evenfall

Hi Professor, You are correct in your thinking and the process. The United States National Geodetic Survey already has all this geodetic work well in hand. Thanks for you insights. Like you say, all the people who decide these things have to choose what their standards will be, or use. What some of the people want to see here is a direct comparison of WGS84 and NAD83 in a couple of specific places just to see what the difference is between reference systems. The stickler is that the Datums though very closely related were developed by different government agencies here in the US to satisfy different primary objectives. It has been noted by Geodesists who were writing thesis papers that there are differences and similarities but no one has really bothered to publish a paper on all the nuances. For the most part, it probably isn't considered proactive when based against all the other things they want to know so it has not been done. In addition, with both NGA (DOD) and NGS (NOAA/DOC) always upgrading their editions of the Datum toward higher accuracy, the freshness of the information if published would have a relatively short shelf life and quickly become misleading to those who are not aware of all the little details involved which affect these sorts of calculations. If your mind is begging the question, why would one government allow two different agencies within it's oversight to develop two different systems that do nearly the same thing only slightly differently? Well It is a good question, and I have two answers. First answer is that the government is big and the right hand does not know what the left hand is doing, Partly true because one of the hands belongs to the military and they like secrets. Second answer is Politics and Agency Culture. Government has branches and funding comes from different places and for different reasons. They all have their own way of doing things. In the most basic way of comparing the two datum, the problem is that some things about it are apples and oranges, You have 2 frames of references that utilize differing frames of references so you have to agree on how you want to compare them. ie; You are using two different models to define earth center so what way should you compare them? Ok from one to the other looking from both directions, the other is 1 meter away in both the horizontal and vertical components. But it does not end there. Both datum use different mathematical models to define things further, so how do we define a term or a baseline for reference? One person said assuming the GPS system has it's own references to which the formulas are applied after the radio work is done, that would show a difference right? Well I suppose so, but it then becomes a 3 way comparison. We would not be directly comparing Datum, we would be comparing the Datum against the Satellite Triangulation methodology. Probably close enough for the answer they want, but the answer is not be all end all because it will vary all over the globe no matter where you measure it, yet they are so closely matched that it would be like comparing 2 BMW M3 Sedans in a full on spec for spec race, with the only major difference being the Stereo Package, and that one is owned by a Man in New York, and the other is owned by a Lady in L.A. They are so similar in what they do that no one bothers to do a comparison, they just know that they need to use one for some things and the other one for other things and leave it at that. So far, and I have checked into it, no one I have spoken to so far is aware of a comparison they feel will give a Certified answer. They know there are programs that calc datum shifts, but they refuse to endorse them because they cannot certify the accuracy of the algorithms being used as up to the specs they would expect. To survey it properly there seems like it would be a problem as well. You spend 2 days surveying an NGS Tri Station with RTK, but that is not your comparison. The NGS grid is adjusted through a Least squares method, so I suppose we need to find a way to adjust our survey to WGS84. So now you are comparing two things that can argumentably be hard to define terms for, because there is more than one camp here and they see this different ways. Bleh. For that matter we could give the question asker a reasonable answer, and they could still play devil's advocate and say How do you know, or how can you be sure? Or what did you use as a reference model? It could go on forever. Quick and dirty you can take a snapshot of the 2 datum in one place and wait for the accuracy to need to see the difference, but the scientists could still ask the question, as compared to what? For most usability, it is enough to know that there is enough of a difference that we should use the one that is recommended for the purpose. Are you bombing or engineering a civil works project? That is the criteria we use for determining which Datum to use. Rob

Art, I bet Jeremy has every word you can think of nailed down. Isn't there a test forum you can check this in? I think there is! Too Funny! Many Congrats Artman, Nice work! Rob

