1831 and 1844 with USHD Magnetic Declination Estimates

If you retrace 19th century compass surveys very often, I'm sure you're aware of the US Historical Declination software developed by the the National Geophysical Data Center and the on-line application on their website.

Estimated Value of Historical Declination Calculator

The values that this software produces were based upon scattered historical observations over most of the 19th and early 20th century and some idea of what the Earth's main magnetic field was doing based upon the fixed magnetic observatories. On the Texas frontier, the record of magnetic observations is not particularly extensive. Surveyors in a county or district tended to simply use whatever value of magnetic declination was already in use, the idea being to follow and fit lines previously run rather than making some absolute determination of the directions of lines. That was based in practical considerations.

The NGDC software produces estimates that are usually fairly good as far as modeling the change from year to year (the secular change), but sometimes they're in systematic error when the record of historical observations is sparse.

So, using the NGDC data often involves some calibration of their models.

I'm finishing up a survey at the moment that involves lines originally run in 1831 and 1844, nearly certainly with surveying compasses. In this particular case, neither the 1831 surveyor nor the 1844 surveyor bothered to mention the declination or "variation" for which he'd adjusted his compass. But I think we can work those out.

So far, I've found three corners that were established in 1844 on a line run in 1831. Each corner falls in a timbered area with hardwood trees that hold their marks well. So it is most plausible that the 1844 surveyor was following the line of a land grant marked in 1831 as he reported he was, the old marked trees having been most likely quite easy to identify after only 13 years.

The average of the line through the three corners over about 3860 ft. was run in relation to a North direction that has a Grid Azimuth of -1°41' in the Texas Coordinate System of 1983 (South Central Zone). In the vicinity of the area of interest, Grid Azimuth = True Azimuth - 0°59'.

The evidence indicates that the lines run in 1844 from corners on the line of the 1831 survey run as follows from each of the three corners:

[pre]
Grid Az Distance
-2°07'29" 7790.50 ft.
-2°05'14" 5110.77 ft.
-2°00'51" 5009.20 ft.
[/pre]

So, that means that the 1844 surveyor was running his lines in relation to a North direction with an average Grid Azimuth of approximately -2°05'

The USHD models give estimates of magnetic declination of 9°10' (for 1831) and 9°18' (for 1844) from which one can calculate the variations that each of the two surveyors would have set off in his compass (if the declination estimates were correct) to yield the actual grid bearings of the lines as marked on the ground.

That calculation is made so:

1831 Surveyor:
[pre]
+09°10' True Az Magnetic North in 1831
-09°52' Estimated Survey Variation
-00°59' Mapping Angle
-----------------------------
-01°41' Grid Az Survey North
[/pre]

1844 Surveyor:
[pre]
+09°18' True Az Magnetic North in 1831
-10°24' Estimated Survey Variation
-00°59' Mapping Angle
-----------------------------
-02°05' Grid Az Survey North
[/pre]

The values of "Estimated Survey Variation" are merely those backed into using the USHD estimate of magnetic declination, the mapping angle, and the actual directions of lines as evidenced on the ground.

In Central Texas, most surveyors adjusted their compasses to variations taken to the nearest 0°15' or, more commonly, 0°30'.

So, what I'd conclude from the above is that the 1831 surveyor probably was running at a variation of 10°00'E and the 1844 surveyor probably was running at a variation of 10°30'E.

If that was so (as seems eminently reasonable in this case), the values of magnetic declination estimated by the USHD models need to have about 0°07' added to them to correct them.