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NAVD 88 height vs NAVD elevation (Land Surveying)

by foggyidea ⌂ @, Peoples Republic of Cape Cod, Wednesday, June 13, 2012, 12:05 (1084 days ago)

Am I the only one that remains confused about this issue?

I read and read, listen and listen but it's still not clear to me what the difference actually is, if there is one! I make sure to tie into published NAVD benchmarks that have elevations also published when I am required to be on the NAVD datum.

Mass Highway publishes their benchmarks with an 'elevation' so typically I will tie into their benchmarks.

i also like to tie into NGS benchmarks, mostly fro horizontal control but on this most recent job there was a Mass Highway benchmark and an NGS bench about 100' apart so I hit both of them.

The resulting difference was about 0.03', using the Mass Highway benchmark and translating the elevations I find that the NAVD 'height' is only 0.03' different (lower) than the NAVD elevation for the NGS bench...

Is this a function of where I am working and the relative low elevations?

just wondering...

Don

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GEOMAGICIAN
"Outstanding in the Field"

Donald T. Poole PLS MA#32662, CFedS#1499 (expired)
www.outermostlandsurvey.com
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NAVD 88 height vs NAVD elevation

by Kris Morgan ⌂ @, Rusk, Texas, Wednesday, June 13, 2012, 12:11 (1084 days ago) @ foggyidea

Yes. Heights and Orthos aren't the same. You must be at the coast for them to be so close.

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"You don't have to be a good surveyor if you find all the corners."

Kris

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NAVD 88 height vs NAVD elevation

by foggyidea ⌂ @, Peoples Republic of Cape Cod, Wednesday, June 13, 2012, 12:19 (1084 days ago) @ Kris Morgan

yeah, about elevation 15 NAVD

Z42 or PID LW1551 per noaa is NAVD 88 ortho height 15.22

I find that it's elevation is 15.25 using a NAVD MA Highway benchmark. (#8057)

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GEOMAGICIAN
"Outstanding in the Field"

Donald T. Poole PLS MA#32662, CFedS#1499 (expired)
www.outermostlandsurvey.com
www.cfeds.org

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NAVD 88 height vs NAVD elevation

by Kris Morgan ⌂ @, Rusk, Texas, Wednesday, June 13, 2012, 12:25 (1084 days ago) @ foggyidea

I'm not so sure that you're not seeing the difference between bench marks either. 0.03' could very well exist between the BM's and has been adjusted. Not saying that's the case, and the "N" value being 0.03' is a good indicator, but I'm just sayin......

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"You don't have to be a good surveyor if you find all the corners."

Kris

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NAVD 88 height vs NAVD elevation

by foggyidea ⌂ @, Peoples Republic of Cape Cod, Wednesday, June 13, 2012, 12:40 (1084 days ago) @ Kris Morgan

It makes me a little nervous when differing government agencies have such similar results, and to find the ortho height virtually the same as the elevation made me question what really is the difference...

--
GEOMAGICIAN
"Outstanding in the Field"

Donald T. Poole PLS MA#32662, CFedS#1499 (expired)
www.outermostlandsurvey.com
www.cfeds.org

NAVD 88 height vs NAVD elevation

by adamsurveyor @, Colorado, Wednesday, June 13, 2012, 14:13 (1084 days ago) @ foggyidea

Wow....that would be hard to keep straight.0.03? Around here the difference between the ellipsoid height and the Orthometric height (elevation), is over 50 feet.

The ellipsoid height is a mathematical difference between a geometric figure that calculatable for GPS purposes. The geoid elevation is more fluxuating for real gravitational purposes (for water flow). I don't like using the term "height" alone for ellipsoidal height. That makes it even more confusing for me since (to me) "height" and "elevation" mean the same thing.

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NAVD 88 height vs NAVD elevation

by spledeus ⌂ @, Chatham, MA, Wednesday, June 13, 2012, 15:19 (1084 days ago) @ adamsurveyor

our geoid heights are 28 meters or so from the orthos

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J. Thaddeus Eldredge
Eldredge Surveying & Engineering, LLC

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NAVD 88 height vs NAVD elevation

by RoadBurner, Colorado, Thursday, June 14, 2012, 14:21 (1083 days ago) @ adamsurveyor

I don't like using the term "height" alone for ellipsoidal height. That makes it even more confusing for me since (to me) "height" and "elevation" mean the same thing.


Same here. If I talk about ellipsoid heights I'm talking about heights above GRS80. NAVD88 elevations are "ortho heights" on the NGS data sheets.

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An armed society is a polite society. Stand your ground.

RB

NAVD 88 height vs NAVD elevation

by adamsurveyor @, Colorado, Thursday, June 14, 2012, 14:38 (1083 days ago) @ RoadBurner

I was making an assumption that he was talking about the "ellipsoid" height when he said "height". Reading below, I think I was making a wrong assumption.

