modern CAD analysis of old compass and chain surveys

I have used BLMs GMM program with little success. I'm in north central Florida where most of the PLSS surveys were done in the 1840s to 1850s. You have to think that if they are off half a degree that's almost 50 feet in a mile. Add on top of that 160 measurement over a mile. It's no wonder that they misclose by a 100 feet. And that is assuming that they made no blunders along the way.

The Bow Tie Surveyor

> Does anyone else attempt to do modern CAD analysis of old compass and chain surveys from the old notes of the day (1800s)? I've had indifferent success with it, and I don't think it's due to local magnetic variation.
>
> If so, how do you do it, and get some kind of useful analysis? It strikes me that there's there's useful information in there, if only it can be dug out.

Well, in Texas, I make extensive use of the US Historical Declination Software that the National Geophysical Data Center has developed. I've found it models the secular change (the change from year to year) very well and gives estimates of absolute values of declination that are quite good if the site is reasonably close to a station where declination was observed before about 1900.

So, the first thing I do is to compute the grid bearing of the "North" direction used in a compass survey on a particular date when the variation at which the survey was run is stated.

Getting things oriented nearly correctly allows the filters of terrain calls to be applied more effectively. That is, if you have the grid bearing of the line determined within a relatively narrow range of uncertainty, what is mainly left are the chaining errors and those can be evaluated better once the orientation is fixed.

In the same size survey in the Adirondack foothills I often find 300-400 feet misclosure. There are unquantifiable errors along with the possibility of mistakes. Terrain, weather, skill of survey party, equipment all played a part. Finding the misclosure is in large part the analysis. It is why a surveyor can't start at one monument and measure from it to establish an interior line. The evidence on the ground has to control when there are problems with the numbers. Here we have great lots subdivided into hundred acre parcels. Typical dispute is one surveyor comes from evidence on the west and another on the east. 60 feet different on determination of the line and both ignore the old farmhouse and fence remains built near the time of the original subdivision. Sometimes improvements built at or near the time of the survey are the best evidence of the line. The misclosure is important corroborating evidence when the line is 60 feet from where the numbers appear to place it. Of course it takes an extensive (expensive) survey to find the line rather than set a couple new ones, so it commonly does not happen until there is a dispute.

In my neck of the woods, (colonial) most of the controlling surveys were slope chained and a proper analysis cannot be done until after you run a preliminary traverse carrying elevations.

That being said, about a third of the misclosing deeds can be rectified by going back to the original description which is going to have the least number of transcription errors.

The next third can be inspected in cadd with the use of an aerial photo for likely problem courses and distances.

The last third can be rectified by adjoining descriptions.

As Duane correctly pointed out, original evidence or the perpetuation of original evidence on the ground trumps all.

Respectfully,
Jim Vianna

Kent,

Thanks for your thoughts.

The convention around here was for the old guys to start an a line with a variation, let's say 7 deg 40'. They run a course, find a post so many links off line, and then correct the line - 7 deg 30'. They did this according to tables in their field books.

I'm not sure where they got their original bearing, but I'd have gotten it from some sky observation. Ephemera were around in those days of course, and once they had it, they likely would have "set" a course from their house to some object in the distance to avoid doing that again. Polaris was always around, for those that were careful to level their instruments.

How can you be sure from the notes of what method they used, and how careful they were to run on "true north", or a reasonable variation thereof? Of course, once you are in the field, you can always determine this.

"Modern" surveyors were often more slack about their basis of bearings, grabbing any something and running with it. One survey I had of a simple school yard by big engineering companies had these variations as far off as 5 degrees between surveyors, none of them hung on anything other than assumed bearings. Ugh.

Yes, I have done it. I had resurveyed part of several sections, (I'm in PLS land) and AFTER finding some original corners, it yielded some different search locations, for additional corners. I even found one that way. It also yielded some value, on some old interior occupation lines, because the old occupation was built on some now missing, but probably in at the time corners.

Welcome back, William!

N

In Sipe's Compass Land Surveying, he compared his compass/chain survey of a West Virgina tract with a T-2 survey (I forget whether he used an EDM or not) and got a check closure of 1:3500, I think.)

> Kent,
>
> Thanks for your thoughts.
>
> The convention around here was for the old guys to start an a line with a variation, let's say 7 deg 40'. They run a course, find a post so many links off line, and then correct the line - 7 deg 30'. They did this according to tables in their field books.
>
> I'm not sure where they got their original bearing, but I'd have gotten it from some sky observation. Ephemera were around in those days of course, and once they had it, they likely would have "set" a course from their house to some object in the distance to avoid doing that again. Polaris was always around, for those that were careful to level their instruments.
>
> How can you be sure from the notes of what method they used, and how careful they were to run on "true north", or a reasonable variation thereof? Of course, once you are in the field, you can always determine this.
>
> "Modern" surveyors were often more slack about their basis of bearings, grabbing any something and running with it. One survey I had of a simple school yard by big engineering companies had these variations as far off as 5 degrees between surveyors, none of them hung on anything other than assumed bearings. Ugh.

PLSS surveys in this area (Florida) were corrected for magnetic declination and the variation they observed during their survey was noted in their field notes. Around here I heard that they used what is called a "Nonius Compass" which allowed for the adjustment of the compass rose to account for declination. I have heard stories that there was a method of using Polaris and the position of the surrounding constellations to time when either upper or lower culmination would occur and thus give them a true north sight to calibrate to.

The Bow Tie Surveyor

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GIS would be closer
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