Skillett, Nee! One that is not too tall and looks really nice! :-) Thanks! Rob

Bicknell, You can do this between NAD27 and NAD83 already with the NADCOM program that NGS has online. But it does not give you the formula to make the datum shifts. There are many factors used in developing a datum and each Datum has it's own. it is a lot to compare. Nobody adjusts between NAD and WGS. To be honest it is pointless to do it. I mean you use a shovel to shovel and a rake to rake, it is almost that clear, We use one for this and not the other. I could get you that number for a location of my choosing, but I won't. Maybe someone will. The Instrument would give you a difference, but the number you get would be a result for that location, NOT a number you could reverse engineer into a formula, because between the Datum there are virtually few places where the ellipsoidal heights would compare due to different earth centers. Depending on where you compare, the difference could range from zero to 1 meter, and this is not going to take in to account orthometric differences that are not allowed for by the WGS in North America if I am not mistaken. See, CORS Data is watching this all the time. But even at any two different CORS stations, the Datum Shift would be different, and dependent on the Latitude Longitude and elevation, etc. It would be wrong to think that number is a rule, as in a hard fast rule, but a skimmer for numbers is likely to misinterpret it as a hard fast rule and not the difference in datum shift for one location. In the end I feel most people would misunderstand what the number means and globalize the result as an answer. Just my opinion but I have seen it happen with less. There are a lot of geodetic equations in play. That is in the realm of Geodesy, and Sorry, I am not a geodesist. It is a bit more than I need to know. Interestingly there is a lot about Surveying that Geodesists don't know too. We are all a bunch of specialists in the end. In fact even at NGS you might have to speak with a geodesist who specialized in Horizontal Datum shift work to get a person who is up on that information. That is 8 components to compare, datum to datum and it is only right for one spot on earth. Sorry. My Advice is Use NAD 83 and be happy there are people who enjoy Geodesy! Thanks for the question. Rob

Skillet, I guess that smile I put at the bottom of my post was not enough for you to catch my attempt at humor... So *Ahem* <<I was attempting to be a little funny there.>> Out of character for me I know... So Sheesh! In fact don't make me become one of the knights who say Nee, and say Nee unto you, until you appease me... Now, go get me, A Shrubbery! Rob

BDT, You can cross your arms and expect the ocean to wash an answer up in front of you if you like, But I won't be joining you! :-D I have already answered this exhaustively. What I am saying is that there are no datum shift conversions bro, Nobody bothers. Use the appropriate Datum for the job is the answer. But I clearly stated that in every essay I ever wrote to help you understand this. :-) I just hope that you will get it. Unless someone goes out of their way for you, and I wouldn't hold your breath to long. If it does happen for you, and it could, Please Consider that someone went out of their way just for you and only you, because that is what it will be. Be Honorable towards them for doing you such a favor. Oh and what I told you are truths which you can easily corroborate, nothing rhetorical there. I tend to steer clear of rhetoric in my conversations and communications. I work in the field but that is no reason why you should believe me. I have no credential that means anything to you, nor do I need to. You are a Skeptic and I am used to those, I see them every day :-) Nobody out there has convincing you as a part of their mission, and I think you have the best answer you'll get unless you roll up your own sleeves and do the figuring. :-) Be my Guest. :-D Good Luck BDT! As Always, Rob

BDT, I realize you are addressing Dave here, but I have attempted to explicitly answer these questions in several different posts I have made. Let me try one more time, and if you like, you can look at my old posts here to research through. As I have said in the past, WGS84 and NAD83 are from different agencies who have different missions. They use different standards because in part WGS84 is trying to be a best fit for the earth, and NAD83 is trying to be a best fit for North America. You could say it is a harder look at just one continent, to describe it as close as it can. As you might expect, there are different folks working at each agency and they are going about this each in the way they think is best, so it is safe to say that there is a differing opinion between them as to what the best geodetic solutions are, and you would be right. From this I am hoping that you understand that comparisons between NAD83 and WGS84 are not routinely performed. Dave has mentioned and so have I that there is an approximate difference 1 meter max, between the datum. Problem is, that this is only true at the earth center interpretation so if you imagine two ellipsoidal reference spheres that are the same size sitting in the same place but with earth centers that are their own and only 1 meter apart then the surfaces (known as the ellipsoidal heights) would not be in the same place, they would overlap slightly, so only a few spots on the globe would be the same from one to the other and all the rest of the places would be between 0 and one meter apart, but mostly somewhere in between the two extremes. the orthometric heights will differ all over as well, and that is not proactive either. Bring two different Geoidal models to the table and compare them and you just have something that is close to mind numbing on a point by point basis. Two compare the one spot like you want is doable I suppose, but the comparison would be unique to that location. Professionally it isn't done because the civil engineering world does not consider the WGS84 Datum as relevant to North America. The Standards are not as high as NGS for North America and it is a Commerce thing not a Defense thing. So it is a sort of quasi political - engineering thing. Due to the quasi thang, nobody has a comparison calculator out there on a web page, because the plan is to stick to the plan and the plan is to use either one Datum or the other, and not compare the two. Partly political, partly engineering. NAD27 and NAD83 Datum Shifts are Compared due to Propriety reasons and due to NAD83 replacing the NAD27 as a standard datum. WGS84 And NAD83 do not supersede each other, they are just two different Datum for different reasons. WGS84 is not a close look at North American Geodesy nor is it used for civil work, Mostly it is a non player in the game. Bottom line, the appreciable differences are there though small, they are mainly only interesting to geodetically related things, that is a focus on things very small and most of the world is focused on bigger. This is one of the scientific studies that fit in the box with superconductivity and absolute zero. The closer you come, the harder it gets. I have tried to answer your questions, I hope to your satisfaction. Maybe Dave has more he can add, but that is pretty much what most of us in the field need to understand to do the work we do. I didn't do the math for you because I don't really have a way to frame both references available to me in order to compare them at the one spot. Calculating ing Datum shifts at a specific place is a lot of number crunching, is not for the meek and besides, Bleh! Rob