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NAVD 88 height vs NAVD elevation

by spledeus ⌂ @, Chatham, MA, Wednesday, June 13, 2012, 15:18 (1084 days ago) @ foggyidea

OK, first off, you should not be using the dynamic height, you should be using the orthometric height which is 15.23 for Z42 or PID LW1551 per noaa is NAVD 88 ortho height 15.22. You want to use dynamic heights? Go visit someone around the great lakes and use IGLD85.

This drops your difference to 0.02'. Why the NGS publishes dynamic heights where we use only orthos is beyond me.

Next, how are you getting from point A to point B? Did you pull out the autolevel or are you still using that fancy GPS? Did you peg test the level? I'm sure the measurements were fine.

Next, that NGS point is First Order Class II. The MassDOT point is second order. I know a few overlapping points in the databases, but typically the DOT survey crew runs levels independent of the NGS survey crew. This goes double the other way around which is why NGS control does not go past Chatham while MassDOT control does.

I could go back to the books and look up the error requirements for the First Order Class II and the second order unlisted class, but a 0.02' difference is pretty good. OK, I went to the book:
1st order class II >0.10 km => +/- 4.00mm per root km (that would be square root)

second order is between 6.00 and 8.00 mm per root km.

third order is 12.00 mm per root km which is equivalent to 0.05' per root mile. (Just for reference to get us away from metric)

Independent loops, a certain amount of necessary error, one agency calling it height, one agency calling it elevation.

Take a look at CYNNIE and the CYNNIE Azimuth marker: each agency publishes them with the same elevation. Not sure how it happened, but that's what they give us.

Finally, onto my favorite portion of the solid benchmark discussion: subsidence & rebound. When were these marks set? The Acushnet CORS ACU6 shows minimal change (0.0000 to 0.0002 m/year), but ACU5 is showing a drop of 0.0022 to 0.0031 m/year. 0.0031 * 3937/1200 = 0.01 foot per year. All they needed was a two year difference with that rate of geologic change to give you a 0.02' difference. Granted, more time would be necessary for a slower rate, but these marks probably were not set at the same time.

When you are bored, check out the velocities of the CORS stations. It's pretty awesome to think they thought the world was more static than it actually is...

You can also check out a 2007 paper by Meyer and others:
The definitions and relationships between elevations, orthometric heights, dynamic heights, geopotential numbers, and ellipsoid heights are not well understood by many practitioners. This is perhaps not too surprising, given the bewildering amount of jargon associated with heights. The NGS glossary contains 17 definitions with specializations for “elevation,” and 23 definitions with specializations for “height,” although nine of these refer to other (mostly elevation) definitions. It is the purpose of this series, then, to review these concepts with the hope that the reader will have a better and deeper understanding of what the word “height” really means.
What Does Height Really Mean?
Thomas H. Meyer
Daniel R. Roman
David B. Zilkoski

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J. Thaddeus Eldredge
Eldredge Surveying & Engineering, LLC

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NAVD 88 height vs NAVD elevation

by GeeOddMike ⌂, Florida, Wednesday, June 13, 2012, 15:39 (1084 days ago) @ spledeus

Howdy,

I don't think this clarifies matters especially well but the NGS Glossary has these definitions:

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

height - (1) The distance, in the direction of the zenith, between the top and bottom of an object, e.g., the height of a building or the height of a person.
By analogy, one speaks of the height of a mountain when one thinks of the mountain as an object with a top and a bottom. However, for historical reasons connected with the use of barometers for measuring heights, one speaks of a point on the object as being at a certain altitude; e.g., "The peak is at an altitude of 3000 meters above mean sea level."
(2) The distance, measured along a perpendicular, between a point and a reference surface, e.g., the height of an airplane above the ground surface.
In this example, the reference surface is the surface of the Earth below the aircraft or a plane fitted to that surface. For the term geodetic height, the reference surface is an ellipsoid.
(3) The distance, measured upwards along a plumb line (line of force), between a point and a reference surface of constant geopotential.
Elevation is preferred if the reference surface is the geoid unless convention or definition dictates otherwise, i.e., measured elevation, orthometric elevation, etc., but normal height, etc. The term height is also applied to elevation of the tide above or below a specified level. The term orthometric height is also in common use. Use of the term geoidal height also is proper since it is the geodetic height of a point on the geoid.

elevation - (1)The distance of a point above a specified surface of constant potential; the distance is measured along the direction of gravity between the point and the surface.
The surface usually specified is the geoid or an approximation thereto. Mean sea level was long considered a satisfactory approximation to the geoid and therefore suitable for use as a reference surface. It is now known that mean sea level can differ from the geoid by up to a meter but the exact difference is difficult to determine. The terms height and level are frequently used as synonyms for elevation. In geodesy, height also refers to the distance above an ellipsoid; it is used in this sense in this glossary, except where custom has established a different usage. "Level" has such a variety of meanings that it is best not to use the term to mean elevation. See also altitude.
(2)An approximation to elevation in sense (1) above.
Most so-called "elevations" are of this kind; the true elevation is often impossible or extremely difficult to determine.
(3) The quantity obtained by adding differences in elevation (definition (1)).
This is the quantity obtained by spirit leveling without correcting the data for deflection of the vertical.
(4) See elevation, angular .

