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Posts posted by Bill93

  1. NGS is working on an update to the vertical datum.  Just as NGVD29 was replaced by NAVD88 and NAD27 replaced by NAD83 when much better data became available, now there will be new horizontal and vertical datums labeled NAPGD2022, but unfortunately delayed somewhat beyond that year by funding and COVID.  When the new vertical datum is released, elevations will change by some amount from the NAVD88 datum values. It has been found that NAVD has about a meter of tilt across the country.  Three feet in 3000 miles isn't too bad, but they can now do better. Most importantly, as we know bench marks tend to go away and you are lucky if 1/4 of them are left in your area.  The 2022 vertical values will be defined as what is measured by GPS (GNSS?) plus a geoid value, so that projects are no longer dependent on having a physical mark nearby.


    We have seen a series of geoid updates to better model the gravity, but those "hybrid" geoids still hold the old NAVD88 elevations on the marks in the data base, and only provide more accurate interpolation between those points.  The 2022 datum will change all elevations to be more self-consistent across the country, using the best new geoid model they can make without depending just on old leveled benchmark values.  For this they have been working on the GRAV-D project to get aerial measurement of gravity across the country and some gravity measurements on the ground.


    NGS needs a tool to allow people to convert NAVD88 elevations they have to good estimates of 2022 elevations. To check and refine the conversion model, they are using comparisons of GPS measurements versus NAVD88 values on good bench marks.  Most of this data is being submitted by state DOT or DNR agencies, with some from various other sources including individual surveyors or researchers with the required equipment, submitted as OPUS Share data (see map of submissions). Look up the GPSonBM project for details.


    I hope that explanation helps, and is sufficiently clear.  DaveD can correct me if I misstated anything.

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  2. You're right about the unrolling.  Sorry 'bout that.


    I'm not a surveyor.  I'm an electrical engineer, now retired, and thus comfortable with most of the math needed in surveying and geodesy as well as error statistics.  My interest in the intersection of surveying and history goes back to high school days.  For decades, I've kept a copy of one or more surveying textbooks around for curiosity about the application of math and the intricacies of the instruments.  If you're looking for a modern surveying textbook, I recommend a slightly out of date (cheaper) edition of Wolf & Ghilani's Elementary Surveying. If you are interested in the older methods, try to find a book from the 1960's or so.  The best of those I've seen is Davis, Foote, & Kelly.


    When a friend told me about geocaching and then I discovered benchmarks on the  site I was hooked, and have learned much more about geodesy since. I joined the Surveyors Historical Society and have gone to a couple of their "Rendezvous" conferences with presentations on how surveying affected history.  I met benchmark hunter moser at the event in Philadelphia, but it's been a while since he has been a frequent poster here. 


    I follow www.surveyorconnect.com and seem to be accepted as a member of that community, where I try to limit my comments to what I really know (plus the members chit chat category). Most people don't realize that while geodesy is about measurement and modeling, land surveying is more about law and evidence than measurement, so there is a lot to learn on that site besides math. And there's a math teacher on that site who knows far more than I do about math and mapping.

  3. Nice looking monument.


    Sounds like you are making progress on figuring this stuff out. It appears you have found the NGS Toolkit, and may need to explore more of the tools. There is lots of tutorial info on the NGS site and elsewhere on line. 


    SPC and UTM are projections onto a plane slicing through the earth. As such the distortion changes as you move on the map away from center. That 1 part in 16000 is the chosen distortion near center, and it reduces toward zero distortion (1:1) as you move toward the line where the projection plane slices the ellipsoid model of the earth, and then increases with the opposite sign as you move farther out to the edge of the intended coverage area. 16000 balanced the worst positive and negative distortion, as measured by the scale factor.


    So far we haven't considered elevation. As you move higher above the projection plane, the projected "image" of the coordinate grid stretches in proportion to the distance from the center of the earth. The Elevation Factor is the measure of this. A map distance corresponds to a different ground distance at different elevations.


    The product of scale and elevation factors is the Combined Factor, also known as the grid to ground factor (or is it the inverse of it?).

  4. If anyone reading this has professional grade GPS (L1/L2 carrier capability), even antiques like my Trimble 4000sse, you will have longer to make submissions that will help NGS make an accurate transformation tool for converting old elevations to the 2022 datum when it comes out.  The deadline was going to be the end of 2021, but the whole program has slipped schedule.


  5. The web site says:

    "Receiver Type: GNSS single frequency with carrier phase tracking"


    OPUS requires L1/L2 dual frequency reception. 


    Some other post-processing software will take L1 only with phase tracking data. I'm not sure if this unit can provide the individual satellite raw data needed for this, as I'm not familiar with the interface formats the product sheet mentions.

  6. Yeah, it's complicated, and I have only limited knowledge.


    NAD83 doesn't really have an x axis, or if it does it isn't important, because NAD83 is tied to the North American tectonic plate and not to the rest of the earth.


    WGS is fitted to the whole earth, and is a snapshot (updated every so many years) of the fit determined by international scientific groups. Its x axis is 0 degree longitude, and that is about 100 meters from the old Greenwich astronomical observatory meridian. There are discussions out on the web about why it doesn't match, which I vaguely recall had to do with deflection of the vertical due to gravity variation versus location, and continuity of time scales.