The Bottom Line in Benchmark Hunting is that you are hunting down a piece of history. It helped, and was likely used in some way to help develop the area it was located in, if that happened, and if not, it helped us physically locate the many features of the country so we could know where they were and to accurately map them, Chart them, and navigate them. You take with you the tools and your wits, to find something nature may have hidden or man, without knowing may have obscured. You take from it a big sense of adventure, and an enlightenment from learning the history behind the Survey Marker you looked for. The payoff is not about taking or leaving a trinket, but rather the experience the hunt, and the adventure. It is quite often many times moreover a riddle and a challenge than a geocache often can ever be, without seeming diabolical at all. So if you like riddles and challenges, where the payoff is the adventure, not the end of the adventure, developing skills that do not leave you relying on luck, things that may push you further than something easy ever would, then you are not "special", you are just someone who is not easily amused and willing to take on bigger challenges for your entertainment. Sure it isn't for everyone, but neither is anything else. It is all about having a good time, so, Have a good time! Rob

What is he world coming to? They try to eliminate paper and then make the Cookie Monster become politically correct? I gotta get Mark Knopfler to rewrite Money for Nothing so as to include the new line, I want my, I want my, I want my datasheeeet! Sheesh! :-)

Nope, They are no fun at all. None! Benchmark hunters are all curmudgeons who hate to have fun and look down on Geocachers! And them Bench markers, well, they are "Special" ROTFLMAO!!! This thread is killing me! What a riot! I love this! You can't write this stuff, really! You'd never think it up in a hundred years! Some pretty interesting perspectives there eh? Proving that the truth in reality is stranger than fiction! It would have been nice to leave the thread where it was just to see where it really would have gone... We could have secretly spied on it from a safe position so as to not infect it with "special" germs... Too bad it has been transferred here to the land of the "special ones". It was great fun to read how some people see this! I am going for some NAD27 benchmarks, I want to be FTF on that action!

Stripemark, You said, "But, if you can't trust the desciption text of the NGS, who can you trust......" I know, It would be nice to think we can trust them, as an official authority, but NGS is an organization which down through the years has been comprised of many people. This could have been one of those people who were like many humans did not get up and look from a different angle to see, for some of the legs were perhaps hidden by others on the tank, and just wrote what they saw. maybe they were Tired or it was almost quitting time, Maybe it was a rainy day and they wrote it in the truck from what they could see at the time... Humans can make errors, and lucky for us, you are a Human who caught the error and could perhaps help make an appropriate correction. If civil authorities have confirmed to you that this is the only tank that has been at the site since well before the 70's, you can use that information to submit an update to the NGS if you like. Be careful of calling a Geodetic position questionable with Handheld GPS. Seventhings has what I feel is the best method to use to become as absolutely sure of what a Handheld is telling us when used with hunting Landmark stations, and I would use his method to double check myself before I over think this much further. Just a few thoughts. Rob

