evenfall

Howdy Back, Plenty of maps will lead a person right to it. Yup. It can be found without Coordinates, In fact Cadastral survey marks typically use the data stamped right on them to clarify their position. It has nothing to do with benchmark hunting at geocaching either. This isn't a benchmark. Anyway, Still not a puzzle. Puzzle solved. Rob

Howdy Back, "I don't do geocaches." But that is a Cadastral Survey Marker and it's data is available from BLM. Geocaching has nothing to do with those, they are not in their database. It was set in 1933, and is located in the State of California, in Township 20 North, Range 25 East, Section 28, and it is angle point 5 on the section border. Puzzle = no puzzle. Puzzle solved. Happy Geocaching. Rob

BDT, As you may remember the Geodesist was upfront about the problems the Meter has with Scaling Factors. You know the terminology; "all things being equal" Well the difference between the datum is not equal, but in order to compare it, you have to look at it through the one's frame of reference or the other's. Say hypothetically that if we look at NAD83 as seen through WGS84. WGS84 will determine it's meter is perfect and correct, but looking at NAD83 from here it looks like their Meter is longer than a Meter. Conversely let's look at WGS84 as seen through NAD83. NAD83 will determine it's meter is perfect and correct, but looking at WGS84 from here looks like their meter is shorter than a meter. Either way, or looking through the eyes of either method will cause a scaling to occur. You have to make them "be" the same, same in reference to the datum acting as the control Datum, in order to properly compare them. It is just something we have to realize happens and accept it. The most important thing we need to remember and take away from this is that we do not compare them. They are real close to one another and we accept that, but we just leave it at that. We just use the datum determined to be most appropriate to what we are accomplishing. Again in the United States and Canada, we stick with NAD83. We don't even think about WGS84. WGS84 provides zero survey data to anyone that pertains specifically to North America in any High accuracy format. They do not give us any known points to adjust, per se. To the DOD, the owner of that Datum, non DOD users do not have a need to know. All the DOD WGS84 Datum Owners let us use is their reference ellipsoid and geoid models. It is up to any end user to accept that and develop their own work from there, as many countries have, or as you know, in North America, we use something else. NAD83 Is the Horizontal Datum of choice for any civil work of any kind in North America. NAD27, in it's various iterations, is number two. Again, It is perfectly fine to reread this topic's thread and revisit what has already been said. It is a lot to take in. Rob

So BDT, I think I covered that. A geodetic Realization is the frames of reference(s) used to make the latest determination, as it the terminology used to refer to older iterations or determinations. It would be ok with me if you would like to refer back and re read some of the things I covered. I know I covered a lot of ground and it would be easy to overlook, or not remember what I had tried to convey. In another frame of reference: In order to have a part fit a particular car you may need to know the make, model year, Build Date, Transmission Type, If it has A/C... Same diff. You own a car? It is a realization of all the particular options you got on that model. Rob

Black Dog, Please. You are tilting at windmills here. Splitting hairs. WGS 84 offers us no Physical Survey Orthometric reference. I said that many times. They do not publish a database. So there is no way to compare that as such on a point to point basis without a Geodesist to figure it out for each point which has already been done for you on, One point. Go do your research eh? Go learn all the various definitions for yourself, Not for what you want to take them to mean, but for what the accepted meanings are. Then go ask a geodesist if your understanding is indeed correct. :-) Rob

