional data down or do you work on ground and scale your GPS up? Either method introduces its own problems which are alleviated by using a local LDP. That is the advantage.
I'll be joining you two.
> I see no problem with multiple projections in a county or single use LDP's. In fact, they provide advantages over your methods. The scale factor being nearly 1, allowing both conventional and GPS to produce and report ground distances being the main advantage.
Possibly you missed this, but I've surveyed in the SPCS since before I had GPS, have drawn the maps in CAD with the SPCS, and have annotated those maps with surface distances without breaking a sweat. You hardly need to jack around with non-standard projections to do that.
Stakeout? Just plug the project CSF into the data collector and stake out SPCS coordinate positions. All of these problems have been solved for about twenty years without resorting to Custom Projections. They've never given me the slightest difficulty.
> how do you mix GPS and conventional data in the field with your method? do you work on grid and scale your conventional data down or do you work on ground and scale your GPS up?
Sure, all the data collectors I've ever used in the field allow one to enter at least a project value of CSF to be applied to conventional distance measurements for survey and stakeout. The measured distance is multiplied by the CSF to reduce it to a grid distance for coordinate computation.
When you log the actual measurements, you can just download them into Star*Net to verify that you got what you thought. In twenty years, I can't recall ever generating a coordinate file that had been scaled to ground to be able to pretend that no CSF was needed. Ever.
I am not doubting that you have it all down pat...except when you dismiss a powerful tool like an LDP, it really has distinct advantages when mixing GPS/conventional measurements in the field. But hey, you cant always teach an old dog new tricks.
> I am not doubting that you have it all down pat...except when you dismiss a powerful tool like an LDP, it really has distinct advantages when mixing GPS/conventional measurements in the field.
No, there isn't any problem with the SPCS really. It works perfectly well and it has the advantage of simplicity. Instead of dealing with a zillion different custom projections, each chosen for some site-specific reason, but otherwise worthless, there are five that cover the entire State of Texas. The SPCS values of boundary markers are part of the written descriptions I write and they are simplicity itself for other surveyors to use if they have any sort of a clue at all.
It's pretty clear that the main value of the Custom Projections are for folks who want to somehow pretend that they are surveying in local coordinates with SF=1.000000, which ain't going to happen if there is any significant change in elevation across the project, anyway.
> > Actually, the surface distances computed using a project CSF will be much better approximations of what a person would actually measure at ground scale than any county-wide projection could possibly accomplish. This approach is the superior one.
> >
>
> I would agree with the first part of this statement. It's about resolution really: a county-wide scale factor compared to a project-wide scale factor. Whether you accomplish this through LDP or CSF, the principle is the same, not unlike the notion of using a county-wide CSF.
I'm afraid I have the advantage of having used the SPCS in everyday work for more than twenty years and have had none of the difficulties that others allege exist, aside from feeding information to clueless engineers and architects. Clueless engineers and architects are just always going to be a problem, regardless.
Some folks complain about being able to inverse distances that approximate surface distances while working in the SPCS. Never been a problem so far. Some folks complain about stake out. Never been a problem so far. Basically, there must be a whole bunch of really, really crummy software out there that a whole bunch of surveyors feel obliged to use, because all of the objections sound like software problems to me.
> Whatever's cheapest, really, and pairs well with living in the past 🙂
I hate to tell you this, but the above process of picking an average project CGF or CSF is something I've done for more than twenty-five years. That's the past.
No, because you are working on grid..You are taking ground measurements and holding some random grid coordinate(your traverse point), applying a scale factor to your total station measurements and calculating grid from there. If you come off two different GPS control points, you are using two different grid points as the origin of your calculations.
The LDP uses the opposite approach and keeps everything at ground measurements and the calculation from grid to ground is always from the Central Meridian you pick. In this way converting grid/ground is more rigorously defined in an LDP. Incidentally, it is only 2 clicks of a mouse to go from a local LDP to SP coordinates in Trimble Office. likely switching systems in Starnet would be just as easy.
I suggest you try one out on a project and see how they work yourself. It cant hurt to see how something works.
Kent, yesterday is the past.
For some of us, 25 years ago was the occasion of our 25th high school reunion, not a geologic era as it is to some.
So no, I don't consider your practices of the previous 25 years to be anything more than recent.
:>)
Don
> No, because you are working on grid..You are taking ground measurements and holding some random grid coordinate(your traverse point), applying a scale factor to your total station measurements and calculating grid from there. If you come off two different GPS control points, you are using two different grid points as the origin of your calculations.
> The LDP uses the opposite approach and keeps everything at ground measurements and the calculation from grid to ground is always from the Central Meridian you pick. In this way converting grid/ground is more rigorously defined in an LDP.
