Re: "Trustworthiness" of vertical transformations

Just a few things to note from my perspective but certainly not 
"official" from the US National Geodetic Survey (NGS).  Probably more 
information than you want to know but I believe it can be helpful to 
have a general skepticism in transformations and vertical 
transformations in particular.  I also apologize since this email got 
quite lengthy.

Vertical datums in the US have inherent errors that cause uncertainty in 
places due to systematic errors or incorrect assumptions.  Here is a 
little information about the historical US vertical datums and Ill note 
the future datum should be far more accurate since it will be derived 
primarily through terrestrial, aerial and satellite gravity measurements.

NGVD29 used 26 tide gauges across North America as constraints and made 
the assumption that sea level was the same in all places, which we now 
know is not the case.  This caused inherent errors in the middle of the 
country where data was made to "fit" through least squares adjustments 
causing inaccurate heights due to the assumption that all tide gauges 
were a height of 0.

NAVD88 tried to rectify this by only using one tide gauge as a 
constraint.  This caused the systematic errors in the leveling to 
propagate across the country.  Today people get NAVD88 heights by tying 
into bench marks that have NAVD88 heights on them which allows for them 
to get consistent heights in NAVD88 and is precise and provides 
repeatable heights, but precision is not accuracy as we are seeing while 
planning for the new datum in 2022.

Here at NGS we are working on a new datum and you can see the systematic 
errors from NAVD88 in this map 
<ftp://ftp.ngs.noaa.gov/dist/bshaw/Approximate_Vertical_Change_2022_sm.png> 
that shows the estimated change that will occur from NAVD88 to the new 
Geopotential Datum (Vertical Datum).  Its called "Geopotential" since it 
will be developed primarily through gravity and not hundreds of 
thousands of kilometers of leveling like past vertical datums.

A note on transformations in general is you should understand that 
almost all transformations are often termed mapping grade which is fine 
for GIS but not for geodetic coordinates.  You should also understand 
the uncertainties of the datums being used.  For instance if you are 
using WGS84 coordinates you should understand that just by using that 
datum you are introducing 3-5 meters of uncertainty in the horizontal 
coordinates, I'm not sure about vertically. Geodetically the best rule 
of thumb for having the most accurate coordinates is to preserve your 
original observations and process them in the datum of choice.  That is 
not always possible since you may not be the person/group collecting the 
data but it is something to keep in mind.

A closing note on accuracies as mentioned below by Kristian is that not 
all people report accuracies in the same way.  Even here at NGS there 
are different products that report in different measures. Some report at 
the one sigma level (standard deviation, square root of variance) also 
described as having 68% confidence in the value. Others report at the 
two sigma level (variance, std dev squared) which is often termed having 
95% confidence in the value.  So I would encourage you to know what 
sigma is used to report the accuracy and also understand this may just 
be the accuracy estimated in the datum which doesnt necessarily account 
for the uncertainty in the datum itself.

Cheers
Brian

On 11/27/2019 3:18 AM, Kristian Evers wrote:
>> The algorithm currently is rather
>> simple: just add the (in)accuracies of each step. I do have some vague
>> remembering from univ that this was not always the "right" thing to do from a
>> math point of view. I guess sometimes perhaps taking the max() would be more
>> appropriate. But given that we can potentially mix uncomparable things,
>> probably not that a big deal...
> Yeah, this can definitely be improved but it's a simple and effective solution that
> doesn't promise too much which is always better that saying that accuracy is
> better than it really is. I think this could be a fun topic to have a workshop on
> at the OSGeo code sprint in the spring.
>
> /Kristian
>
> -----Original Message-----
> From: Even Rouault <[email protected]>
> Sent: 27. november 2019 01:06
> To: [email protected]
> Cc: Nyall Dawson <[email protected]>; Kristian Evers <[email protected]>
> Subject: Re: [PROJ] "Trustworthiness" of vertical transformations
>
> On mercredi 27 novembre 2019 09:34:33 CET Nyall Dawson wrote:
>> On Tue, 26 Nov 2019 at 20:52, Kristian Evers <[email protected]> wrote:
>>> You can get the transformation accuracy using the API function
>>> proj_coordoperation_get_accuracy(). I think it would be cool if
>>> information about transformation accuracy where readily available in
>>> software like QGIS (nudge, nudge :-))
> I should mention that the information about accuracy should be taken with a
> grain of salt for several reasons:
> - the EPSG guidance note 7-1 [1] underlines that the exact definition of
> accuracy varies according to geodetic agencies
> - in a number of situations, PROJ will have to synthetize the resulting
> accuracy when chaining several steps. The algorithm currently is rather
> simple: just add the (in)accuracies of each step. I do have some vague
> remembering from univ that this was not always the "right" thing to do from a
> math point of view. I guess sometimes perhaps taking the max() would be more
> appropriate. But given that we can potentially mix uncomparable things,
> probably not that a big deal...
>
>> That's where I'm coming from now... I'm just wondering if and how we
>> should expose vertical transformation functionality. Do you have any
>> ideas on what functionality you would expect an end-user application
>> to expose for vertical transformations?
> Depends on the use cases. If you want to aggregate point cloud datasets that
> are referenced against several vertical CRS into a single dataset, you will
> probably want to convert them into a single Geographic CRS (ITRF2014, WGS84
> G1762) or a single CompoundCRS appropriate for the area of study so that they
> can be stashed together.
>
> Even
>
> [1] http://www.epsg.org/Portals/0/373-07-1.pdf , §6.5.4.1 Coordinate Operation
> accuracy
>

-- 
*************************************
Brian Shaw
Geodesist
NOAA/NOS/National Geodetic Survey
Phone # 240-533-9522

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