kc2ixe I know it is a PITA but if you carefully re read most of what I have posted in this thread, you will get a good idea of what all the different parameters and reference frames are, believe me, it is not just the meter. I am pretty much done explaining this. I have had my work fact checked and It is correct. Trust me, I know Surveyors who have more formal training than I who have a hard time with this concept so don't feel alone. It is a super difficult subject to explain and understand. There are many, many things you have usually got to have a really good grasp of before this particular concept begins making sense. In the clearest language I can use. WGS84 Is what your GPS and Most all GPS uses to derive basic locational Parameters, Primarily known as Ellipsoidal Heights. The Ellipsoidal Height is an Ellipsoidal Height is something that is a smooth surfaced 3 dimensional object that closely resembles the size and basic shape of the earth. For Civil Survey Work In the United states of America, Mostly it ends there. I will repeat. It ends there. WGS84 goes in the trash right there. Kaput. What we do with Pro GPS at this point is after we derive ellipsoidal triangulation with GPS via WGS84. Three components, Latitude, Longitude and Height above or below reference ellipsoid, then we post process it into NAD83. First, A different reference Ellipsoid that the Geodesists at NGS use because they feel that their Ellipsoidal Model is a Better fit for North America. Then we apply a different Geoid (gravity) Model than WGS84 uses because the Geodesists at NGS feel it is a better fit than the one used with WGS84. Then we apply the sum or difference of our observed ellipsoidal height above or below the reference frame and compare it to the Geoid Height to derive Orthometric Heights. Of course Orthometric Heights are also compared to Other known and documented orthometric heights which are in a look up table, and the highest accuracy ones are part of the NSRS. WGS84 Does not give Orthometric Data for any Country. That is Military info in that Datum. It happens to turn out that the scientists who develop all these reference frames have Meters that differ in length too. It is because of what happens when the meter is scaled by the various reference frames. Yes because of Scaling there Are Differing models for the length of a Meter too. Oh and not to mention the Length of a foot too. It all works out in the end when you use the models that were used before. The 2 datum are close but they are not the same. Further no Datum is the best Datum in the world, and WGS84 is not I repeat NOT more accurate nor considered to be More Bettah than NGS's NAD83 especially in this Continent. WGS is a datum which is trying to be a best fit for the entire earth, NAD83 is trying to be a best fit for North America. WGS84 is not the so called king of all Datum. It is not more right that other Datum. (My Datum and your Datum were hanging out the clothes, My Datum punched your Datum right in the nose...) Your consumer grade GPS uses a lot more of the WGS84 Schema that Pro Gear has to to derive it's location. It uses all of WGS84 and then applies a Moleninski Transformation. Pro Gear can do that too, but there are other, more accurate ways available and for real survey work, those are used. If you want to know more, Google will happily bring you tons of Technical Documents to read! Really! :-) In the end, We in the survey world are told to Use NAD 83 and everything will be just fine, so we do, and it is. My Best advice as someone who has made a buck or two off this stuff is if you are doing something in the Behalf of NGS, just follow their direction and advice and don't worry about the other stuff. You will be fine and well taken care of. If you choose not to believe me or trust in my advice. Ok. Don't. All I have attempted to show everyone is that there is a difference, what the difference is, and why it is important. (P.S, it may seem insignificant but at this level, the geodetic Millimeter level, it is) I think I have accomplished that, and so have the others who helped out in this thread. If someone want's to not follow industry standards from there on, they are on their own! I wish them well! :-) Good Luck! Rob

Highpockets, What I mean is that LC1605 is a replacement, or a RESET Station Mark for one that was there before it which had the same name, Q 197 and it's PID was LC1450 Station Naming Conventions were not exactly supposed to be arbitrary. As a rule of thumb, in most cases, and I say this as it was more true back in the day before GPS that it is now, If a Station has a Verbal, Phonetic Name, It was given so because it it a Horizontal control Station, (Optical Triangulation in that day), and if it has a Letter followed by a short series of usually 3 numbers and maybe other letters after that, sometimes, it was generally meant to be Vertical Control. Since GPS came along, it is not uncommon to find Stations which have been given both types of survey control, Regardless of the Naming Convention. This particular naming convention only holds true for stations which were originally monumented by the NGS and the older names that that Agency had back in the Day, which was know as CGS or USCGS. (United Stated Coast and Geodetic Survey.) Coast Survey is now a separate arm under the National Ocean Service. The Naming conventions for all the other Agencies which set monuments varied with their policies and localities. Rob