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NAVD 88 height vs NAVD elevation

by Farsites, VA, seldom Toronto, Wednesday, June 13, 2012, 16:30 (1084 days ago) @ GeeOddMike

I think that has heightened and elevated our awareness
;-)

Good reading on this thread though.

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NAVD 88 height vs NAVD elevation

by Carl B, Baldwin, Long Island, New York, Wednesday, June 13, 2012, 16:32 (1084 days ago) @ foggyidea

Mr. Fog
2 cents - In this neck of the woods (Long Island, N.Y.) I use VERTCON to convert elevations NGVD 1929 to NAVD 1988. Average variation is minus 1.1 feet.
Carlton

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NGVD

by spledeus ⌂ @, Chatham, MA, Wednesday, June 13, 2012, 18:04 (1084 days ago) @ Carl B

Carl, have you ever checked the elevation difference between geodetic benchmarks with sheets on both datums? Not to scare, but the documentation of Vertcon outlines the quality of the model: it is not suitable for third order or better elevations (or heights). The distortions are impressive in some places.
[image]

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J. Thaddeus Eldredge
Eldredge Surveying & Engineering, LLC

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NGVD

by Carl B, Baldwin, Long Island, New York, Thursday, June 14, 2012, 11:39 (1083 days ago) @ spledeus

Thanks for your information and graphic.
All our bench mark monuments are based on NGVD 1929 datum. I use VERTCON to
determine convertion to NAVD 1988 datum for Elevation Certificates.
Carlton

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NGVD

by spledeus ⌂ @, Chatham, MA, Thursday, June 14, 2012, 17:09 (1083 days ago) @ Carl B

When Should it Be Used?
Users with a need to transform height data between NGVD 29 and NAVD 88 can use
the VERTCON tool. Because the VERTCON model can be considered accurate at the 2
cm (one sigma) level, it is suitable for a variety of mapping and charting purposes.
As a model, it can not maintain the full vertical control accuracy of geodetic leveling. Users needing high accuracy should adjust their observations using published NAVD 88 values.
NGS Toolkit, Part 9: The National Geodetic Survey VERTCON Tool
Donald M. Mulcare

It is not the intent of the notice to declare when to use a datum transformation or by what method but only to declare that when a mathematical transformation is appropriate, VERTCON is recommended. … Note that VERTCON is not appropriate to transform between NGVD 29 and NAVD 88 for first-, second-, or third-order heights. [Federal Register / Vol. 72, No. 132 / Wednesday, July 11, 2007]

Problem Lines in VERTCON: In rare cases, local distortions of 20 cm (0.66') or more were found in the NGVD 29 network I forget the exact reference of this one.

Just double check for those critical ones. We get the added pleasure where the FEMA contractors used VERTCON to transform from NGVD to NAVD for the new maps. In reality, coming off a NAVD benchmark, I should be staking out a flood plain contour that is 0.26' from the published flood plain contour (In Chatham, the flaws in VERTCON will change per given location). And good luck to me explaining that one to Conservation.

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J. Thaddeus Eldredge
Eldredge Surveying & Engineering, LLC

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NAVD 88 height vs NAVD elevation

by foggyidea ⌂ @, Peoples Republic of Cape Cod, Thursday, June 14, 2012, 15:09 (1083 days ago) @ foggyidea

I see that "Roadburner" answered my question. I was thinking thatthe two benches were on the same datum and that NGS ortho height equaled NAVD elevation.

I wasn't concerned about the 0.03', as much as surprised that the two benches checked so well, coming from totally seperate entities.

Dtp

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GEOMAGICIAN
"Outstanding in the Field"

Donald T. Poole PLS MA#32662, CFedS#1499 (expired)
www.outermostlandsurvey.com
www.cfeds.org

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NAVD 88 height vs NAVD elevation

by spledeus ⌂ @, Chatham, MA, Thursday, June 14, 2012, 17:00 (1083 days ago) @ foggyidea

0.02'
Don, we do not use dynamic heights.

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J. Thaddeus Eldredge
Eldredge Surveying & Engineering, LLC

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NAVD 88 height vs NAVD elevation

by foggyidea ⌂ @, Peoples Republic of Cape Cod, Friday, June 15, 2012, 04:34 (1083 days ago) @ spledeus

oh yeah...but I hadn't pegged my level, or anything! ok, it's just a GPS tie since my error budget is a tenth...

--
GEOMAGICIAN
"Outstanding in the Field"

Donald T. Poole PLS MA#32662, CFedS#1499 (expired)
www.outermostlandsurvey.com
www.cfeds.org

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