I don't advocate for the modification of Scaled, or Modified, State Plane Coordinate Systems, but due to the proliferation of this procedure, I think there may be a way to name such systems that may make them easier to recognize. The principle behind scaling a system is to apply some scale factor to offset the onerous combined factor sometimes (often) associated with SPCS in general in order for grid inverses to closely match measured horizontal surface distances. We often consider scale factors as a ratio, ie. 1.00001 or 0.99999. We could just as easily note these values as parts per million (ppm). In this notation 1.00001 becomes +10ppm and 0.99999 becomes -10ppm. The simple math for this is to multiply the ratio x 1,000,000 and then subtract 1,000,000 from the result. Thus:
1.00001 * 1,000,000 = 1,000,010. Subtract 1,000,000 and the result is +10ppm
0.99999 * 1,000,000 = 999,990. Subtract 1,000,000 and the result is -10ppm.
Now for the Naming convention.
So let's consider our State Plane System produces an average CSF for our project site of 0.99993. We want to scale by the reciprocal of this value (easy math without a calculator is to take this value and subtract it from 2), the result is 1.000070 or, from our formula above, +70ppm. If we scale about the grid origin (0,0), our projection name could be:
State, Zone, Year, Unit, Scale, Origin
For example for Texas Coordinate System of 1983, North Central Zone, in US Survey Feet, scaled by 1.000070 or +70ppm about the grid origin (0,0), the name could be:
TXNC83S+70,0,0
This is no substitute for good metadata, but puts the user on notice that the system is scaled and also puts the user on notice as to whether this system would be appropriate for other projects. A user could have a library of such systems, +30,+100, etc, as reuse these systems the need arose.
Furthermore, I've seen DOT projects with the modified factor carried out ten places or more. Unnecessary. We do well to survey inside of 5ppm. A factor carried out to the millionths is likely sufficient for any cadastral work mortal surveyors are likely to encounter, thus 6 places beyond the decimal should be fine (perhaps even extravagant) and simplifies the metadata. Just some random thoughts.
We run our CSF to 8 places and work up to 20 PPM... go figure...
We did too until I started thinking about how the significant digits applied. Thanks to Loyal.
[sarcasm]we have specs- no thinking needed...[/sarcasm]
😛
I know of a surveyor that has been into GPS as long as anyone and he refuses to quote location beyond 1 decimal place.
Fugawe Coordinate Systems
> I don't advocate for the modification of Scaled, or Modified, State Plane Coordinate Systems, but due to the proliferation of this procedure, I think there may be a way to name such systems that may make them easier to recognize.
Just saying that they refer to the "Fugawe Coordinate System of the Internet" should get the idea across. After all, the scaling of State Plane Coordinates is invariably evidence of some cluelessness somewhere in the vicinity.
For What Logical Reason Do You Scale About 0,0 Or The
SPC reference point?
I am asking for a non-stupid reason.
Paul in PA
For What Logical Reason Do You Scale About 0,0 Or The
The principle behind scaling a system is to apply some scale factor to offset the onerous combined factor sometimes (often) associated with SPCS in general in order for grid inverses to closely match measured horizontal surface distances.
The reason to scale around 0,0 would be to make scaling less complicated.
The reason to scale around a local SPC would be to minimize the differences in SPC scaled values and true SPC values in a small area.
Again, I can not stress enough. I am not advocating for this solution, but recognize that it exists.
Fugawe Coordinate Systems
We have numerous projects requiring multiple products. I have to publish LLH in two different reference frames, current SPC and provide surveys with descriptions that meet code.
For these projects we compute the CAF for the township and apply it to 'whole coordinates'. It is easily repeatable and works in all of our software packages and data collectors. Data management is simple as we name files what they are.
At the end of the day my client gets the products he needs (all of which exceed his needs and the standard of care) at a reasonable fee. My database of observations continues to grow and all is well. If I need the unadulterated data it's still there. Call it what you will, I call it a good business model...
Fugawe Coordinate Systems
> We have numerous projects requiring multiple products. I have to publish LLH in two different reference frames, current SPC and provide surveys with descriptions that meet code.
> For these projects we compute the CAF for the township and apply it to 'whole coordinates'. It is easily repeatable and works in all of our software packages and data collectors. Data management is simple as we name files what they are.
Okay, you have to compute your surveys in two different reference frames (presumably NAD83 at different epochs),
and you have to express the positions in those two epochs of NAD83 as grid coordinates of:
a) the projection of the State Plane Coordinate System covering your project and
b) some other projection that meets "code".
But on top of that you compute a Combined Scale Factor for the township in which your project is situated and multiply the actual grid coordinates in those two projections by the inverse of the CSF?
> Call it what you will, I call it a good business model...
It sounds like a nightmare to me. There pretty much have to be engineers involved in that one.
Fugawe Coordinate Systems
Computing the CSF for a township takes less than 5 minutes. That includes checking the vertical extremes. The remaining computations are automated. I can export whatever I need. The crew can also convert in the data collector. Scratch paper works in a pinch.
I am certainly not a one trick pony. I have been creating and using custom projections for 15 plus years. I even localized once just for fun. The needs of the project drive the method.
