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vofsar

Ngs Coords.

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Black Dog,

 

You are in a way correct, It won't be you or I who decides, But the Survey Field is age old, and is arguably a profession that shares it's spot with the oldest of all professions in the world. It has a Culture to it as you may have guessed and it is serious about following standards and practices. It is the adherence to following these standards and practices properly that make the system work at all. In other words it has already been decided and has been that very way for years before you or I came to play. I therefore do not see this as an open Question or just my opinion, I see it as the way it is and has pretty much always been. If You don't think I am correct in my thinking, Please refer to the FGCS Bluebook and read it over, then see if the methodologies are not extremely detailed and clear. Those rule have been in constantly upgraded development for years.

 

We as fellow Surveyors know that we work best in this Culture if we just fall in step with it, and that is why we do this, we follow the ways of the culture. Simply put, the adage is do it this way or it will become severely screwed up. We, not being the ones who go down in history as being the ones who screw something up "get it". We fall in step with the norms and follow along. Those who study Culture realize that Culture changes too, but it does this very slowly and never without a bit of anguish along the way.

 

Before now when a document stated NAD 83, we just accepted that we should use NAD 83 and I am telling you it is like this to the professionals in the field, but now you say it needs to be even more official than what you have already been instructed by the datasheet and your knowledge that the NGS does not concern itself with Other Datums than the ones it owns? The NGS has to come down on this just for you or you refuse to believe? Is that what you mean?

 

Um, Ok. If the only official Horizontal Datum of the NGS currently are NAD 83 and NAVD 88 what more do you need to be told? If an Official NGS Datasheet says:

 

*CURRENT SURVEY CONTROL

ED0700 ___________________________________________________________________

ED0700* NAD 83(1986)- 34 52 34. (N) 083 57 31. (W) SCALED

ED0700* NAVD 88 - 576.857 (meters) 1892.57 (feet) ADJUSTED

ED0700 ___________________________________________________________________

 

From this I take it they are not asking me to substitute any other Datum here, they Officially want me to use the ones they are working with, and they Officially expect me to do this too. (and I will if I want to be right.) It simply goes without saying. Is is like common sense. There is no other current survey control no matter what the supposed accuracy or formatting. Please make note that this quoted text from an "Official Datasheet" is calling the Datum outright and implying the desired display format of D.M.S by using that convention in the first place. At this level of accuracy, there is but one Datum we should concern ourselves with, and if it is not listed on the data, then we should not concern ourselves with it. The NGS does not adjust or even work with the WGS 84 Datum and they really have no desire to. They do not even offer tools to convert back and forth to that Datum. You see? Get it? They want nothing to do with it. Not their Datum. It uses different everything than the NGS does. The only similarity is that they use the same satellites to do the work.

 

It has always been the practice to use the Datum asked for, Not any datum we want. If I am handed Building Site Plans that ask me to survey in NAD 27 then that is what I do. They designed it that way. If the Vertical Control is in NGVD 29 then I set my control to match. When I work with NGS Data, I use the Datum they Prescribe. It is as simple as that. If someone has a problem with this standard or practice, perhaps a different hobby or even profession is in order? Maybe.

 

This sort of thing was never needed to be written down, it was just one of those things that was always done and not questioned. Ingrained in us as surveyors just as I am trying to ingrain it to all who read this. I guess part of my career which is ingrained in me feels wrong to question this, I mean, like why would you even want to do that? Why would I knowingly want to do something to increase the factor for error. That is counter culture to me.

 

If I could foster a perspective I would want it to be such that we work as accurately as we can from the start, By using the time tested standards we know will work well, rather than be flippant and say we can interchange Datum because they seem close enough. What may seem, is not necessarily what is, and you never know when you could buy a GPS with even more accuracy than you now have. Would you need to unlearn poor habits that didn't seem to matter in order to learn the right ones that do matter? Why not just be correct from the beginning? Does a worthy Ethic need to become an Edict in order to have the credence it should? Of course there is an official Edict but I feel the Ethic is a better motivation.

 

If it is only a sense of authoritarianism, such as following a official rule is the only thing that someone will revere, then I suppose we will have to ask that a rule be made so that people will then be expected to follow it. But the Jails are full of people who simply love being their own people, Rules be damned, so will it help? I doubt it. We in my industry have never needed this sort of motovation, and have always respected the rules, even the unwritten ones because they are the underpinnings of integrity that makes the system as good as it is, and it is very good. It took a lot of work and a lot of respectful Surveyors to get where we are.

