Sp to Ground Again

R = Radius of Ellipsoid at Site (20,906,000 is probably close enough)
P = Average Ellipsoidal Height at Site (6100)

Elevation Factor = (R + P)/R

= (20,906,000 + 6100)/20,906,000)
= 1.00029178

Dave

Any SPC Derived From GPS Should Come With Elev Factor

Scale Factor and Elevation Factor are both a part of the metadata.

SPC data is provided on the particular state SPC base ellipsoid. I guess it appropriate to start with SPC 101.

36-30' N Lat at an elevation of 6100 feet is insufficient information as a Lon and/or State SPC zone is also required.

Once you have a state zone and it's metadata even scaling an elevation from USGS is precise enough.

Once you have GPS SPC data you never want to separate whatever elevation you have from the N & E.

Similarly you want to always carry an elevation on your field traverse work. Elevation checks are the best comparison of your GPS and traverse coordinates.

You may have to emphasize to your field crews that all data observed is equally important.

Paul in PA

OK - it's time for me to be a curmudgeon.

You clearly don't know what you are doing. Have gotten a bunch of conflicting opinions that you don't know what is right or wrong. It's time you hired some expert advice that knows what they are doing.

Just because you are licensed in 4 states and have been surveying for 37 years doesn't mean you know everything. And geodesy is something that many surveyors are weak on.

As I stated earlier, SPC's are SPC's period. Do not monkey around with your coordinates over this long a distance! Use your combined scale factor(s) (they will be different at every station - you do know this don't you? A single correction will not work over this long a distance.) to adjust your distances.

OK - it's time for me to be a curmudgeon.

I'm with Dave the Curmudgeon.

OK - it's time for me to be a curmudgeon.

No Dave, I'm the resident curmudgeon.
And you know what really annoys me?

This:

"...for there responses."

Whose responses? The ones that are over there?
Could it be that they're the ones?
Is it their response?

Sheesh,

Don

Or this one. My Bible. I still use some Excel spreadsheets I wrote in college for work I do these days for local geodetic projections. Mr. Stem is one of my heroes.

http://www.tech.mtu.edu/courses/su3150/Reference%20Material/SPCS_Manual.pdf

Earlier this year another poster had some difficulty with the nuances of SPCS and a lively thread ensued. You might look here for [msg=241560]some help[/msg].
I have to say I give you credit for coming on here to try to learn. Many of us are self taught and learned by studying the NGS papers which already have been mentioned. I for example learned much of what I know about SPCS back in the seventies by reading Joseph Dracup's paper from 1974.

Don't be discouraged by the few who don't think what you are doing is proper. And, heck, I don't think switching there for their is worth calling out another professional over. And I surely do not agree that you are being even borderline unethical in attempting to use the SPCS to double check some work that has already been done (that's if I read your original post correctly). Actually this is a good way to start out. You already have some existing values to give you a sanity check.

Keep in mind too that all the pundits on here or is it hear?? had to learn these things at some time too. Either from their colleagues through mentoring, by their own self-study and, for some, through formal education. And just because a declaration is made insisting that this way or that way is correct, it is only their opinion. My own take?: LDPs are very close to modified or ground level SPCS and are okay but not necessary. Dropping digits from modified SPCS is good but not imperative. Scaling about a point different that the SPCS origin? confusing unnecessary step to some. I for one think SPCS are here to stay in some form or another. And as long as we get our OPUS reports expressed in SPCs, and they include a combined factor for the point, "ground level" SPCs will be useful to the practicing surveyor.

Good luck with your project and keep learning. If you will study the NGS papers already referred to, especially the illustrations, things will become clearer.

"Scaling about a point different that the SPCS origin?"

Scaling about the SPC origin is about the stupidest thing one can do.

Why bother with SPC at all in that case?

2xcntr best rethink his advice.

Paul in PA

"Scaling about a point different that the SPCS origin?"

> Scaling about the SPC origin is about the stupidest thing one can do.
>
> Why bother with SPC at all in that case?
>
> 2xcntr best rethink his advice.
>
> Paul in PA

No, Paul, actually I agree with you. You just misunderstood what I was trying to say. You know I always agree with your takes.

Here is what I said: "Scaling about a point different that the SPCS origin? confusing unnecessary step to some" I said it is confusing and unnecessary for some. That's not you and me for sure. Only making a point that their is more than one way to skin this SPCS cat.

I think if you take a true state plane coordinate and divide it by the CAF, you will get a so-called "modified coordinate" or ground coordinate that is scaled about the SPCS point of origin. It will be different enough from a real SPC that it is easy to detect. When you scale about a point on or near your project, you end up with values that are very close to true state plane and unless they are further changed they can be mistaken for the real thing.