Highpockets, STA311+45 is 31,145 feet from the beginning of Stationing. Stationing is a method of measuring from where an engineer decides a point called ) is going to be located, which can be at the beginning of a project or maybe even the beginning of the road. In this case I dunno. Station continues forth from Station 0 to Station 1, and that is 100 feet away. 311 stations is 31,100 feet. +45 is a location 45 feet beyond the last full station. I could use the same method to describe the beginning of a Horizontal or Vertical Curve, an Intersection, a Fire Hydrant, a Catch basin, a Light Standard, a Driveway Approach, Where to paint a gore point in an on ramp or where to begin a double yellow line or end one, a street sign of any kind, and Yup, Even a Bridge. This is a builders plate for the Bridge. It says some things that may only make sense to the Illinois department of Transportation. At the end of it is likely a bridge loading specification that the bridge is supposed to be able to meet or exceed. This plaque has nothing to do with the survey marker. From the datasheet: IN THE TOP OF THE NORTHEAST END OF WINGWALL AT THE SOUTHEAST CORNER OF CONCRETE BRIDGE is where you will find the survey marker. This one is a reset to one that was there before on the same wingwall only 3 feet closer to the road than the one you are looking for. You may notice where it once was. A wingwall is a concrete retaining wall that runs perpendicular to the bridge and on either side of as well as under the bridge, often on both sides of, parallel to the waterway. It is often retaining earth which is being stabilized for the area the bridge approaches are in and may also be structural components of the bridge. They often have slopes leading to and from the wingwall with the wall it self being a vertical drop off as a typical design element. It is good idea to be really careful when you find yourself in these area, do to the inherent dangers of slipping falling etc. Be careful and good luck. Rob

Geo, It would be easy to reverse engineer this. Load the waypoints into your GPS. Run a go to at them until you are pointing at it and compare the coordinates on the datasheet. It will be close enough to sort it out, Believe me. Match up the PID with the displayed location. Done deal. I mean you are dealing with some rather large objects here eh? The one in the road, run a go to on the coordinates and go with what is or is not there. Report it as you observe it. Unless I am missing something, Things will either be as they should, and the one in the road is likely missing, Or? Either way it will help you sort out what is what. Go with the positions the Datasheet gives you. Good Luck, Rob

Bicknell, I had hoped the answer was in the links I posted, and in many ways it is. Let me help sort it out. NGS is the overseer of Geodesy and Survey accuracy standards in the country. Each state has it's own program in most cases to help sort it out on the state level. They generally follow NGS guidelines but have their own paid staff, as well as their own Standards and methodologies. Often they come under the "umbrella" of NGS standards. (Many counties and Cities carry this further to their own areas of influence. These days we are using CORS and HARN networks, They are real time stations and the record geodetic movements in 3 axis's 24/7 and report movements and their directions as "velocities". Though these CORS Stations are in the NGS database and NGS can look at the data real time, they are often not sponsored by NGS, in fact Many are owned by States, Counties, Cities, Other Government Agencies, Universities, Private Survey Companies, etc. You see, It is networked in a lot of different ways and used locally by locals to establish and maintain the accuracy of local control networks as well. It really is multi function in it's roles. In spite of and recent or not so recent adjustment to a Station Mark, not visited, we can apply velocities to the data to help us understand what may be happening in any given area. As for the datasheet. Well some have been adjusted since a recent earth moving event in their area, some not. Like we have discussed in other threads, look at the datasheet for the NAD83 (XXXX) XXXX will represent the year of the latest adjustment recorded on the datasheet. If you need newer positioning, then you can include the data with local realtime observations and apply the changes, then run a least squares for it in the network. One way you could anyway. If we use it and don't do that, the published data has the date, so they know when the value we are using was last updated and they know to use that. Some Stations will not be adjusted any more at the NGS Level because it has been determined that the quality is simply not high enough to bother any longer given the resources available. It is still important as a geodetic position and good to know it is still there. We just see that it is an older adjustment er, Realization if you will. We know what reference frames were used to determine the position. There are uses for lower order survey on different jobs and so it is still of value. Funding and manpower have us using the 80/20 rule in regards to survey control a lot. In other words we are investing the most energy into our highest quality. BUT! the datasheet for the station is still the published data and it is usable. This maintains the value of even the lowliest of station marks. The rule is to go with the data on the datasheet. It will tell you how old it is and if need be the changes that have occurred in a locality can be applied to the data to bring it in line with current situation if that is needed. The thing is, it isn't always necessary in fact, usually not. Some survey markers have a different role than others. Just like people. Third order Landmarks are no longer included in adjustments. A and B order stations are High priority. CORS and HARN adjust themselves 24/7. The realtime data can be applied trough the network if we need , if not, the date tells us when the survey was last adjusted and we either hold it there or we can apply the observed changes to it and go with that. Ultimately, it could be resurveyed too. It depends if it is chosen or needed to be. The link from what California does to handle the Geodetic movement is a pretty good example of what is done in that State, and that area of the country is where the lion's share of the action is. That is the agency watching the situation. As you read, they have chosen the stations they choose to use in order to monitor the earth movements they have. It is enough to give them a good picture yet at the same time remain manageable. I think if that website outlines how they are handling the movement of geodetic control, and the are in partnership with NGS, (both are true btw) then I think you can take what they are saying as to how they deal as the truth. That is the answer. If they submit the latest observations to NGS, then perhaps the stations in question will be included in the NSRS adjustment which will be commencing this June. Well, that is if they meet the criteria for the adjustment. If you like, the California site has people there who you can write to if you need further details. It is their baby. The same would go with any respective State or other localities' site. Hope that helps. Rob