 

I am not trying to be arguementative, nor am I implying that you may be either, but it seems a shame that some people would not respect the practices and standards as they are. The Devils Advocate is not a useful position to play in this.

 

Here is the alternative. Here on geocaching the joke always seems to be on the USPSQDN for their not founds. Soon there may be a segment of snickerers out there who make light and mutter about Geocachers refusing to log corrections in the correct datum in lieu of an official rule.

 

Is there a way we can avoid this? Sure. Hold the work you do in High esteem as you well should since you will put your initials by it. And when you do, Be proud that you followed the standards that those before you did, and you are following in the foot steps of some great, knowledgeable people who pioneered this and made it possible. You too helped make it better by following the Standards, Practices and the history of the work.

 

Further I think anyone interested in following the Standards that the NGS would like people, including us to use would do well to familiarize themselves with the information on this page:

 

http://www.ngs.noaa.gov/FGCS/BlueBook/

 

It does outline the use of Degrees, Minutes, Seconds, and Decimal Seconds as the preferred methodology, amongst many other preferred methodologies... In the field we just are not accustomed to questioning the practices as much as were are focused on trying to follow them. Perhaps you are right, Maybe it needs further instructions and more succinct rules, even if the Blue book is not enough. But lets give Casey and NGS time to work up their FAQ and such. We may be pleasantly surprised.

Edited by evenfall

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Yes, I've seen the blue book and have indicated it to people here in the forums several times, usually Annex P.

 

I think what we'd both like to see is something like an additional chapter to the blue book, although not really part of it, that would be a reference very specifically for people making mark recovery reports.

 

Such a thing is really a necessity, especially since the NGS has opened up the mark recovery system to the general public, many of whom don't know much about standard surveying and NGS practice. People are usually more than willing to follow instruction, if it's given specifically and unambiguously.

 

This was the reason that some of us did a project to augment the Geocaching FAQ page because of all the questions that keep coming up.

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Such a thing is really a necessity, especially since the NGS has opened up the mark recovery system to the general public, many of whom don't know much about standard surveying and NGS practice. People are usually more than willing to follow instruction, if it's given specifically and unambiguously.

Here here!

 

I'm not a surveyer, but I think you're right on, and I'd be happy to review anything that comes up.

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I've been merrily entering coords in the DDD MM.MMM format for months, but after reading this thread I made an attempt to use DDD MM SS.S. However, lo and behold my unit only will give me DDD MM SS. From what I gather, that last decimal place is, uh, desireable.

 

I would suppose the thorough thing to do is to covert my DDD MM.MMM to DDD MM SS.S on a web site, then submit. That's a bit cumbersome, but I'm willing to do so. Is there a better course of action?

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Using an online converter seems unnecessary. MM.MMM can easily be converted to MM SS.S in a few seconds with nothing more than a calculator (or for that matter a pencil stub and brown paper bag).

 

Leave the whole minutes (the MM before the decimal point) alone. Those remain as unchanged minutes.

 

Then multiply the decimal minutes (.MMM) by 60 to get seconds.

 

Example: 078 36.568 = 078 degrees (unchanged), 36 minutes (unchanged), 34.1 seconds (.568 X 60 = 34.08 rounded up to 34.1)

 

Quick real world reality check: 36.568 minutes is a bit over 36½ minutes. One-half minute = 30 seconds. The calculated 34.1 seconds is consistent.

 

-ArtMan-

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That is within my calculation skills, and definitely easier than the high-tech solution. Thanks, ArtMan.

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Embra,

 

Really, Degrees Minutes and Seconds to the nearest your GPS rounds is fine. This is not a big deal nor difficult. No need to hassle the conversion, But Artman's solution will certainly work if you would like to use it. Or, simply take your GPSr and go from what ever display mode you are using and set it to D.M.S, or Degree minutes and Seconds Mode. It is all there is to it. Nothing High tech to it at all. This will apply a conversion to the new display format on all waypoints currently stored in your GPS, and you will easily see the numbers change. You can change them back, they will follow you to any format in the box. The GPS itself uses a different methodology for knowing where it is actually at despite any Datum or formatting. This is more about the formatting than it is the degree of accuracy you have. The GPS will automatically convert this for you.

