Which height from the NGS data sheet should I be using?

> I've got three heights or elevations on the NGS data sheet and I'm not sure which one to use.
>
> NAD 83(2011) ELLIP HT
>
> NAVD 88 ORTHO HEIGHT
>
> GEOID HEIGHT
>
> When I enter the ellipsoid height into my data collector (SurveyPro 4.x) under geodetic and rectify it to SPC, the new ortho height doesn't match what the NGS data sheet says it should be. It's off by tenths or sometimes feet. The current coordinate system on my data collector is US State Plane 1983, Florida North 0903, and GEOID03 (Conus) using no scale factor. Here's a NGS data sheet of a county monument for the area I'm in. Am I doing it all wrong or is this normal? I'm somewhat new to surveying, so pardon my ignorance.

Your data collector is taking the ellipsoid height you give it and applying the geoid height to compute the ortho height (what most people call elevation). I and I think most people do it the other way around by keying in the correct ortho height (elevation) and then let the data collector apply the geoid height to get ellipsoid height.

The ellipsoid height is the height system that GPS uses that is referenced to a mathematically perfect ellipsoid. Orthometric heights are referenced to the geoid surface (sea level). The Geoid model is supposed to be the difference between the two, but it is not an exact figure and varies between the model you use. The most current geoid model is GEOID12A.

update your geoid model...

In the case of the monument that you linked to, you would need to use the published NAD83(2011) Ellipsoid Height and use Geoid12A to compute the orthometric height. Geoid03 is not the appropriate geoid model for the 2011 datum.

I agree with a previous post that says that it would be more common to start with the orthometric height (when available) and use the model to derive your ellipsoid heights.

> .... I'm somewhat new to surveying, so pardon my ignorance.
You deserve credit for asking the question.

The advise others have given is good. But once you have settled your data collector issues you will still have other fish to fry....

AE5564 ** This station is in an area of known vertical motion. If an
AE5564 ** orthometric height was ever established but is not available
AE5564 ** in the current survey control section, the orthometric height
AE5564 ** is considered suspect. Suspect heights are available in the
AE5564 ** superseded section only if requested.

The elevation on this monument should not be considered reliable.

What ortho height are you comparing against?

The data sheet linked does not give a NAVD88 ortho height, due to being in a subsistence area.

AE5564 ** This station is in an area of known vertical motion. If no
AE5564 ** orthometric height is shown in the current survey control section,
AE5564 ** all orthometric heights are considered suspect and are only
AE5564 ** available in the superseded section if suspect heights were
AE5564 ** requested.

Even going through the NGS menus and asking to include suspect heights does not bring up one.

AE5564 ** No published orthometric height exists and therefore all are
AE5564 ** considered suspect. This station did not take part in a recent
AE5564 ** survey which established orthometric heights in the area. Therefore,
AE5564 ** any previously published orthometric heights have not been validated.
AE5564 ** NGS does not recommend using suspect or superseded heights as control
AE5564 ** unless they can be validated or a new NAVD88 height established.
AE5564 ** If this station were to take part in a new project and submitted
AE5564 ** to NGS a new height could be published.

It depends on what you are trying to do. There are so many different reasons that cause you to need to establish elevations. Decide what you are doing first, what monuments control your project and why. From that you can decide how to do the survey. There are way too many questions.

Are you tying into existing datums (a FEMA map, city mapping....ect.). What was established to control the datum? If it's existing bench marks you need to hold; then locate them and adjust your ellipsoid to the elevation shown on the bench marks using a Geoid model.

If the project is new; then you might be able to use the latest Geoid model and ellipsoid height which will probably not match existing bench marks (at least not in my part of the country), compare apples to apples.

Whatever you do keep a record of how you did it.

Methinks that isn't the station he's using, but one example of a different station in the same county. 🙁

> > .... I'm somewhat new to surveying, so pardon my ignorance.
> You deserve credit for asking the question.