Bicknell, Though NGS is a National Agency, they often work in unison with the many State agencies which, quite obviously find themselves at ground zero when these things happen, and since there are similar yet different approaches these links should give you the gist of it. http://www.profsurv.com/ps_scripts/article.idc?id=1149 A Nice Overview. http://csrc.ucsd.edu/projects/hpgn2000.html Specific to California http://ees2.geo.rpi.edu/rob/www/gps/g0026.html A Washington State Study I am fairly sure that from reading those three links you will se how they continue to watch and study, as well as how they react to the changes they observe this and correct for the movements over time. Regional adjustments can happen and do. Hope that helps. Rob

BDT, Just remember what I said early on in the thread, "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." This is what happened for you and you got real lucky. This is a Golden BB. Casey, in quotes; "asked one of the big-wig, PhD in Geodecy guys (most of the books and papers referenced in the thread were actually at his desk!) around here to take a look at this thread and see what he thought of it." So please realize that you basically got a favor equal to a Holy Grail sort of answer here. Someone had to go to an extreme to do this for you. And for the most part, everything we tried to tell you is the truth about it. I hope you can finally accept this answer. You even got past the bunny rabbit guarding the Cave. Realizations means that each time they set out to re observe WGS84 and NAD83, (separately and by different agencies) they took what they felt were newer and improved reference frames, (and other ways of looking at this problem) which were used as baselines to observe it with. Each time they got a new, different result and they named it WGS84. Yup, the same old name. Both in the case of WGS84 and so on with NAD Datum. (they have had reasons for not renaming too) Each realization used different reference frames so the results were different, Hopefully better, yet each with a different purpose in mind. That Purpose in mind is in the name of each Datum. World Geodetic System is trying to describe the World, and North American Datum is trying to best describe a Continent. WGS84(G873) (G873 being the GPS Date when that particular realization was made) is the latest realization of that Datum being used I believe, but since it's initial inception, the changes made to it from one realization to the next has the WGS Datum becoming closer to the ITRF than it had originally been, but it is still called WGS84. So to the Geodesist, it becomes necessary for him to know what to compare. Without going into it, the NGS has similar realizations and that is why you see the year they did that in parenthesis next to the NAD83 designation, such as NAD83 (1993) as a for instance. This too is important to a Surveyor, as they have to use the version, or realization which is there. If I tie a survey into NGS control and that control is NAD83 (1993) control then my work will Jibe with that. If I go to another part of the country where a lot of the latest adjustment in that area is (1999) then that is the control. Yes our instruments and GIS systems will know of these different iterations. We get the Data From where you might imagine. The realization in the end is the affected adjustment. It can include new observations but it does not have to. They each are continually trying to improve their Datum, but they do not agree upon doing it the same way and they are not trying to meet in the middle. Again, I just want to remind you that NGA does not provide adjusted Orthometric data to the Public, as they have not unclassified it. So on the point of that fact, Beyond what you have seen NGS give for a CORS Station, which is basically providing an orthometric height to WGS84 in a Back door way via the ITRF conversions, Most Surveyors would have no way of comparing datum at any arbitrary station. It simply isn't done. Also keep in mind that this Geodesist did one point for you and his work is true for that one point only. His result is not uniform, and will not fit for every other place on the Continent or Globe. Please don't read that answer and go away thinking you have a global answer, applicable to all locations on the continent. It is very specific to the coordinates used to solve for it. That is the only place in the world that the result is true, as based on the terms or realizations used. Rob

