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GPS on Benchmarks deadline extended

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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.


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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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