I agree they deserve credit for asking. But with all the talk on here lately about education verses experience and professional verses tradesman, Am I the only one wondering about this persons qualifications?

No

> > > .... I'm somewhat new to surveying, so pardon my ignorance.
> > You deserve credit for asking the question.
>
> I agree they deserve credit for asking. But with all the talk on here lately about education verses experience and professional verses tradesman, Am I the only one wondering about this persons qualifications?

My grand kids ask questions about things they don't understand too. The lack of "qualification" does not indicate a reason for lack of an answer. Is one expected to go out on his own and find all the answers before he asks a question?

Carry on.

I'm not a RLS or anything, I was an instrument man for a little while and I'm just trying to learn a thing or two.

That's correct, I should have picked a better example.

Good for you. Don't let the negative comments bother you.. you came here asking an honest questions and there's isn't a problem with that.

Howdy,

As others indicate, carefully review the datasheets for points of interest. They contain a wealth of information. Included are a number of different heights.

You mention three of the possible heights on the sheet.

Let h = ellipsoid height (in NAD 83), H = an orthometric height (in NAVD 88) and N = the geoid-ellipsoid separation also called geoid height (current model consistent with NAD 83 is GEOID 2012A). These three heights are related by a simple equation h - H - N = 0 (zero). Due to errors in all three types the equation does not generally yield zero.

H is the height with respect to the geoid, h is the height with respect to the ellipsoid and N is the difference in height in the sense from ellipsoid to the geoid. Sounds simple? Unfortunately,the geoid (defined as an equipotential surface of the earth's gravity best represented by global mean sea level determined in a least squares sense) is not something we can dig down to and measure from. It's value is derived from scientific consensus and used in the creation of models.

Using GPS yields a ellipsoid height which can only be transformed into an orthometric height via a geoid model.

Your question indicated an uncertainty about the appropriate height type to use. If performing leveling using a level or total station, use an orthometric height. When working with GPS I use ellipsoid heights (actually Cartesian coordinates). You should refer to your data collector's documentation.

The plane coordinates of a point do NOT include height information. I do not think that surveyors working with state plane coordinates work with ellipsoid heights except to the extent that ellipsoid heights are used for the elevation factor.

Having gone on and on and digressed too much, I close.

Cheers,

DMM

Your example is fine, pay close attention to what the data sheet it telling you. Some are data sheets for bench marks and will have a scaled value for the xy which could and probably is waaaay off from the actual location, but the elevation is good, some will have good xy but the elevation is not good like your example. Here's one with a good elevation but you shouldn't use the lat long for anything, might even be difficult to find the station using it, probably better to use the quad sheet and the description found on the data sheet.

PW0182 ______________________________________________________________________
PW0182* NAD 83(1986) POSITION- 44 56 08. (N) 106 56 08. (W) SCALED
PW0182* NAVD 88 ORTHO HEIGHT - 1098.869 (meters) 3605.21 (feet) ADJUSTED
PW0182 ______________________________________________________________________
PW0182 GEOID HEIGHT - -13.96 (meters) GEOID12A
PW0182 DYNAMIC HEIGHT - 1098.502 (meters) 3604.00 (feet) COMP
PW0182 MODELED GRAVITY - 980,245.2 (mgal) NAVD 88
PW0182
PW0182 VERT ORDER - SECOND CLASS 0

> I agree they deserve credit for asking. But with all the talk on here lately about education verses experience and professional verses tradesman, Am I the only one wondering about this persons qualifications?

Wondering about their qualifications to do what??? To ask questions?

Stephen

Hey thanks for the very informative post!

Question for you if you don't mind; I updated my GEOID from 2003 to 2012A, however I'm not seeing any difference in the translation from ellip height to ortho height. It's still the same 36.52ft on both '03 and '12A GEOID's when it should be 36.00 according to NGS, any idea if that's normal when using a nearly decade newer GEOID? I also noticed that the Ortho calls for 11.1M / 36 feet on the data sheet, however 11.1 meters is 36.42ft

Here's the monument I'm doing my testing on.