Buck, We do have to keep in mind a few things, perspectively speaking. The USPSQDN is a volunteer organization and they are performing a public service just like Geocachers. They are not paid, nor are they professionals when it comes to Survey Markers. They do have a program where they help out the OS, National Ocean Service by reporting changes to Buoys and water hazards. Since NGS is a sort of division in the NOS, they get the survey markers reported too as a public service, and have for over 30 years. It really does us no good to put a sharp stick in their eye. Going beyond that, this is a national organization who is comprised of people who own boats, Big Boats. Think Yacht Club if you will. If you have ever lived near the water, then you know what kind of pull a Yacht Club can have in a Community due to the political power of it's affluent members and the way it works in the community. The USPSQDN has a National Board and Officers and they actually have National Political power in that they Lobby for Boaters rights and privileges and have moneys to support the causes they like. In other words, they are highly organized. Some of their members are friends of people in high places, maybe even are people in high places and so you see where the hidden connections could be and what the hidden ramifications of not showing appreciation could do. I am sure none of them really care much If we quietly go about the business of correcting mistakes when we find they have made one, but we would be best off if we leave it at that level. We are not the new Sheriff in town, Just another set of eyes! :-) We know they do not always get it right but that is just our challenge to try to resolve, It does not have to be a good deed done in public, by letting our pride well up a bit too far. After all, some things are better left unsaid. :-) Rob

Xangxa One of the things I've worked hard to explain in the past is consistent with the problem you explained here. Not being on the correct display format is very misleading. Geocaching likes Decimal Minutes. NGS Datasheets like Degrees Minutes and Seconds. The uninformed hunter mistakenly does not know there is a different in formats, or does and misreads it. This happens when they try to use a real Datasheet written in DMS and compares it to their GPSr which is defaulted to DMM formatting. If you try to make DMM output on the GPSr numerically match the DMS location on an official NGS Datasheet that 28 Yard error is consistent with distances of error I have observed when I have checked this myself, give or take a little. I have consistently seen it be a couple hundred feet of error. Making the numbers look the same can result in hundreds of feet of error due to the differences in output formats, when the formats being used are not the same. My thoughts on this have been to use the NGS Datasheet when I hunt, and use their DMS Format with NAD83 Datum. What I am saying is that if I truly set myself up on the same page as the source for the real data, I reduce my own error as I am doing things exactly the same way the work was done in the first place, and I can see at a glance that all numbers Jibe. I personally do not use any other formats other than the ones given by the issuer of the data. That said, If I were Geocaching my formats would be WGS84 and DMM output formatting. Then my formatting = their formatting and it is one less thing to have to double think. Rest assured, if the station you are looking for is horizontally adjusted as it were, the accuracy on that datasheet exceeds the accuracy of a GPSr Handheld by over a country mile. I cannot speak to what GSAK does because I am not a user of that program, Sorry! As I did say, Using the DMS format is consistent with what NGS does in the first place. If you reformat, you run he risks of introducing an error, especially if you make a mistake in the process. Your milage may vary. Dave D Just posted a nice explanation about the differences between the NAD83 Datum and the WGS84 Datum, it is well worth reading! All stations which were originally monumented during the time when the NAD27 Datum was in use have been upgraded to use the NAD83 Datum. The NAD27 Datum is now considered superseded for use with these particular monuments, However, the NAD27 datum is still in use and it should be the datum used if so specified by the document in question. Like it is said. Always use the Datum on the Map. Good luck! Rob

Thank you Dave, That supports what I have been explaining a great deal. I appreciate you weighing in on this. Rob

Prospectors, If you like, you could post with the PID of the station marks that you found, and we can look at the datasheets and guide you toward understanding what there is to be known about the stations YOU found. Then it is specific to your situation and not just a generic overview. It helps us all to relate to the situation rather than generalizing without anything specific to relate it to. We can give you better more specific answers that relate directly to your experience this way. We could even post the datasheet here and sort of tear it apart and explain it to you. Are you Game? We'll all be here... Rob

