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NGS Benchmarks versus OPUS derived Elevations

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Bill93
(@bill93)
5,000+ posts Member
Posted by: @reddog

The objective here should not be trying to determine what is the most correct, true, accurate or precise NAVD88 elevation. The objective should be trying to MATCH THE DATA THAT WAS USED TO BUILD THE MODEL that established the flood elevation reported on the FIRM map.

This.

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Posted : August 5, 2022 11:08 am
Peter Lothian
(@peter-lothian)
500+ posts Member
Posted by: @bill93
Posted by: @reddog

The objective here should not be trying to determine what is the most correct, true, accurate or precise NAVD88 elevation. The objective should be trying to MATCH THE DATA THAT WAS USED TO BUILD THE MODEL that established the flood elevation reported on the FIRM map.

This.

Double this ^^^^.

While FEMA has updated the mapping of the flood plain lines, the model used to define those lines could still be based on surveys run before GPS elevations were available.

I worked on a flood plain study for FEMA in 1990 after a major flood event in Maine. We only measured cross-sections at the bridges and culverts that had been destroyed and replaced in the prior flood. All the cross-sections at the "undamaged" constriction points were re-used for the new flood model.

 

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Posted : August 5, 2022 1:14 pm
Murphy
(@murphy)
500+ posts Member

NC Flood Inundation Mapping (FIMAN)

Quality mapping saves lives, saves money, draws businesses, and in general make it easier to make good decisions.  This entire thread is another indictment of our inability as a country to make wise and long term decisions.  The tools needed to generate the real time map linked above also provide free statewide orthometric imagery, free LiDAR that can be immensely useful for planning, and over time provide snapshots of the past. 

FIMAN has predictive capabilities, the accuracy of which is astounding and have been verified numerous times by expert geodesists as well as anecdotally by PLSs and visual observance of high water marks. 

It took back to back hurricanes to prompt North Carolina into action, but we quickly realized that its value far exceeded the cost.  What will it take in your state?

 

 

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Posted : August 6, 2022 7:16 am
Gary liked

MI-Other-Left
(@michigan-left)
10+ posts Member

@murphy

That's some pretty fancy stuff.

It would be nice if all federal, state, and local gov't were at that level of accessibility and usefulness, etc.

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Posted : August 6, 2022 11:47 am
Sergeant Schultz
(@sergeant-schultz)
500+ posts Member

I try to have my elevations match the RM(s) on the FIRM.  Put it back on FEMA.

 

 

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Posted : August 6, 2022 3:42 pm
FairbanksLS
(@fairbanksls)
500+ posts Member

I think it’s only fitting that we have vertical pincushions if we have horizontal pincushions.  I would base my elevations on the recovered NGS Benchmarks.

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Posted : August 6, 2022 5:05 pm

GeeOddMike
(@geeoddmike)
1,000+ posts Member

Shown below are the input file for Phase 1, Output for two points in previous post, and header information for output file showing corrections applied. In this run all possible corrections were applied. Corrections are listed in the header section. 

BTW, there is another NGS tool for transforming published NAVD88 orthometric height differences into the leveled height difference by removing the orthometric correction.

Extracted from the tool's documentation:

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Posted : August 6, 2022 11:24 pm
GeeOddMike
(@geeoddmike)
1,000+ posts Member

I received a PM asking me to provide information about a number of monuments. While willing to point to resources and provide examples of the tools available, I do not provide additional free services. Nor am I interested in taking on fee-based services.

To expand and elaborate on information I provide the following.

To find a PID given only a designation, one uses the following link. You enter the designation, select the state in which its located and submit. Note that the date is not generally part of the designation.

Once you know a PID, use this tool to find projects associated with it:

L 435 is PID SS1269 the only level project in which it appears is L24479/6 (the /6 means that it is part 6 of L24479).

K 455 is PID RX0124 there are two level projects associated with this BM. A line observed in 1936 is L7684/5; the BM also appears in a 1961/2 level line L18718/3.

Using the Leveling Project Page

you can display and retrieve a multitude of information after entering the level project designation.

See previous posts for screen captures of the Phase 1 output for L18721. See previous posts for a link to the Phase1 tool. The complete output file is shown below:

Note that this is output where all corrections were applied. 

 

 

 

This post was modified 2 hours ago by GeeOddMike
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Posted : August 7, 2022 6:59 pm
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