> Question for you if you don't mind; I updated my GEOID from 2003 to 2012A, however I'm not seeing any difference in the translation from ellip height to ortho height. It's still the same 36.52ft on both '03 and '12A GEOID's when it should be 36.00 according to NGS, any idea if that's normal when using a nearly decade newer GEOID?

In places were new data was collected and added to the model between 2003 and 2012 you would see changes. But in many places there has been no reobservation for many years. If no additional data was added between 2003 and 2012 there is no reason to expect the geoid model to change.

Note that a readjustment of the datum does not mean that all the monuments were reobserved. In fact, only a very small percentage of the whole - especially of the physical brass disks - ever gets reobserved. For most, they reuse the old observations.

> AA8925 *CURRENT SURVEY CONTROL
> AA8925 ______________________________________________________________________
> AA8925* NAD 83(2011) POSITION- 30 24 05.97741(N) 086 59 09.92821(W) ADJUSTED
> AA8925* NAD 83(2011) ELLIP HT- -16.155 (meters) (06/27/12) ADJUSTED
> AA8925* NAD 83(2011) EPOCH - 2010.00
> AA8925* NAVD 88 ORTHO HEIGHT - 11.1 (meters) 36. (feet) GPS OBS
> AA8925 ______________________________________________________________________
> AA8925 NAVD 88 orthometric height was determined with geoid model GEOID96
> AA8925 GEOID HEIGHT - -27.10 (meters) GEOID96
> AA8925 GEOID HEIGHT - -27.29 (meters) GEOID12A
>
>
> AA8925 SUPERSEDED SURVEY CONTROL
> AA8925
> AA8925 NAD 83(2007)- 30 24 05.97773(N) 086 59 09.92841(W) AD(2002.00) 0
> AA8925 ELLIP H (02/10/07) -16.134 (m) GP(2002.00)
> AA8925 NAD 83(1999)- 30 24 05.97736(N) 086 59 09.92919(W) AD( ) 1
> AA8925 ELLIP H (05/31/01) -16.135 (m) GP( ) 4 1
> AA8925 NAD 83(1990)- 30 24 05.97686(N) 086 59 09.92721(W) AD( ) 1
> AA8925 ELLIP H (09/29/95) -15.926 (m) GP( ) 4 1
> AA8925 NAVD 88 (09/29/95) 11.1 (m) GEOID93 model used GPS OBS

For some reason, the ortho ht is computed using Geoid 96. That is unusual - I can't say why they have done that.

Nonetheless, the quoted 11.1m ortho elevation does not differ from an ortho elevation computed from the NAD83(2011) ellipsoid elevation and the Geoid12A geoid ht. ie/ [-16.155 - (-27.29)] = 11.13m. Which stated to the nearest 1/10 of a meter is 11.1m. The same.

Howdy,

Not sure why NGS shows the computed value of H at this point to the nearest tenth of a meter. It is computed from h and N. The computation did use the GEOID 2012A model value (-16.155 - (-27.29)) = 11.1 the the nearest decimeter.

As indicated by other posts, the data for an NGS data sheet is retrieved from the NGS data base at the time of the query. They are NOT forced to agree with the h - H - N = 0.

I suspect that as the ellipsoid height was not determined using height modernization guidelines its height is truncated to the decimeter. NGS has policies for how it displays information. The basic policy is not to show values at greater accuracy than supported by the data. This is why when you look at the data sheet you see the geoid-ellipsoid separation shown to the centimeter; the output of the Geoid2012A toolkit item is shown to the millimeter.

Not all points are equal. It was arguably easier out determine the best points based on their "order." This system was based on proportional accuracy (e.g. 1 part in 1 million) as well as a base error. This system was replaced by network and local accuracies determined by analysis of least squares results. Your point seems well determined though not connected to many points.

HTH,

DMM