Geo, I would write Cheryl and ask her if she thinks the 1961 monumenting narrative information can be added to the Datasheet, or at the very least if she can share more file information with you. The Datasheet leaves too much to be desired on GG0688. You may which to inquire about any box score data for either of the stations you mentioned. Both are second order survey, and it isn't unusual for that information to be available. In this case it would seem it isn't. Without that, there is not any information what they intended to offset, or how and by how much. I am certain that they may refer to the state line but without knowing how far they offset, you cannot locate the border. Not much of anything. Getting that will help explain the things you are wondering. Hope that helps for now. Rob

Professor, It is interesting to see how tidal effects and plate movement seem to change things with each new Geoidal model they derive. More interesting, WGS84 makes this sort of thing difficult to track in terms of Orthometric Heights, as NGA does not offer this Data. Only the ellipsoidal and Geoidal references for the Datum. In the US, NGS is the keeper of Orthometric heights but they are not for use with the WGS84 Datum. So for us in the US, the WGS Datum is highly uncheckable. We have no local control for use with it. Rob

The term we use is driven to refusal, It is everything like Bill said and looks like the pic Mike posted. An electrical pavement breaker with special attachments as such is used and the stainless steel rod, of which can have many sections added to it, to whatever length is needed is driven, and driven until it stops going in the ground. Here, this link will give you the basic idea. http://www.holtwood.co.uk/Berntsen-Install.htm Enjoy, Rob

Geo, There is no need to probe. It is there. If the surface mark is disturbed we will immediately know when we survey. Besides, If your probe is anything like mine, it is good at finding Rocks too. No one is going to alter the underground mark. Really. Frost heaves and other forms of land movement, perhaps from soil saturation, liquefaction, earth tremors, what have you might disturb an underground mark but the surface mark would immediately denote this. Please remember that the surface mark that was placed above this is, on average 9-12 inches in diameter if round, and is sometimes square in the same size range. They are often and most usually 3 feet long, which when buried will make the bottom 3 feet deep. Just trust that the underground mark is there. It is :-) If the surface mark is gone, you may be hard pressed to locate the subsurface mark without surveying the point in. Rob

Fwiw, I have no problem with any of these methods, but I would like to advocate that you try not to omit any data fields from the data sheet. Download it all and take it all with you. Many people here have learned how to read the datasheet and have found it has a lot of great info on it that tells a lot more of the story about the station than one may think. If you are looking for clues, many of them are on there. As a thought about the Paper. The datasheet you print, if you get it from the NGS website is an official datasheet. Some of have had the "Official-ness" help us access and egress form this situation or that. So I am not sure how official it looks on the screen of your PDA but keep that in mind just in case someday. You never know. Rob

Not Trying to overstep my bounds here, but Z15 is a former Surveyor, and a lot of us here in the Forum know this because he used to mention it in his Signature. I am not trying to blow his cover as of late, but I want to clarify that the reason he knows about Underground marks is because he has had to work with them professionally. He is not saying that he dug these up as a Geocacher, and he is not likely to advocate that you should either. So if you have it in your desire to dig up an underground mark, please reconsider. Like Mike said, a survey party placed an underground mark in a deep hole, and then just above it they positioned what was to be the primary Mark for that location. From there, a different survey party came and performed a number of surveys at that location to obtain the needed data to perform the Geodetic positioning. The old practice, since abandoned for better ones had the underground mark acting as a back up to the surface mark in case it is lost. We professionally assume that if a previous party put in an underground mark and the surface mark is in position and not lost or disturbed, that the underground mark is there and in position. If the surface mark is fount to be lost or disturbed then the Underground mark serves as a backup. We dig for it and check it's quality. It is instructive for you to know that there is no way for you to dig a hole and check the status of an underground mark without disturbing the survey quality of the surface mark so please do not do this! There is a $250 fine for tampering which in today's terms really could mean that if you are found doing this you could be liable for the costs incurred to replace or repair what has been undone. I am saying it has happened. The best advice is to be happy that the station is there and that you know it is a Triangulation station with an underground mark. Thanks for your interest in these stations and I hope we have helped you understand what is going on with them. Oh, and sorry Mike if I have blown your cover! :-) Rob