Fugawe Coordinate Systems
> Computing the CSF for a township takes less than 5 minutes.
Oh, I don't doubt that for a minute. The weird thing is keeping a whole bunch of different mix-and-match coordinates around just to be able to send them to some engineering client who doesn't know any better.
The Fugawe Coordinate Systems don't make any real sense. They seem to be specific to American practices. I'd doubt very much that they are needed anywhere else in the clued-in world.
Lol, just like every DOT project of recent vintage......
Fugawe Coordinate Systems
The scaled to ground coordinates are the simplest method of obtaining near ground values. I don't end up with numerous random coordinate systems. I end up with a systematic treatment of coordinates based on an identifiable repeatable system. It is useful for many of the products we publish. On others it doesn't make sense.
While I understand and respect differing views, you seem bent on being an insulting jerk. Why not be part of a grownup productive conversation instead?
Fugawe Coordinate Systems
> While I understand and respect differing views, you seem bent on being an insulting jerk.
Well, maybe I'm wrong about the US being the place where the design and construction of engineering projects require generating a bunch of jackleg coordinate systems, but I'd be willing to bet that the engineers in other countries are actually able to design and build in grid coordinates on standard projections with scale factors other than 1.000000. As a land surveyor, I know perfectly well that the zillion coordinate systems that some clueless engineers and their technicians require aren't needed for land surveying purposes. That is more of a statement of fact than an insult.
> Furthermore, I've seen DOT projects with the modified factor carried out ten places or more. Unnecessary. We do well to survey inside of 5ppm. A factor carried out to the millionths is likely sufficient for any cadastral work mortal surveyors are likely to encounter, thus 6 places beyond the decimal should be fine (perhaps even extravagant) and simplifies the metadata. Just some random thoughts.
If you are going to change the coordinate values to make the inverses match ground distances, then you need to report the scale factor you used to the same number of digits that are in the coordinate values. In order to use only five or six places, you would need to have the software you are using truncate its scale factor. It is easier to let the software do the scaling and just publish the value used to the same 8 places. It doesn't affect the measured distances between points per se, but it just puts it in the metadata so that the follow-up user can use the same values you used.
I agree with you that six places or even less is close enough to get good enough inverses between points to within the same order of precision that you can measure in.
I'll leave the argument to others as to whether or not coordinates should be "modified" at all.
Absolutely, Tom.
In reference to shortening the scale to 6 places, this has to be done when the modified system is created. It would be extremely hazardous to shorten the scale to 6 places for an existing modified system as the resulting coordinates from 0,0 will vary by a significant amount due to rounding. A half part per million applied to a coordinate in the range of 1,000,000 feet would result in a half foot discrepancy.
Shawn
Scale from WHERE?
Just Look at the Projection Parameters of your SPC ZONE to see what's going on!
Here's a good one in my neighborhood:
NAD83
Nevada East
Transverse Mercator
Origin Latitude 34°45'00” North
Origin Longitude 115°35'00” West
Scale Reduction 0.999 900 Unitless
False Northing 8,000,000 meters
False Easting 200,000 meters
Now lets see WHERE you would be scaling from...
Latitude 37°29'44.94551” SOUTH
Longitude 117°50'42.26002” West
Plot that sucker on Google Earth!
That's right, SOUTH Latitude!!!
"You scaled from WHERE?"
:-O
Loyal
Fugawe Coordinate Systems
I would be careful not to bet more than you can afford to lose. I've used the technique described above in North Africa, Korea, Panama, Jamaica and a smattering of others. There is some foundation in what you are saying. The United States is one of the few places to choose stability of datum over Global technical perfection. The fact is most of my manipulation is for surveying, not engineering.
My job as a Surveyor is to locate, preserve, interpret and leave evidence. One aspect of that is to provide directions and distances between monuments. Working at elevations from 2500 to 9000 feet, the distortions between SPC and ground exceed my error budget. While I am perfectly capable of computing ground distances using average factors at each end of the line, I also understand the elementary math that allows a simpler, more efficient method.
We do not create 'a zillion coordinate systems' when using this method. We generate a near ground product with an established relationship to SPC. When I do create a custom projection and coordinate system, it is usually to match some legacy data for a client or to facilitate overlays without manipulation.
The bag of tricks I lean on was assembled by listening to others, reading and considerable hard work. I would love to hear other methods, but so far all you've had to offer is telling everyone how stupid and clue-less we are.
Fugawe Coordinate Systems
Kent is a boundary surveyor who is quite good at what he does, from what I can tell, and he can dictate how he treats coordinate values and how they best work for him. What he doesn't understand (okay, not "understand" but what he doesn't care to recognize) is that many surveyors have to take on jobs and give their client what they want. They need to meet certain standards or specifications that may even be created by engineers. Not every job takes a boundary expert who will testify as to what they want.
What the rest of us have to be on top of, and aware of, is to provide enough metadata so that other surveyors or anyone else can apply the data and get the relation to spherical coordinates. (and I think there are some of use that just aren't as bright, who can take the insults because we don't ever recognize we are being insulted). o.O