 

You and I are not capable of giving more accuracy than this on our consumer grade equipment. This is not going to be used as survey data. The good news is that the GPSr will handle this for you, and though it will only go to 2 places right of the Decimal, this is level of accuracy is within 5-10 feet of the place the Mark actually is supposed to be, and not the possibility of 500-1000 feet, which scaled locations can be, and that is a big big improvement.

 

Remember, in this case we are only trying to improve the "to find" ability of a highly accurate "Vertical Control" position. We are not getting involved in a way that will affect the Actual Survey Data at all.

 

My professional equipment can resolve a LOT more accuracy, but remember, you are not surveying, you are only adding quality to a previously scaled position. I, when working in a professional capacity will not use this "to find" data to survey with, as I am looking for this particular type of station to check elevation, not location. It is not so anyone can survey with it, it is just so we can find the darn thing! :-)

 

Rob

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Along these same lines;

 

Does anyone make a hand held GPSr that will give seconds to 2 decimals? e.g. 179° 59' 59.99"

 

I have not found any. I asked Garmin if they had any that did this and were DGPS. They said NO because thier GPSr are only intended to be accurate to +-3 meters. So if you are taking decimal degrees ddd.ddddd or ddd mm.mmmmm and converting to DDD MM SS.SS, you are implying accuracy not intended by the manf. of the GPSr. Hand held units are only capable of positional accuracy of seconds to one decimal place. To get more accuracy you would need expensive RTK equipment.

 

At least this is what info I get from Garmina and Magellan, 2 of the most popular.

Edited by Z15

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I'm wondering if there is a source for used survey gear. I could go for a cheap 3" accurate GPS that's been replaced in the survey world with their new sub-centimeter units. :)

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Its not that easy. You need a bunch of gear, not just one unit.

 

Typical RTK consists on a base station with radio and a roamer with radio. + You need a computer to process info. Even if you had a hand held that give your Cm acc, you still need a base station to broadcast the differentiual corrections to the roamer. The current free DGPS out there (WAAS, USCG) will not give you less then 3 m acc. You have to establish your own stations to work off of. There are companies that sell this service.

 

When I worked for the DOT, I had 1 base station and 2 roamers + all the associcated gear (tripod, trucks etc). We had Leica systems and the cost new was approx. $50,000-$70,000.

 

You might find this interesting

Edited by Z15

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Please excuse me for resurrecting an old thread, I am working my way from the oldest to newest in this forum.

 

If I understand correctly, there are vertical stations and there are horizontal stations. My question deals with this dichotomy, why are vertical and horizontal stations separate? Why don't the surveyors determine both dimensions accurately when placing a benchmark/monument/disk/etc.? It seems odd to me, to be accurate one way, but not the other. Am I missing something?

 

Also, correct me if I'm wrong, because there are three dimensions. Is a vertical disc equivalent to elevation, while the horizontal is equivalent to latitude + longitude?

 

-stroh

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I'm sure the professional surveyors here will give a more thorough answer to your question, but briefly, yes vertical control is elevation, and horizontal control is latitude and longitude.

 

As far as I know, the reason why there are separate kinds of control is that it takes different techniques (or at least it used to) to establish the two kinds of stations. Vertical control points were established by "leveling", which was a matter of setting up a perfectly level telescope at a point and using it to read the markings off a graduated rod. This allowed surveyors to compute the height difference between the points the rod was rested on, and thus transfer elevation data from point to point. Station marks were set periodically along the line that had been leveled, and their elevation recorded.

 

Horizontal control points were established by triangulation, which often meant erecting towers so that the surveyors had lines of sight between several distant points, and then using a theodolite to accurately measure the angles between the points. A few baseline distances could be measured by chaining, which was a tedious process of using tape measures, or in some cases precisely machined metal bars laid end to end. With an accurate baseline, and accurate angles, the locations of the triangulation points could be calculated.

 

Horizontal control was much more difficult and expensive to establish than vertical control, so the majority of the older control points are vertical control. Nowadays, differential GPS can establish horizontal control relatively easily, so modern control points will often be both.

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Green Toad,

 

Let me see If I can help...

 

You said: "If I understand correctly, there are vertical stations and there are horizontal stations. My question deals with this dichotomy, why are vertical and horizontal stations separate? Why don't the surveyors determine both dimensions accurately when placing a benchmark/monument/disk/etc.? It seems odd to me, to be accurate one way, but not the other. Am I missing something?"