Someone looking for this? http://www.geocaching.com/mark/details.aspx?PID=HU1178 And this? http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=HU1178 Rob

I really gotta tell ya, Once you learn how to read a Datasheet, It really is pretty simple and just gives you all the data you want in it's entirety. It just takes getting past the initial hang of it. NGS Datasheets, Fresh from the NGS website are the best resource to use for benchmark hunting. Bar none. For most of all definitions you will ever want to know, go here: http://www.ngs.noaa.gov/cgi-bin/dsformat.prl Get the sample that is a best fit for you or get them all and click all the links, as there is a lot there being defined. Once you learn the ropes, it is really hard to imagine how much more straight forward you could make it. And most of all from the mouth of a noob! Thanks Domiller! Enjoy, Rob

Bill and all, Not to put a fork in this But Consumer grade gear isn't going to give anyone the accuracy you are trying to see, but I feel you are correct in the general thinking that not all modes of handheld accuracy is currently created equal. In the last 5 years we have seen Accuracy improve with the removal of Selective Availability and I remember reading in the past that the hopes for consumer grade accuracy were going to be in the 1-3 meter range in the future. The Crux of what I have tried to maintain here is that there is a danger in simply saying NAD83 = WGS84. The problem is that on a blanket basis, that is not a fact and simply is not true. It is important to know when it is similar enough to matter and not matter. Many people have a misconception, not one that will get them killed or harmed, but since we are working in the realm of geodesy here in the Benchmark hunting side of geocaching, we are looking for Geodetic monuments, and so it can matter. It can matter because the future is alleged to be bringing everyone higher accuracy, and if it does, the more accuracy we use now will make it easier for users in the future. It is safe to say in 2005, that for any navigational purpose that a human can use as transportation including their feet, WGS 84 and NAD83 are similar enough to be used interchangeably. When you get on your hands and knees with a tape measure near an object that is meant to represent Geodesy, that last meter, or yard if you will is within the grey area that would be better defined by using the Datum which has been asked for. I contend that you and I may see accuracy like this on consumer grade gear in the not too distant future so I advocate being careful. Just do yourself a favor and use the correct Datum. It may not seem to make sense but the future could reveal a difference and that will make it easier for all concerned. The Big deal is that for the purposes of this discussion, We are working at a level with objects where they are part and parcel of what defines a Datum. If I were looking for a latte stand on my GPS via the software that helps me find local business, and I was in an area of unfamiliar territory, I am sure several different Datum could get me close to my beverage. But since we are looking for object which represent high locational accuracy, and with the intentions of improving on that accuracy as time and technology permits, it is important to think of the Datum in an appropriate way. It is a hard concept to get our minds around, I know, and I have the unfortunate advantage of being accustomed to accuracy, which most people are unable to reveal. It is hard to convince you all of this when I know you have not seen it, but I can tell you, and with repeatable results that I can check the value of survey I have set on a daily basis, and the accuracy seems higher than the published values. I am talking about being right on the nut, day after day, same exact spot, same exact numbers and in little time to derive it. I can hop on a D-9 Caterpillar Bulldozer with a Huge "U" blade , equipped with GPS and cut grade at 4 miles per hour to better than 1/10th of a foot accuracy in real time. I could surpass that accuracy to 2/100 of a foot on a Cat 140 H Road Grader, but that higher ability is inherent in the Machine and the material being graded, not the onboard GPS equipment. 2/100ths of a foot is about the width of a paperclip if you were wondering. They will then come behind me and pave that road. The same can be said for an excavator digging a ditch and laying any kind of a pipe, and Pipe is laid to such accuracy. I am saying that I observe the system as having what seems more inherent accuracy than they publish that it has, and I believe that most of us here see better accuracy out of our consumer gear than the published values claim for it. The seems to mean that the GPS system itself is operating with seemingly more accuracy that the Government publishes for it as well. I said seems. I am leaving all the Manufacturers an out, because I cannot speak for their claims. and no Manufacturer is going to claim higher accuracy than the Government provides... I will agree with Holo, in that the cheapest, or should I say cost effective way to convert datum in consumer grade GPS is likely a Molodensky transformation, and I cannot refute that either. (pro equipment often uses Helmert amongst other schemas) I have tried quizzing people in customer support at GPS manufacturers but unfortunately, I have not reached a person who could speak to a good many of my questions, and have not found them yet. To further make it painful the GPS is still too much of a Black Box for my liking, too many of the answers we seem to find are over simplified for the use we have for them. But, it is what it is. Datum Conversions are post processed by either the GPS or a software in a GIS system where any transformation can be made form raw data. Basically all a GPS can get from a satellite is X,Y,Z, and current network time coordinates and post processing has to apply what is known about that location as based on the selected datum, after the fact. In the end, I want to say that if we were not dealing with geodetic survey markers as a point of discussion, a comparison of the WGS84 and NAD83 Datum would not have been necessary and I likely would not have pressed that we carefully define the terms. NAD27 is a good ways different from either NAD83 or WGS84, or any other Datum. It is obvious that if called for, we must observe that datum as radically different from any other. Just the same, it is important to stick to the time honored rule. Not many people know Datum from David. We may think we know better but "always use the Datum being called for by whatever it is you are working with", Use the Datum on the Map, Use the Datum being specified, even if you think you know what you can get away with and you will be inherently more accurate in the end. That trumps a 10,000 word discussion in one sentence and it is correct. It is a safer way to go, it requires zero over thinking, and It really is not that hard to do. Rob

