<p>Not officially, far as I can tell.</p>
<p>The <a href="https://doi.org/10.5636/jgg.34.309" rel="nofollow noreferrer">1980-1985 IGRF (gen. 3)</a>, <a href="https://doi.org/10.1016/0031-9201(87)90150-6" rel="nofollow noreferrer">updated 1985 IGRF (gen. 4)</a>, and <a href="https://doi.org/10.5636/jgg.44.679" rel="nofollow noreferrer">1985 DGRF (gen 6)</a> publications all gave integer coefficient values for 1985 (-29876nT, -29877nT, -29873nT, respectively).</p>
<p>It is possible to end up with 0.1nT precision from 1nT coefficients as the rate of change (secular variation) coefficients are generally given to 0.1nT, but the only published SV valid for 1985 (gen. 3) happens to round to an integer at 1985.</p>
<p>Higher precision coefficients did exist when original candidate models were computed, and some of these were published independently before the final IGRF release was determined as some combination of candidate models - you might have one of these early candidate models, rather than an official IGRF release?</p>
<p>I can't check the original software that far back as the archive only goes back to gen. 5 - if you've got code from early generations, please let me know!</p>

CC BY-SA 4.0
2024-05-23T20:50:26.360Z
unfortunately the history for the coefficients only go back 8 years when we switched to git. I did already look at all IGRF gen 1-13 from here https://www.ngdc.noaa.gov/IAGA/vmod/igrf_old_models.html
Slack Groverglow
CC BY-SA 4.0
2024-05-23T20:50:42.013Z
The motivation behind the question was which measurement is more "correct". The less precise official coefficients, or the more precise, but with mysterious origin coefficients.
Slack Groverglow
CC BY-SA 4.0
2024-05-24T09:12:49.000Z
I'm the current maintainer of an archive of some of the original working on IGRF and World Magnetic Survey, but with system and staff changes over 60 years we've not got a comprehensive record from the earlier stuff. Officially IGRF-13 is most "correct" and supersedes any older version.
WJB
CC BY-SA 4.0
2024-05-23T14:47:29.943Z
2024-05-23T14:47:29.943Z
WJB
<p>As  per <a href="https://www.ngdc.noaa.gov/IAGA/vmod/coeffs/igrf13coeffs.txt" rel="nofollow noreferrer">NOAA</a>, DGRF values for 1985 are rounded to the nearest integer for 1995 and before. I recently stumbled upon some software code where for 1985 the harmonic coefficients had 1 extra significant digit ie <code>-29872.6</code> instead of <code>-29873</code>. I know this is not an IGRF vs DGRF discrepancy because the historical IGRF for 1985 is <code>-29877</code></p>
<p>This makes me wonder, have the coefficients before 1995 ever been published with extra significant digits compared to latest published values?</p>

<geomagnetism>
Precision of Geomagnetic Reference Field coefficient values for 1985
Slack Groverglow
# Precision of Geomagnetic Reference Field coefficient values for 1985

(Asked by: username_0 on 2024-05-22)

As per [NOAA](https://www.ngdc.noaa.gov/IAGA/vmod/coeffs/igrf13coeffs.txt), DGRF values for 1985 are rounded to the nearest integer for 1995 and before. I recently stumbled upon some software code where for 1985 the harmonic coefficients had 1 extra significant digit ie `-29872.6` instead of `-29873`. I know this is not an IGRF vs DGRF discrepancy because the historical IGRF for 1985 is `-29877`

This makes me wonder, have the coefficients before 1995 ever been published with extra significant digits compared to latest published values?

**Tags:** geomagnetism


---

## 1 answers

### Answer by username_1 on 2024-05-23. Score: 2 | Accepted answer

Not officially, far as I can tell.

The [1980-1985 IGRF (gen. 3)](https://doi.org/10.5636/jgg.34.309), [updated 1985 IGRF (gen. 4)](https://doi.org/10.1016/0031-9201(87)90150-6), and [1985 DGRF (gen 6)](https://doi.org/10.5636/jgg.44.679) publications all gave integer coefficient values for 1985 (-29876nT, -29877nT, -29873nT, respectively).

It is possible to end up with 0.1nT precision from 1nT coefficients as the rate of change (secular variation) coefficients are generally given to 0.1nT, but the only published SV valid for 1985 (gen. 3) happens to round to an integer at 1985.

Higher precision coefficients did exist when original candidate models were computed, and some of these were published independently before the final IGRF release was determined as some combination of candidate models - you might have one of these early candidate models, rather than an official IGRF release?

I can't check the original software that far back as the archive only goes back to gen. 5 - if you've got code from early generations, please let me know!

> **Comments:**
> **username_0**: unfortunately the history for the coefficients only go back 8 years when we switched to git. I did already look at all IGRF gen 1-13 from here https://www.ngdc.noaa.gov/IAGA/vmod/igrf_old_models.html  
> **username_0**: The motivation behind the question was which measurement is more "correct". The less precise official coefficients, or the more precise, but with mysterious origin coefficients.  
> **username_1**: I'm the current maintainer of an archive of some of the original working on IGRF and World Magnetic Survey, but with system and staff changes over 60 years we've not got a comprehensive record from the earlier stuff. Officially IGRF-13 is most "correct" and supersedes any older version.  

---