 

First, Holograph addressed this really well... Horizontal control, Lat/Lon uses a different Methodology to derive than Vertical control. He covered this with you. But the history is interesting and I will only go back so far. Until 1992 we used a system of 26 tide stations which were mathematically averaged to determine mean sea level. This was the basis for Vertical control.

 

As science became available to observe the earth in better ways we learned that too many things affect tides to rely upon the average of them to suffice as a reference standard. Gravity, Wind, and how much water IS in the ocean at any given time are factors. Then we found that Wind is weather, and that isn't static. and gravity, well, it isn't static either. In fact the gravity has more to do with what elevation is about than wind or water, so we tossed the old model.

 

The New Orthometric model was accomplished by leveling. Well so was the old model, but we now use just one point in Quebec to base all the leveling off of. it is known as Father Point/Rimouski. Accomplishing this caused all the numbers to change towards more accuracy..

 

Today in the GPS era we have a reference ellipsoid that the GPS Satellites use as a base model for the surface of the earth it is a smooth ellipsoid that represents what is now considered mean sea level and has nothing to do with the sea. It is just an averaged earth surface to the ellipsoidal size of Earth.

 

Then we have a Gravity reference called the Geoid. It is a reference of the gravity measured over all the areas of earth. It is sort of generally ellipsoidal too but it is lumpy and unequal It is high in the mountains and low in the valleys, even lower in the oceans... It is not the actual terrain but is is close.

 

Finally there is the actual ground. When we measure this with GPS, the figuring considers ellipsoid height, Geoid Height and orthometric height, with Orthometric height being the actual dirt. The formula works like this: The relationship geoid height is the vertical distance from the ellipsoid to the geoid level surface. These heights obey a simple equation h = H + N

 

It is a straightforward procedure to algebraically subtract an interpolated geoid height, N, from a GPS ellipsoidal height, h, to obtain an orthometric height, H: H = h - N . What I mean by interpolated is that the geoid height can be above or below the ellipsoid height. The actual surface, the orthometric height can go below the ellipsoid as well. The ellipsoid is a smooth sphere like non undulating surface, the Geoid is comparatively lumpy and both are existent in the same space. So interpolate is meaning to recognize the positive or negative comparison to the ellipsoid. Your GPS is going to measure your height above or below these references in order to tell you what the elevation is.

 

Now for your horizontal work, the Latitude and Longitude are based on the ellipsoid and so you make the grid we use and impose it on that ellipsoid. It seems pretty simple, and that is what GPS does.

 

But the earth is not a perfect ellipsoid so then what? Well In the first place we used optical surveys and triangulation to base relative physical locations marked on the ground. We used a different model for our ellipsoid then too. Science reared it's progressive head again and we tossed the old model, then we had to make all the positions we had before fit the new model because we had very good physical measurements with which we could compare just like in the vertical models.

 

We went from a place where we considered a place in Kansas, called Meades Ranch as the center of all horizontal survey in North America, to using a model we considered the center of the Earth, or pretty close to it. GPS Simply Positions in the horizontal to a grid drawn on the ellipsoid in a basic way. the size and shape of the ellipsoid has a lot to do with where the positions actually are and the science behind the ellipsoid is quite involved too as you well might imagine.

 

Taken all at once the GPS can formulaically get close to a three dimensional position. But, and I mean a Big But, when you get to the critical, highly accurate observations, we cannot take them all at once because we are applying Calculus to these to statistically accurize a network of observations and the vertical observations will skew the horizontal ones and visa versa. This is why some datum treat the Horizontal and the vertical separately.

 

In the end if you fix a horizontally averaged location against all the others in the average and you lift one up and push another one down in the vertical plain, you will observe that the positioning in the horizontal plain will have been altered. The Mathematics just won't play, so we keep them separate.

 

"Also, correct me if I'm wrong, because there are three dimensions. Is a vertical disc equivalent to elevation, while the horizontal is equivalent to latitude + longitude?" Yup. You got it! :-)

 

Simple GPS is not enough to position these locations, GPS has to add the Formulas contained in a Datum to get the accurate positioning these stations represent. The Horizontal Datum for these discs is called NAD83, and for the vertical is NAVD88. Some discs are Vertical only Some are Horizontal only, some carry Data for both, but the Datum for either Horizontal or vertical is kept separate always. WGS84 does not apply to these monuments in any sense.

 

Hope that Helps,

 

Rob

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