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Like to see your opinion on this new explanation of Elevation Concepts

GISJoel
(@gisjoel)
100+ posts Member

Hi Surveyors,

Your thoughts? https://community.trimble.com/community/find-answers/mapping-and-gis-solutions-community/blog

This is meant for the GIS crowd - from Trimble's team that manages the TerraFlex/Positions/Pathfinder Office/TerraSync/ and now the ESRI Collector world (when using a Trimble device to collect positions for a ESRI collector experience.  If you have a GIS guy/gal in your crew, this is the way Trimble speaks to them.

Thanks.

 

joel

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Topic starter Posted : April 5, 2020 7:08 pm
MathTeacher
(@mathteacher)
1,000+ posts Member

Some folks might object to the term "mean sea level." Think about this. If the mean sea level in Tampa rises by 0.2 feet, does the geoid model for the surrounding area need to be changed?

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Posted : April 6, 2020 4:25 am
Bill93
(@bill93)
5,000+ posts Member

Some suggestions and comments.  I soon got tired of the exercise and quit.

"between an observed GNSS elevation and the elevation of a reference coordinate compared to the published coordinates recorded for historic reference monuments"

Ellipsoidal height Elevation at the surface of the ellipsoid is zero [Similarly in the following sentences.  Reserve the word elevation for orthometric]

 (the most common of which is referred to as the WGS84 ellipsoid) [GRS80 used by NAD83 is about as common.  Same shape, small shift.]

Your GNSS receiver usually outputs a global MSL [The discussion of MSL should emphasize from the start that it is a loose term and that actual average sea level is both hard to measure accurately and different at different places.  Better to introduce the geoid.  Receiver may have a crude model of the geoid.]

A geoid is a highly accurate model of the local gravitational forces potential in a specific region of the world. [No. It is a model of the geopotential. The acceleration of gravity, and therefore the force acting on a mass, is not constant over the geoid or other isopotential survace.]

different to from

effect on the observed sea level at any given location relative to the ellipsoid.

Geoids measure represent [measure is an action taken by a person]

effects of variances variations [variance is a word used in statistics with a specific different meaning]

Each location Various countries or regions in the world has its have defined their own local geoid(s) [ZStatementimplied a different geoid model in each state or city.]

and this is referenced against the geoid model

ReplyQuote
Posted : April 6, 2020 6:36 am

MightyMoe
(@mightymoe)
5,000+ posts Supporter

I'm not sure why any of this should be considered new.

Geoid Models flesh out the crude ellipsoid models and allow a GPS to kinda get on elevations. Geoid 18 seems to be better than any before, at least from the little I know about it. In other words it's closer to NAVD88 elevations using ellipsoid heights than earlier models. Thank surveyors like Bill 93 for doing the work to get it there along with the NGS and their gravity surveys. 

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Posted : April 6, 2020 8:03 am
Paul in PA
(@paul-in-pa)
5,000+ posts Member

There is no such thing as Mean Sea Level, especially in reference to Denver. One might calculate mean seal level at some specific tidal point. From Denver there are an infinite number of directions to the seas, therefore an infinite number of elevations above those infinite number of MSLs. Consider the simple case of the Panama Canal where the sea level difference from Atlantic to Pacific varies from 0.3' to 1.1' based on the season of the year.

All in all it is complicated and when the earth stops spinning all bets are off.

Paul in PA

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Posted : April 6, 2020 12:11 pm
GeeOddMike
(@geeoddmike)
1,000+ posts Member

It is always a challenge to find the appropriate level at which to address these issues. That said, I believe you would benefit by reviewing the NGS Geodetic Glossary of the terms appearing in your document. It is located here:

https://www.ngs.noaa.gov/CORS-Proxy/Glossary/xml/NGS_Glossary

I randomly comment on some of your content.

1. I agree with Paul in PA that the term mean sea level should only be used locally (as in Tidesandcurrents.noaa.gov data). Recollect that the vertical datum once known as MSL 1929 was renamed National Geodetic Vertical Datum of 1929 for that reason.

2. I disagree with your statement that the current US vertical datum “... incorporates the latest geoid model.” It does NOT.  NAVD88 predates significant use of GNSS for heighting and has not undergone readjustment. When using NAD83 (####) coordinates (Lat, Lon, Eht) to derive an NAVD88 orthometric height once uses the hybrid geoid model GEOID(##). 

3.  As I assume this document is intended to address concerns of novice or others with no background in geodesy, some mention should be made about the differences between gravimetric and hybrid geoid models. The hybrid geoid models, whose names start with GEOID, are intended to fit the relationship between the height systems (h, H and N) to best fit the NAVD88 datum. The gravimetric model is free of this intention.

4. Depending on where you are in the world the ellipsoid is either below or above the geoid. I personally prefer to present the relationship of the height systems as h - H - N = 0. The ellipsoid (h) is the distance from the ellipsoid surface to the point of interest; orthometric (H) is the distance from the geoid to the point and the ellipsoid-geoid separation (N) is the distance from the ellipsoid to the geoid.

5. Some ideas I would like to see incorporated are the “non-parallelism of level surfaces” which means that even over relatively short distances a constant “offset”  does not reflect the relationship of the surfaces. The ellipsoid is based on  geometric quantities while the geoid surface is defined by its geopotential (physics) but the value sought by surveyors and mappers is at the terrain level.

 

I attach some entries from the Geodetic Glossary and a graphic showing the h, H, and N for one of the most remote places I’ve been. 

4FA5235F DE25 484F B45D 1C07F0A5B989
912E669B 9363 416B B25D C13B917128BE
52C9CC31 298C 423D BCC7 E59A3CA7B4D0
1F50BADB 201D 40A6 B862 B61B01F45B18
8F7B8F02 49FF 45FC A967 2458D1EFBA39
BB6CDBC2 9D4D 48AE 809B AAEA39C631E9
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Posted : April 6, 2020 2:40 pm
MathTeacher and Loyal liked

GeeOddMike
(@geeoddmike)
1,000+ posts Member

While doing some computer link maintenance I came across this excellent resource on heighting at: https://www.nrcan.gc.ca/height-reference-system-modernization/9054#geopotential_number

Like most subjects there are various ways to approach the issues like heights. Most (?) surveyors could care less about the geopotential and other complexities involved in height determinations. For those with such a practical bent, it is sufficient  to do the “bookkeeping” work of knowing that using an NAD83(2011) ellipsoid height to approximate an NAVD88 orthometric height one uses the GEOID18 model of the ellipsoid-geoid separation and NOT the gravimetric model USGG18.

 

 

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Posted : April 10, 2020 2:27 pm
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