Magnetic Bearings / Sanborn Maps

[D Barberio]

I'm working with a legal description from 1900, which states bearings to the 1/4 degree and states the magnetic declination. The description calls for courses "8 feet westerly of the westerly bank of Thompson Creek...". The courses approximately fit the historic westerly bank, except for the first course, but if I assume the first course was mistakenly stated as a magnetic bearing, rather than true like the description states it is, then all the courses approximate the bank pretty well. My question is how likely could this happen? Did all the compasses used near 1900 have a declination arc so you could directly read the true bearing, or was it sometimes the case where a surveyor would note the magnetic bearing in the field book, and then calculate the true bearing mathematically, thus increasing the odds that a "transposition" like this could occur?

Also, has anyone used the Sanborn fire insurance maps for supportive evidence? Anyone aware of any court cases which involved them in anyway?

Thanks.

[mpallamary]

Magnetic bearings fluctuate. You need to locate a isogonic projection map to see where magnetic north was as of the date of the survey. They are very common back east where land is heavily wooded and there was no other way to get oriented. Look out for local attractions also. These are compass measurements. If you can tell who the surveyor was or look at local maps, you can compare and get a sense of the needed adjustment. Most surveyors read the magnetic directly and did not adjust. That is why the map is important.

Good luck.

Her's a good slide viewer.

https://maps.ngdc.noaa.gov/viewers/hist ... clination/

[Peter Ehlert]

Magnetic Bearings and declination were know by surveyors long before that description was created. Typically if Magnetic Bearings were cited the Declination correction was used... hence Magnetic Bearings = True Bearings to the extent of the technology of the day, and the capabilities of the Surveyor. Correct for the change in Declination to "follow the footsteps" as Mike said above.
Two possibilities:
1. Perhaps the actual description was created by a Non Surveyor and the data was complied using mixed sources (or parroting the words of others without understanding the terms.
2. Or the bank has moved... and that would lead to another discussion :-)

[D Barberio]

Peter,

It appears my question wasn't clear. The deed cited that the bearings were true, and it also cited the magnetic declination at the time. So that, along with the bearings being cited to the 1/4 degree, suggests strongly to me a compass was used, magnetic bearings were determined, and the stated declination was used to transform them into true bearings. I'm wondering about the likelihood of neglecting to transform the bearing of the first course ( and therefore it would still have been stated as a magnetic bearing in the deed) while all the rest of the courses were stated as true. I'm figuring this would be more likely if the compass didn't have a declination screw (or something of that nature) so that the compass wouldn't read a true bearing directly, like my Silva compass that reads true bearings directly as I already have the declination set in it. The surveyor would have to mathematically change all his/her field recorded bearings (or at least make a transformation in their head), and could have much more likely failed to do so on one of the courses.

By comparing Sanborn maps from 1883 through 1941, the bank was never in a configuration that supports the first course and its relation to the rest of the courses along the bank.

[Peter Ehlert]

see conversation at http://surveyorconnect.com/threads/magn ... ps.324672/

[Stan_K]

I suggest you are looking at the math and not the evidence. The deed says, "8 feet westerly of the westerly bank of Thompson Creek..." This sure sounds like a call to the creek with an 8-foot offset. If your deed created a fixed alignment at the time it was created, you have a deed that does not close. You have already identified the "first meander course" as the probable error. If your deed follows the creek, as-built it.

Look at the words in the deed, the adjoiners, and the physical evidence.

[D Barberio]

I am actually looking at the math as further extrinsic evidence to the location of the creek. The creek in this area does not exist anymore. It was replaced by a concrete culvert ( a "tube") during the Depression by a WPA project. The evidence for the creek location is the storm drain itself (presumably it was placed in the bed of the creek), the creeks approximate location determined by 1883-1941 Sanborn maps (scaling to still existing old buildings), and the "westerly bank" courses in the deeds. The courses substantiate the approx. location per the Sanborn maps when the above-mentioned first course is assumed to be mistakenly stated as a magnetic bearing and is transformed into a true one. I'm just querying what others thought about the probabillity of this "transposition" occurring in a compass and chain survey in 1900 or so.

[E_Page]

What you asked in the OP is entirely possible. So are other explanations.

Have you ever worked on a project that required you to use a project scale factor? Typically, this might be a project in which you are using control with CCS positions, where the project area is fairly limited and the combined scale factor is far enough from 1.00000 to make a difference to whatever you're doing. If so, have you ever been reminded at some point after getting started for the day, or had to remind the I-man "Did you put in the scale factor?"

I've been in that position before and had to remember to make corrections to a few observations when processing the DC file. I can also tell you from experience that the same can happen with regard to declinations and compasses. We used staff compasses fairly regularly when I was with the BLM in AK. Often, the declination would need some minor adjustment as compared to the last location we had used it, but once in a while, someone may have reset it to zero, either to check a magnetic bearing, for some sort of demonstration purpose for a new person, or just because they were screwing around with it. I can easily see a chief reminding a compassman "Did you check the declination setting?", and then for whatever reason, the correction was never made in the notes for that first bearing read with the declination at zero. Never assume that the old crews were screw ups, but conversely, never assume that because it appears that they knew what they were doing and generally did very good work, that they never made blunders. The surveyor that never made a blunder never surveyed.

The discrepancy might also be explained by a local magnetic anomaly, whether it was some natural ore deposit by that first occupied point, or someone standing too close to the compass with a hammer or axe. It could be transposed numbers or poorly written or interpreted numbers in the notes.

Probably more important, are you able to identify that first bearing as a mistake with reasonable certainty? If so, can you identify a reasonably likely explanation as to how the mistake occurred and it's magnitude? It seems that you've done these things if you are able to identify the historic bank location and come up with the hypothesis you did in the OP. It sounds like you've looked at a variety of evidence and found this description to be at variance with your other evidence. It further sounds like you've analyzed it to see if you could find a reasonable explanation why it wouldn't fit and deduced a probable source and amount of error. that when corrected for, makes things fall in place. Sounds like a bit of good detective work to me.

I do not recommend holding the record bearing if it results in locations that are less likely than the location with the reasonable explanation you have. Based on the info you've given, I'd feel more comfortable correcting that first bearing by the declination amount so that the meanders agree with the historic bank location.