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vofsar

Ngs Coords.

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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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