    The 2022 datum will be more like  IGS and WGS, and there are documents and seminar recordings on the NGS site explaining it.


    I hope something in this ramble helps sort out the stuff you read, and that Dave D comes along to correct my mistakes.

  7. I've seen hundreds of bench marks and only one chiseled square. Apparently there wasn't much leveling done in my home area other than on disks and occasional items in concrete. That one was PX0154 in downtown Cody, WY on a building foundation or wall. And it looks like I failed to report it after our trip.

  8. What did you think was odd about its location? The USGS pdf  says it is on a bridge, a very common location.


    The map in the pdf shows the routes used by the leveling crew that made the measurements. They often used roads and railroad routes, so the lines could represent either, or even cross-country routes sometimes. The lists on the following pages list the points measured, so you can trace out the route from that.

  9. Note on page 9 the mark 15 JWM 1950 has the notation "Tied by NGS 1950" which most of the ones in this pdf do not have. It is typical that few were so tied.


    That notation is a bit strange because the agency was still C & GS in 1950, so someone added the note much later..

  10. It's a US Geological Survey mark. Only some small fraction of those were measured to the standards of the US Coast and Geodetic Survey and submitted to that agency, now called National Geodetic Survey.

    The list on Geocaching is a 20-year old snapshot of the NGS data base. So it isn't surprising when a USGS mark is not in the list here.

  11. I just now checked with my Android phone and find the benchmark count on the profile/geocache page just like usual.

    I never paid much attention to the statistics page so wouldn't know if it changed.

  12. I did some extensive averaging experiments with my Garmin a decade ago, and found that there were computational errors (roundoff of intermediate values?) that limited the resolution to a foot or two. Averaging might still reduce the error below that.  I was getting differences of several feet from day to day on 2-hour averages, and did a lot of days, but never felt I was getting to the foot level.

    Professional grade receivers get their accuracy by using carrier phase rather than the code timing that most handhelds use, receiving multiple signals from the satellites (GPS L1 and L2, for instance), sometimes receiving multiple constellations (GPS, GLONASS, GALILEO, BEIDOU), correcting for iono/tropo delays by comparison to known stations which are relatively nearby that received signals from the same satellites at the same time, and using time averages. The known stations are either

    a) fixed CORS stations or proprietary network stations run by Trimble, Leica, etc.  This processing is done by NGS program OPUS or vendor proprietary software. The fixed stations are typically a few 10's of km away and multiple station comparisons are made.

    Or b) processed using proprietary software against a local base receiver whose coordinates are taken as known. The relative position vector is quite good if the distance between is less than a few km.


    OPUS and some of the proprietary processing methods are known as Static, because you leave the receiver in place for a while.  Some of the processing methods are known as RTK (real-time kinetic) because a radio link brings the fixed-station data to the receiver for immediate processing.


    There is no way you will ever get good enough results with amateur equipment to get a mark into the NGS data base.  Their procedure is called "Bluebooking" after the original manual for the process.  It requires extensive measurements (multiple 4-hour sessions?) with professional grade L1/L2 equipment and a lot of arcane file submissions. They expect to be repeatable within a cm or two.


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  13. The uptick in COVID cases is giving us doubts about travel, hotels, and indoor meetings with people from all over.

    We're vaccinated but worry that may not the stop the next variant that a huge number of cases makes more likely to emerge. Darn.


    Edit: I see they've moved the registration deadline to Sept 5, so we h ave another week and a half to see how the trend goes.

  14. In my hometown a stone post was placed in the cemetery, a place without wire fences or water pipes to distort the magnetic field. Later, steel coffins became much more common, so that might not be such a pristine magnetic environment.

    True bearings were given to the courthouse peak and a church spire, both of which are now gone so we can't use the bearings.

    The stone post is cracked but still there level with the ground as of a couple years ago. I did an OPUS share on it just to perpetuate the history.


  15. That means the mark had an NGVD29 elevation, but it wasn't considered good enough data to be put into the NAVD88 adjustment. Look at the SUPERSEDED data down lower on the data sheet for the NGVD29 value.  VERTCON applies an interpolated difference between the two datums to arrive at a usable, but not super precise elevation in the newer NAVD88 datum.


    There are some stations that have classical observation data for both horizontal and vertical, but it isn't common because of the way sites were selected.  Triangulation stations were usually on high points whereas elevation marks were along railroads or roads where possible to take advantage of easy terrain.  When GPS is used, those tri stations may end up with 3-dimensional coordinates.

  16. NGS won't give anybody permission to keep a disk, but nobody will come after someone for keeping a disk that is destroyed by the building being torn down or other non-vandalism reasons. It happens all the time.


    It would be great if a local surveyor could do a RESET to preserve an elevation reference.  NGS has no funding to do such work.


  17. I don't know how many garage sale trowels I've ruined while digging down to a bench mark or other monument because they won't stand much prying force.  I recently saw one of these, a gardener's Nisaku NJP650 weeding and digging knife. 

    Somewhat pricey at $20-25, but very sturdy, sharp edges for cutting roots or other vegetation, and inch and cm scales as a bonus. Much handier than a machete for those occasions when you don't need the length. It comes with a sheath but probably isn't legal to carry in public.  A downside is that it is sharp enough to cut utility lines that the old trowels wouldn't.



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