So maybe I used the wrong terminology? Clue me in Paul.

May want to rethink that, Mr. Cntr -

I think if you take a true state plane coordinate and divide it by the CAF, you will get a so-called "modified coordinate" or ground coordinate that is scaled about the SPCS point of origin.

Since all state-plane zones have non-zero false eastings (and many zones also have non-zero false northings), when you divide/multiply SP northings and eastings by the whatever-you-like-to-call-it factor you are scaling around 0 northing 0 easting, not the actual projection origin. Big (usually hundreds of kilometers) difference!

Happy scaling!

GB

I Cannot Put Your Thoughts Into Words

Only you can do that. The statement I quoted may be able to stand alone, but only you can clarify it.

Paul in PA

I Cannot Put Your Thoughts Into Words

Well , Other than my miss-speak on the "scaling about the point of origin" what else about my post do you take issue with. Obviously, I meant the 0,0 point not the false north and east at the legal point of origin. So thanks to you two who so politely corrected my miss speak.

Same Stupid Comment Applies to 0,0 Points.

Paul in PA

Same Stupid Comment Applies to 0,0 Points.

You can argue all day long about the value of various techniques. At the end of the day modifying SPC a out 0,0 is efficient and repeatable. There are times it is inappropriate and times it is the best solution for the task at hand.
Labeling the valid practice of others as stupid is ignorant, unproductive and unprofessional...

Same Stupid Comment Applies to 0,0 Points.

At the end of the day modifying SPC a out 0,0 is efficient and repeatable.

I can't say I've ever loved doing it, but so many jobs REQUIRE that it's done, and it does allow for easily converted numbers and files.

Often it isn't the surveyor driving the choice for "working plane" or "ground coordinate" systems, there are often many other users of the data produced during a project.

"Scaling about a point different that the SPCS origin?"

Easy to detect? Not so much. I was involved with two projects in the 80's where they did scale about the origin, but left the coordinates otherwise intact (i.e did not chop 1,000,000 off of the easting, etc to make it look different). Somewhere what was actually done was lost (no metadata). These were both huge construction projects that cost megabucks (one was 500 million, the other over a billion dollars).

Then, after many years, GPS came along and people started bringing in external control. Missed by 100 feet or so. I was brought in to figure out why. It was pretty obvious (to me), but they had no idea.

"Scaling about a point different that the SPCS origin?"

Then, after many years, GPS came along and people started bringing in external control. Missed by 100 feet or so. I was brought in to figure out why. It was pretty obvious (to me), but they had no idea.

No doubt it's a regional difference, here with larger zones and elevations, the scale factors have been used for so long it's second nature to most everyone. Can't think of any large construction project using state plane unadjusted, there would be too much distortion.

Most of the coordinate systems have been driven by DOT and they require it be done a certain way (coordinate scaled from 0, 0), one thing not done is dropping 1,000,000 cause it loses the connection to state plane. Using the scaled coordinates allows all kinds of info to be brought into the job easily.

They want it as close to ground as they can get it and since almost all projects are 10 miles long that is the box you work in.

The worst thing I saw: two projects connected to each other using the same scale factor but not fitting by about 5'. Project #1 used USSF and #2 international feet-that was messy for a while because there was no metadata to figure THAT one out (international feet was the "wrong" one). Of course the scale factor didn't cause the mess, state plane coordinates would have had the same issue.

So far I've found no clients wanting to use any of my LDP projections, everyone wants state plane adjusted or a rare few want just state plane. The most recent large project I took it on myself to send everything in state plane because I couldn't get anyone to sign off on any kind of coordinate system and info is being sent to all kinds of people.

Give your clients all the options, explain the pro's and con's and let them decide what to do. Or in the case of the DOT do it how they tell you to.;-)

"Scaling about a point different that the SPCS origin?"

Hi John,

Yes, well I am talking about the here and now.... not the eighties. I don't think one would have too much trouble spotting 100 feet these days. Quick OPUS and you will have a value in no time accurate to a few CMs. But like I said before to the OP, there are many acceptable ways to get the work done and we all have our preferences.

But maybe I should have said easier not easy

BTW... For many years (over 25) I worked with numbers sent along to our aerial mapping company from customers who did their own control or hired locals to do it. When our analytical department could not make them fit they would pass the numbers to me. Believe me I have seen every possible way there is to Eff up a state plane coordinate project.

"Scaling about a point different that the SPCS origin?"

You make a good point about the DOTs. In the two states, MO and KS, most of our DOT mapping was done in, that is exactly how they specified it to be done.

I did use Blue Marble to create custom projections for many pipeline clients, however. They wanted as much mapping on one projection as possible. Sometimes they just needed to count items in an attached database, not measure anything, and so that worked okay.