Bicknell, If you look at the CORS on the West Coast, primarily West of the Rockies, and most specifically California, you will see a lot more velocity. Geodesists have learned that North America Plate movement affects Survey Accuracy and they try to keep their database appraised of this. WGS84 does not adjust it's own accuracy for this movement in the minute ways that NGS does by the way. In fact it doesn't even try to do it on this scale. They are more Basic and Global in their thinking. The Mission is different. Interestingly many of these real time stations are operated by County Seats and Municipalities. They use them to keep their own civil measurements as accurate as they can, and tied into NGS Sponsored Datum. It is pretty cool the way it is all networked. It means that I can use a city monument, or a DOT Marker, in that if it has been tied in, I can be pretty happy with it's geodetic accuracy when away from actual NGS Survey. Generally speaking, Though Many Countries in the world are using WGS84 as a basic Datum, they have to apply their own local methodologies to accurize the datum for use in their Locale. NGS does this for their NAD and NAVD Datum as well, but the Methodologies though similar, are different in the most basic ways than what NIMA/NGA used to formulate the WGS Datum. The NGS Datum are not rip offs of the WGS, they are truly developed differently and different enough to be called by a different name. Until the scientist decide different, they are not alike at the Geodetic level. Has anyone ever noticed this? Maybe it is not anything more than a Rhetorical Coincidence. :-D Rob