Art, JV2798 DESIGNATION - V 167=DAM The Disc had a stamp before it ever had a PID. The 1992 visit was not to this mark if the one they found had no stamp. I would have to say that for me, I'd say they didn't find it. Why? JV2798 NAD 27 - 39 00 27.78100(N) 077 15 18.29200(W) AD( ) 2 JV2798 NGVD 29 - 82.099 (m) 269.35 (f) ADJUSTED 2 2 See, the station had been both triangulated and leveled back in the 30's. The superseded data proves it. Probably leveled in '30 Triangulated in '31 by CGS. That superseded data shows that the 92 recovery likely did not add to the data. I'd trust that the NAD83 and NAVD88 Adjustments were done by NGS in house. Who knows what the '92 visit found. You found a station with a stamping that matches the one described by the hard data in the datasheet. I would say the station designation trumps the narrative station recovery. I'd feel free to submit it as you found it... Who cares what the 92 recovery says. Your recovery is going to say you found a stating with the correct stamping at the right coordinates which compare favorable to your GPSr. Right? :-) If there was a problem, there won't be when you are done with your filing. You have the original. It really is pretty obvious eh? :-D It is a found. Done deal and Congrats! Great work! Rob

Munin, This is a Horizontal, second order station. Since it is horizontal, we can place our instrument over the stem and optically use it's center of the remaining stem for survey purposes. If it were vertical control, we would physically rest something on the station to take our measurements. So, that said as a future reference, Horizontal Stations like this would be considered Found Poor, but usable, and if vertical control, then Not Found or destroyed, though the burden for proof of destruction at NGS may be difficult to achieve so a not found would have to be used to place mark the station as not usable for vertical work. Your work and your journal here of how you approached this station was a wonderful essay on the critical thinking behind hunting the details down. I really enjoyed it. Good luck with the other stations, Rob

Callaway, It would be a day hard to top for most anyone... Really nice! Congrats! Rob

Buck, I would not go as far as to say what you said, and I am not sure you can substantiate it. I'd be happy to let you try if you like though! :-D The meter has it's history in the physical length as a line from the Equator to the North Pole as taken through Paris. it was first defined as 1/ten millionth of that measurement. That is a Geodetic Measurement. Science needed a more stable object than the earth and a physical unit for comparison so they made one several times and adjusted it a bit because of further observations. The meter today: The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. It remains the main unit of geodetic measurement today. While it is true that the meter is now defined differently, it has not lost it's relationship to the work of Surveying. Yes, the Surveyors and engineers DO rely on it quite heavily. Less so in America, but is underlies a lot of it. We just use the US Measurements because it works best for the US thinker's frame of reference. That is a cultural resistance thing more than anything else. A lot of us Survey is done and Has been done in the US standard of feet with no connection to the Standards of Geodesy too. After the fact it is a bit of work to make it all fit well too. I will reiterate that all NGS Control is based on the Meter, In fact this is true of Geodesy in every country in the world. So all Geodetic control issued in the US is based on the meter because the NGS is the purveyor of that control. The math is done metrically and converted to US standards after the fact in most cases. It is the way Geodesy has always been done. And that is the most accurate Horizontal or vertical control we have, period. There is no other Higher order for it in America, including the work done by NGA, the WGS Datum Folks. They use the metric system too. Holo, Maybe we need outriggers or stabilizers like they use on ships to help with the topic drift eh? :-D Rob

Bill, Yes it is common to convert things to feet because Americans Think in feet and use feet. Construction Staking is no exception. Everything is planned to feet. Meters are sometimes used on highways, but that is not always the case. Many States are dropping the requirement for it as too many things go wrong. American's make mistakes with measuring systems they do not understand but it goes beyond this. They are actually resistant to learning it. Rather than fight them on it, it is just easier to convert. Further there are no real tidy metric numbers that fit what America has already built in feet. All the conversions to metric were more crazy making than converting metric to feet. The reason you see metric system usage in NGS is because of Geodesy. The meter itself is a derivative of geodetic measurement. It takes ten million meters to go from the north pole to the equator. It was a given distance and the French Academy of Sciences decided that the appropriate sized unit would be that distance divided by ten million. Since the meter was designed to describe geodetic measurements, it still is. In construction Staking, we so not add the SPC info on the stakes as it is not what a constructor needs to do the job. It is written in codes, but it basically tells them what to do, how far away from and what quantity from where we place our staking. Decoded, it might say something like "25 feet away from here on line with these two stakes, is the location of the Top Back of Curb. The Cut to Top Back of curb is 3.07 feet to Finished Grade. It can be a lot more elaborate than that but that is the side of the stick facing the work. On the back of the stick I'll write the Job point number and the elevation for the Hub, with or without Tack that I drove in the ground in front of the Lath I wrote the directions on. In the GIS the SPC of that spot can be derived, so can the Lat / Lon, but it is not important to the constructor who's main gig is to move dirt and get paid. Construction Staking is a ways down the food chain from Geodetic Control, but the construction staking could not be meaningful without the Geodetic work in place. In summary, the Metric system is a good fit for Geodesy, but a difficult fit for the American way of thinking. Rob

