This weekend's find

Yes, this is an NGS project that Pam Fromhertz, NGS Rocky Mountain Regional Geodetic Advisor and Dr. Derek van Westrum gave a presentation in February at the PLSC Conference. It is the third geoid slope validation survey conducted by the NGS. Here is the title and abstract of their talk.

The Geoid Slope Validation Survey 2017, Walsenburg ƒ? Durango, CO: The basis to evaluate the new vertical datum in mountainous terrain

Abstract:
NGS is conducting the third of three extensive field campaigns called Geoid Slope Validation Surveys. Two have already been completed and the last one is being conducted here in Colorado from Walsenburg to Durango in 2017. Marks have already been set every mile along US 160. GPS, gravity, 1st order leveling and Deflection of the Vertical data will be collected all along this route.

NGS is planning on new datums to be implemented around 2022. Both horizontal and vertical will differ by nearly a meter here in Colorado. The new vertical datum will be based on a gravity survey, GRAVƒ?D. The GSVS will validate the method of the new vertical datum. Here in the mountains it will be interesting to see how well things fit. In this talk we will discuss the new datums and the GSVS campaigns.

I don't have a link to the particulars of this survey, but I imagine it will be similar to the earlier surveys. Pam Fromhertz's contact info can be found https://www.ngs.noaa.gov/ADVISORS/&apos ;">here.

The first survey was done in Texas in 2011 and the second in Iowa in 2014. Here are links to the NGS home pages for the two previous surveys.

https://www.ngs.noaa.gov/GEOID/GSVS11/
https://geodesy.noaa.gov/GEOID/GSVS14/

Thanks for the info

Some additional info here:

geodesy.noaa.gov/web/science_edu/presentations_library/files/gsvs17-backgroundstatus-201601.ppt

I can't wait to see the results of this baby!

Loyal

Thanks for the link to the PowerPoint presentation, Loyal. This should be interesting indeed. The survey goes over two mountain passes, La Veta and Wolf Creek. The last two bullets of the summary slide are telling. I can't wait to see the results either! It will probably be 8 years before I can apply the new model in other mountainous areas of Colorado.

ƒ?› To date, two such surveys have been conducted successfully in reasonably flat terrain.
ƒ?› Colorado has now been selected for a final, challenging test.

Here's what they are using. I can even see several of the concrete rings on Google Earth.

Had no idea, but one of those is a stone's throw from my house. Was gonna stop by and snap a pic on the way home yesterday, but got caught in a monsoon on the moto. I should be able to go visit A1521 this afternoon.

From the 2011 Texas line run. Set in the base of either an old street lamp or a payphone. About 4-500' from the house where my daughter was born. Funny reading the report from that line- apparently our de rigeur summer weather made a big impression on those NGS fellas.

That's https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=DN7654

Surprisingly, there's also another A 1521 in Texas, AE6984 with a totally different description.

I was driving down US 160 today heading to Durango from Pagosa Springs. I saw the GPS units setup in the ROW. It was then that I realized that this thread was talking about this project. Very nice drive. Small world.

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Two things I found interesting upon reviewing the PowerPoint:

1. NGS appears to have abandoned the idea of motorized leveling.
2. They appear to be using poured-in-place concrete marks instead of driven rod marks.

Jim Frame, post: 431612, member: 10 wrote: Two things I found interesting upon reviewing the PowerPoint:

1. NGS appears to have abandoned the idea of motorized leveling.
2. They appear to be using poured-in-place concrete marks instead of driven rod marks.

Interesting they would not drive rods to refusal. I would like to know more about how they arrived at this decision.

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BushAxe, post: 431622, member: 11897 wrote: Interesting they would not drive rods to refusal. I would like to know more about how they arrived at this decision.

I wonder if the presence of a deep SS or AL rod somehow messes with the gravimeter.

I believe all of the marks were set with the assistance of a crew(s) from the Colorado DOT and allowed to settle over at least one winter. Some marks were set in 2015 and went through two seasons before being occupied for this campaign. The previous results from the Geoid Slope Validation projects in Texas (first and easiest) and Iowa have been quite impressive. Gene provided some excellent links above, and as said, Pam Fromhertz would be the best contact if anyone wants to know more.

BushAxe, post: 431622, member: 11897 wrote: I would like to know more about how they arrived at this decision.

Here's my guess:
They want the 2-ft concrete base for the equipment.
If they are pouring concrete anyway, it's easier and cheaper to pour a post than to drive a rod AND pour a ring.
Although the rod would be more stable in the long term, they don't expect people to rely on passive marks in the long term.

What surprised me, knowing very little about gravity distribution, was how close the spacing was.

Bill93, post: 431661, member: 87 wrote: Although the rod would be more stable in the long term, they don't expect people to rely on passive marks in the long term.

I hadn't thought about the transient nature of those points, so I think you're right. They don't need long-term stability for those.

With regard to the concrete part, a driven rod is almost always accompanied by concrete around the PVC sleeve and surface access cap, so mixing concrete is just part of the deal. When I first started installing those we would hand-mix the concrete, but after awhile I decided that it's easier to haul a portable mixer along. It's been a few years, but my recollection is that it takes 6 each 60 lb. sacks of dry mix to surround a 6"x 4' sleeve in a 12" hole.

Bill93, post: 431661, member: 87 wrote: Here's my guess:
They want the 2-ft concrete base for the equipment.
If they are pouring concrete anyway, it's easier and cheaper to pour a post than to drive a rod AND pour a ring.
Although the rod would be more stable in the long term, they don't expect people to rely on passive marks in the long term.

What surprised me, knowing very little about gravity distribution, was how close the spacing was.

I asked Pam Fromhertz about the monuments. Her reply is that it is a research project and not intended for the NSRS. The project has 2 monument types. The standard monument is a concrete mark, averaging 1'x4'. That was likely a cost-saving decision. On average, every 10th monument is a rod driven to refusal. All stations will have GPS, leveling and a gravimeter observation. They need to be stable for the gravimeter.

Gene Kooper, post: 431720, member: 9850 wrote: not intended for the NSRS

All 200-plus stations in the Iowa GSVS have NSRS data sheets with PIDs, such as
https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=DP4644

I haven't checked, but probably a similar ratio of rods to posts.

Bill,

I imagine that this survey will follow the procedures of the earlier two and have NGS data sheets and PIDs. At least I hope so.