Geodetic Leveling

Loyal,

I attended this class last year about this time. Charles and Ben taught that class as well, along with some help from Malcolm Archer-Shee, the author of DSWorld, and a few other programs. The class was outstanding, and I would highly recommend it.

Jimmy

I was at the LSAW Fall Seminar, over this past weekend.

I sat in on the 2022 NATRF Datum; Dave Doyle @base9geodesy and I gotta tell you; Dave is never a disappointment! A greater understanding of Geodesy was the goal and Dave delivered.

I hope he keeps up with this; generations to come will benefit.

Dougie

Posted by: RADAR

I was at the LSAW Fall Seminar, over this past weekend.

I sat in on the 2022 NATRF Datum; Dave Doyle @base9geodesy and I gotta tell you; Dave is never a disappointment! A greater understanding of Geodesy was the goal and Dave delivered.

I hope he keeps up with this; generations to come will benefit.

Dougie

I don't remember (for sure) exactly when I first met D?ý, but it was nearly 40 years ago. Over the years, I have attended dozens of seminars, work shops, and after hours discussions, presented or moderated by Dave. Nobody presents the information better, or is able to get the important points across to us muddy boot surveyors than Dave can.

Loyal

I always remember what Heiskanen and Moritz wrote in Physical Geodesy

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Also of possible interest is the Geodetic Tool Kit item: LVL_DH.

I share the admiration of Mr Doyleƒ??s continuing efforts to highlight the role of Geodesy.

?ÿ

Posted by: GeeOddMike

I always remember what Heiskanen and Moritz wrote in Physical Geodesy

Leveling without gravity measurements, although applied in practice, is meaningless from a rigorous point of view, for the use of leveled heights as such leads to contradictions (misclosures); it will not be considered here.

It often is quite good enough for application purposes to ignore the gravity changes for runs of a few miles/km, but wise to consider how much change you are ignoring. In particular, a run with a lot of elevation change may have significant gravity corrections, and more so if it is north-south.

An easy and approximate rule of thumb is to look at an NGS data sheet near each end of your run for orthometric and dynamic heights.?ÿ Calculate

[(Ortho_1 - Dynamic_1) - (Ortho_2 - Dynamic_2)] / 2?ÿ?ÿ?ÿ as a rough estimate of how much you will be off by ignoring gravity changes.?ÿ If that is significant, then do the proper orthometric corrections.

BTW. the venerable Manual of Geodetic Leveling is available on line at:?ÿ https://www.ngs.noaa.gov/PUBS_LIB/GeodeticLeveling_Manual_NOS_NGS_3.pdf

Lots of good information about many aspects of this type work.?ÿ

The geodetic leveling adjustment manual written by Rappleye is available from commercial sources but not any free digital versions that I could find. It is mostly of historical interest but has some details about observation and condition equations.

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Good suggestion to check published values near project. As for the computation, why not just use LVL_DH? You can use either PIDƒ??s or height and gravity as inputs. Shown below are inputs and output using sample values on page. Note that data sheets provided in output.

Duh.?ÿ I should have known that NGS had a handier method.?ÿ I'd never used LVL_DH.

For those that want to look at the above Special Publication by Howard S. Rappleye

you can go to the NGS library (library.noaa.gov) and you will find it (10MB). This Publication

first came out in 1948 and was reprinted in 1978.

Manual of Leveling Computation and Adjustment, Special Publication No. 240

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JOHN NOLTON

I played more with the approximation I posted above, and while it does a fantastic job on the example in Wolf & Ghilani's book, it seems to fall apart for some other cases I tried.

Thanks to Mr. Nolton for finding the digital version of the manual.?ÿ

Finding it from the link he posted was a bit of bother. I ultimately obtained my copy from:?ÿ https://library.noaa.gov/Collections/Digital-Collections/USCGS-Special-Pubs?ÿ

Annoyingly I was not able to specify the author or publication in the Search tool at the base site. Instead,?ÿfrom the link above navigate to no.240 (1948) using the drop-down listing above the "View Publication" button.?ÿ

It is hardly current but covers many aspects of leveling and adjustments with a specificity not available elsewhere.?ÿ

Once again, thanks Mr. Nolton for finding this document.

Cheers,

DMM?ÿ

FWIW,

The dynamic height is the geopotential number scaled by the constant normal gravity at 45 degrees latitude. You will note that using LVL_DH in addition to using surface gravity specific to a site also includes the provision to account for the difference due to orthometric height.

See:?ÿ http://geodesyattamucc.pbworks.com/f/HeightSystemsSneeuw.pdf for full text of this document on height systems by former U Calgary professor N. Sneeuw.?ÿ?ÿ