Hi Dave, <grinning> Yeah I had heard... I even know of a county where they are very, no, make that VERY adamant that all vertical control be done in NGVD29 to this day. It is a pain. Politics and bureaucracy as usual. Makes one wonder how we update anything when half the world insists on staying in the past. It goes to show that the old saying is true. All politics are local. Yup! Too Funny! For us here at geocaching the international foot will work well enough. Rob

Long time no see Dan, Nice to see you drop in! :-) Rob

Zhanna, In many cases these sketches are on Microfiche and copies can be made by NGS, You just have to contact them and ask if any are available for the PID in question. Sometimes they are, sometimes not. The only way to know is check and see. Rob

Keep in mind too that the Earth is round and perfect squares do not necessarily describe something round really well, so grid north does not exactly point true, except for on it's base meridian. Where is that Base meridian? Well, it is not where you might think... It comes from a different way of looking at the world. A way that does not take into effect the curvature of the earth as well as we would like, but a way that works if we keep that in mind when we use it. On the grid based idea, the State Plane Coordinate System was born. It is a form of Lambert Conformal Conic projection, only it was given an accurized baseline for each state, Rather than 2 edges of a world map which is the usual manner with Lambert. It was a way that a flat map could fit a territory, even if the way we thought of it didn't hold true for every construct about the territory that there is. Of course anywhere off the baselines is a a geodetic distortion, but it helps describe things accurately, or well in a measurably accurate way if you don't account for everything, and locally when compared to that baseline. NGS Developed the SPCS back in the NAD27 days and there used to be this thing that came out of it called the US Survey foot that was part of it. The Grid foot, or U.S. survey foot = 0.3048006096012192 meters, exactly. We as a country no longer use it. 1986 came along and so did NAD83, NGS went 100% metric, dropping the old survey foot. Now you convert Meters into international foot with the Formula: Feet= Meters/0.3048 exactly. Read more here: http://www.vterrain.org/Projections/sp_feet.html Follow the links you find there too if you like. There are your Grid Feet. Oh and the Grid they are working with too. For most of us, it will be helpful not to put too much energy into it. It is just another mind numbing construct, albeit needed to fulfill some purposes, like using a grid system to describe something instead of Lat/Lon via Trig, in the survey world. Like UTM only different. Enjoy, Rob

More on fun and hi-jinx in the workzone... I was kneeling down and on one knee pounding nails with shiners on them into the asphalt pavement. These were to become survey points for a road widening that was to occur. during construction there would be no good instrument setups over the bank as it was where the work would be. The survey points had to go in the road. I was in a traffic controlled area with cones set up. A big moving van punched a 24 inch cone at me going about 50 mph. I heard it and saw it. Probably was looking up at that moment. Can you say duck tuck and roll? It would have clobbered me. A buddy of mine had to take a ride to the hospital after a car hit one of those big orange barrels they use in high speed areas. The problem is, he never saw it coming. He was in the work zone too, standing on the back of an asphalt paving machine. He was standing on the screed as the screed operator. He was where he was supposed to be, doing what he was supposed to do. Knocked him hard. wind out and all. They took him in and had him X-rayed, nothing broke but had it hit him higher it would have been a concussion. He got a few days off to heal from the bruises. Saved a buddy not long back, He almost walked around the corner of his vehicle without looking or thinking... I saw his head wasn't in it and yelled to him, If I hadn't, he would likely have walked into the oncoming car. It would have hit him with zero time to react for anyone. It's really just stupid stuff that you could never imagine let alone make up. Traffic just adds insult to injury. Be safe. Rob