No, sorry, that's completely wrong. The reductions to grid for the control network are rigorously done in Star*Net. The stakeout is done from rigorously derived SPCS values using a CSF that is in error by at most a few ppm, which amounts to no error at all over normal stakeout distances. The angles and measured distances logged to points set out are then included in the adjustment and the SPCS coordinates of the points are rigorously computed for error propagation.
The accuracy of results obtained by this method is always going to be better than using a custom projection that neglects the variation in CSF across the project.
> No, sorry, that's completely wrong. The reductions to grid for the control network are rigorously done in Star*Net. The stakeout is done from rigorously derived SPCS values using a CSF that is in error by at most a few ppm, which amounts to no error at all over normal stakeout distances. The angles and measured distances logged to points set out are then included in the adjustment and the SPCS coordinates of the points are rigorously computed for error propagation.
>
> The accuracy of results obtained by this method is always going to be better than using a custom projection that neglects the variation in CSF across the project.
The same rigorous application of a CSF can be performed with LDP, just as it can with SPCS. Both are projections with distortions. This just happens to be a case in which "mine is bigger than yours" isn't terribly desirable.
A well designed LDP is subject to the same variations in scale as SPCS, it simply begins much closer to unity than SPCS tends to do.
LDP combined with publishing geodetic coordinates allows for very concise metadata. Geodetic coordinates truly are supported by most (all) software.
Fear is a stinky perfume.
> > The accuracy of results obtained by this method is always going to be better than using a custom projection that neglects the variation in CSF across the project.
>
> The same rigorous application of a CSF can be performed with LDP, just as it can with SPCS. Both are projections with distortions. This just happens to be a case in which "mine is bigger than yours" isn't terribly desirable.
>
> A well designed LDP is subject to the same variations in scale as SPCS, it simply begins much closer to unity than SPCS tends to do.
Thanks for restating the point that I've already made, which was that to get accuracy from a custom projection that is as good as rigorous use of the SPCS can get, you have to do all the reductions that rigorous use of the SPCS requires. In other words, there is absolutely no computational advantage when you're using software like Star*Net to do the reductions.
IF you are using software such as TBC all the reductions as you call them are done on the fly on every point. It's just a built in feature of how the software works. An SPC projection is based upon the same base geodetic system as an LDP. You pick a projection type and Lat, Long for the origin. There is no difference between the gears turning in the background for an SPC or LDP. You apply a scale factor to the projection in an LDP which makes your grid output work out at some picked elevation. The scale factor "adjustment" is the same as the CSF in the way that you are adjusting the distances in the base system to give you the "ground" distances in the LDP. You pick your CSF at some point or elevation. For a basic LDP you can pick the scale factor by taking the inverse of the elevation factor at some desired point (lat, long and elev/Ht). So scale factor x Elevation factor = 1 at that point and elevation. With an LDP set up as you move around all the factors are calc'd in the software for every point (the gears in the background). The results for your adjusted grid are every bit as good as it would be in an SPC.
Kent, I'd expect that since you have never actually used a field/office software with the capability to do the calc's for an SPC or LDP you can't realize the benefits. I don't doubt the ability of StarNet. I used it a long time ago. Do they have a data collector version - seamless transfer from field data into the office software and playing around with the data. I don't think they do, if they did we wouldn't even be talking CSF as it's not even something you need to deal with in the software I use. You can print out a points listing with it listed if you like.
I have no doubt that what you do is very good, correct and accurate. But, man, you are working a few times harder than the rest of us to do it. It's no where near as hard as your way of doing it.
> Thanks for restating the point that I've already made, which was that to get accuracy from a custom projection that is as good as rigorous use of the SPCS can get, you have to do all the reductions that rigorous use of the SPCS requires. In other words, there is absolutely no computational advantage when you're using software like Star*Net to do the reductions.
I'd say reductions are only about a third of the total equation. So you can reduce terrestrially measured data with SPCS or LDP rigorously. I'd call that a draw. However, what happens when you have a call for 100.00 feet to replace a missing monument. With LDP, I compute using 100.00 feet. With SPCS, you'll have to apply some project wide combined scale factor. The result is the same (homogenous scale factor applied to a single measurement - 100.00 feet) but the methodology is clearly much simpler with LDP. If you are retracing very many calls, this can be a very significant advantage of LDP. Third, you must report the calls (I've never provided a StarNet report as my final product... ever). So I have to make a map (graphically or literally) of these positions and the distances must be related to a single grid surface. If the distances are not related to a single surface, Euclidean geometry doesn't work and operations like closures don't work. My surface is closer to the ground. Yours is often far from the ground, requiring an additional homogenous scale factor be applied. Again, the result is similar, but clearly the methodology to the map user will be simpler. Distances reported match distances measured, North is near North. Clearly LDP is better.
Point is, the rigorous application of individual scale factors can be applied to SPCS and LDP, however rigorous application of individual scale factors to reported distances is not desirable. As a result a single homogenous scale factor near unity in an LDP is bears consideration over much larger single homogenous scale factor determined from SPCS.