Hello to all, I have read every word of this and I do not concur with the conclusions being drawn. To say there is no difference between WGS84 and NAD83 Derivation Schemes in a Handheld unit is simply postulation at this point. I would strongly caution those reading this thread to keep in mind that the words "implies" and "probably" are words used to describe best guesses in the face of a lack of information or proof and not facts. I have seen no hard data from Garmin or Magellan, nor anyone else who manufactures GPS units for sale to the general public stating that their GPS does not use _ACTUAL_ NAD83 Datum Parameters for conversion in their units. This would be hard to determine on a Saturday. I am saying that those are unsubstantiated claims. Implication and Probability should not be enough to persuade any position. In the real world it is what it is, so since we don't know what really is, I feel it is not safe to assume anything. I would invite anyone to come forth with hard facts with supporting source material from the Manufacturer's that support the idea that some Datum are not carefully converted when very similar. My reasoning behind this is pretty easy to check. In a Garmin eTrex Vista, there are 110 Datum loaded for conversion when used against, and in addition to the WGS84 reference ellipsoid. There are 11 separate iterations of NAD27 loaded in it, with specific names for specific areas in which it is meant to be accurized for. Yes, I did say that Garmin's eTrex Vista claims it can position you accurately (within specified accuracy) of 11 separate iterations of NAD27. Using this as an example, do you really think it is safe to assume that since NAD 83 is only one meter different than WGS84 that the GPS manufactures would go to all the trouble they have and then throw that particular baby out with the bath water? Do you think they actually would and not tell the public? Do you think that if it could be proved they did as such that they would be misrepresenting the facts? That the devices they sell cannot actually do as is claimed they will do? Boy, That is not how I would sleep well at night. I would hope they can prove that there is a basis for all 111 Datum loaded in that little device. If someone were to prove that GPS Manufacturers were actually omitting the conversion to NAD83 on the basis that it is so similar to WGS84 yet selling their equipment to the public touting that they are offering such a Datum conversion, is it plausible that I or any of the rest of us would be able to Sue them for misrepresenting that to me? Do you think the NGS could take action for proving that the datum is not being represented properly and no note of this has made the public aware? The Datum, all of them in some way are Proprietary and there has to be permissions to use them, even in the public domain. Something tells me the Legal departments of these companies would not go so far as to open themselves up to such claims. If the GPS says it can convert to the Obsrvtoria '66 Datum, then it should have a program loaded in there that knows how this is to be done within reasonable accuracy when based on the WGS84 reference ellipsoid it uses for positioning itself. If not, then it is not true in what it claims it can do and that would be a legal liability. I realize that 1 meter is not enough to be revealed in your handhelds. I realize that many of you want to use WGS84 because you can. Some I get the feeling may even feel that though the fields of Survey and Geodesy are old fields with time honored standards and practices, these practices are not so honorable to them and they would rather do what they want, perhaps even in spite of those traditions, and interpret anything as they want for the sake of making things easier on themselves. It is an inkling of an idea, but some I sense would rather just do things however they want. I am not sure that is a great path to travel, but who am I to change the minds of those who are looking for ways to get out of having to believe a scientific construct when their minds are already made up? I have tried on many occasions to explain this as based on the science of it. You know, Here is the way Science presets the situation? Not what I want it to be, but on the basis of what it is. I have watched while many in the geocaching community have tried to find a way to support killing NAD 83, it is as if many of you hate the fact that the Datum even exists. So a way has to be found to prove it is nothing more than a Crock. I have got to tell some of you guys, this scenario is looking a lot like a chapter out of the book Animal Farm by George Orwell. It is a Fact that as Datum on the Geodetic level, NAD83 is not WGS84. I will say this very carefully one more time. You are not doing yourselves any favors, nor anyone else the same, by claiming it is. You would be wrong. Worse, people will read what you are saying and draw some very incorrect conclusions. Seems to me it would be better to keep them separated, something I have advised all along... It is so easy to do that too. In the end I suppose it simply does not matter. The construct of what science believes or what is generally thought to be held as most true is simply just a construct. No one has to do anything in any other way that the way they wanna. Of course there would be no way or methodology to compare one another in this line of thinking either but hey? <shrugs> I think if any of you wanted to sit down and pick up the phone, Call NGS and ask to speak with a Horizontal survey specialist, a Geodesist or something, They would spend a while trying to explain what the differences are and why they observe them in different ways than WGS84. I can tell you that in 25 years they have not adopted WGS 84 and it's methodology over their own. I wonder why. Rob

Rose_Thorn, I came across the site that does Benchmark Hunting in the UK while back, and I was able to find it again. Here you go: http://www.trigpointinguk.com/ Enjoy, Rob

Yeah some of us are just old fashioned paper lovers who prefer the typed smiley's over the emoticons... I only use emoticons in completely arbitrary circumstances which can be related to absolutely nothing at all, except for when I don't. I like analog clocks and radios that have dials on them too. But I will be keeping my iPod, thanks.