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2 Qs- Local Grids & How To Use Benchmarks


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Q1- A friend got a diagram of his property lines from a surveyor. The diagram starts with false coordinates, and gives "easting" and "northing" coordinates. I stood at one stake and took a GPS reading, then tried to convert the diagram so I could enter it in Lat/Long on an aerial photo. I got everything about right, but the property is rotated about 7.3 degrees CW. This isn't the difference between mag and true north for this location, so what system is the surveyor using, and why are there no absolute references used?

 

Q2- Almost all the benchmarks around here are vertical references only, and were set in 1942. What good are all these vertical points without precise knowledge of where they are? How does a surveyor use these things? Why did the year 1942 produce such a bumper crop of benchmarks?

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Photobuff-

 

Q1; You are not seeing false coordinates, you are seeing the use of the state plane coordinate system. Every NGS Datasheet not only includes the Latitude and Longitude, but the State Plane Coordinates (SPC) as well as UTM Grid Coords.

 

SPC is a local coordinate system used in zones all over the US for control that is based on a grid that is flat, and does not take into consideration the curvature of the earth. They are kept to small local zones so that the curvature of the earth does not affect them. SPC was created by the NGS back in the 1920's and 30's when they were known as USCGS. We actually use the SPC a lot more than we use Latitudes and Longitudes for local control. Some GPS can utilize SPC as a Datum... What you are seeing as 7.3 degrees is interesting, but I am not clear as to why. You don't have instruments that will reveal why either, But the surveyor or firm is licensed so.... Perhaps they can explain it, since they performed the survey. There may also be references to the Cadastral method of surveying on your diagram as well.

 

Q2; 1942 was the year the Geodetic Surveyors passed through your area to perform leveling. Leveling or Differential leveling is the surveying process we use to determine the elevations of things. You can google for further info.

 

Vertical references were typically never given precise horizontal locationing data. The elevation was a Bench Mark, the location was a Triangulation station in the purest form. Back in that day we used Mean Sea Level as a reference, today we use a combination of references to determine elevations.

 

They are all part of a formula that uses a reference ellipsoid that approximates sea level globally, a reference that measures the gravitational strengths of the earth, and physical measurements as well. The way we look at it today removes the sea from the equation due to it's lack of stability. Too many things affect the sea to make it a valuable reference for geodesy. Before Satellites, it was all we had.

 

Hope that helps answer your questions,

 

Rob

Edited by evenfall
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It would be helpful to know what state, and perhaps what county your friend's property was in.

 

In some portions of the West some local grids are aligned with the railroad survey. The railroads were granted land on either side of the tracks, and they found it convenient to lay out property lines aligned with the tracks. If you look at a topo map of the region of Texas that lies near the southeast corner of New Mexico, you will see evidence of a grid that is skewed about 30 degrees.

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This is a big 2 questions.

 

The way I understand it to date.

The State Plane Coordinate system is tied to the benchmarks.

The scew is from what is known as the annual westward slippage or proper motion.

Proper motion

 

Pole shifts

 

The magnetic field as well as the North star have been progressively moving west.

You will see lots of argumets on this one.

But since 1834 the magnetic field has slipped west 7.5* + -

This is equvelant to about 27*.

 

If you go back and study the charts you will see this.(if you can find them).

Luckily I have some 100+ year old evidence to back up my theory.

 

When the lands were first surveyed out a compass and chain were used,as time progressed more precise instruments were used.

 

Originally departures of Latitudes and Longitudes were used and still are if legally done.

 

This is just the beginning of the theory and practice of Surveying.

 

There is volumes of data about this but few have gone into depth to study it.

Or those that have and found these major errors are reluctant to speak out.

 

This is not to say that all are like this and I am not putting any person down for it,it is my observations of current practice,by some.

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When you say rotated, I assume you mean a few degrees from true north.

 

When were the property boundaries originally established, and what were the tools and techniques of the time? Seven degrees does not strike me as a large skew for old surveys. Look at the map of Ohio, for instance.

 

In Western NY I would imagine most properties were either first surveyed in the early to mid 1800's or else created by subdividing one of those early tracts. So we must ask what the surveyor used in the 1800's to set his directions. At that date, it was probably a magnetic compass, with some value for the magnetic declination that he was given that may not have determined some distance and years away from the work he was doing.

 

In other cases something set the predominant direction of land boundaries other than the compass. SOme one mentioned railroads. Other times it was a river or lake shore that ran close, but not on a cardinal direction.

 

Do some more historical research on your area. It will probably be interesting.

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I got everything about right, but the property is rotated about 7.3 degrees CW. This isn't the difference between mag and true north for this location, so what system is the surveyor using, and why are there no absolute references used?

 

Before we all dive in here trying to guess until we are dead, there is some respectful reservations I would like to ask us all to consider.

 

Here is something which we can all agree is an understatment. Surveying is confusing. If one does not know what the last surveyor did and why, then replicating the results which by the way is the goal here will be difficult. I work in the profession and I have been there done that. That said, I feel we lack enough information to even begin to speculate.

 

Going on, if one has everything right, then the results will duplicate.

 

If we are observing a rotation in a survey originally performed by professionals, then more research needs done. Something is being missed or misinterpreted. As of this writing, all anyone can do is guess unless more data, real data about this specific survey is provided. The Big But, here, is that this geocaching forum is not the correct place where legal property boundaries should be discussed. it is both off topic and since no one here is likely to be licensed to survey in the State of question, then it is dangerous to discuss.

 

The appropriate way to ask this question is by asking the firm who conducted the survey. If you approach this right, they may be more than happy to explain their approach and meanings without charge. Geodetic Surveys are different animals than Legal property surveys. Both are important and are means to different ends. Beyond utilizing similar skills and instrument by Surveyors, they really are different animals.

 

Beyond that, State Plane Coordinates are commonly used by surveyors all day long, everywhere you go. Nothing secret there. It is not a secret nor difficult to understand.

 

Magnetic References are really weak references for any kind of a survey these days, yet if I describe the declination I had at the time I did something, then if you came along 50 years later and observed the current declinations, using a compass, the notes should jibe. Pole shifts are well, a nice distraction for the dinner table when Anthropologic discussions are being held.

 

As Holo pointed out, there are many versions of Grids which are being used in local ways. Again, anything beyond stating that is guessing. Many People work in the Survey field and are well trained. Local knowledge is always a big factor.

 

Geo's reference to major errors isn't really safe to say because without the proper training and research, what any error which may or may not be, or actually is, is hard to ascertain without the qualifications and experience needed to determine what and if they exist in the first place.

 

As a for instance, Downtown Seattle has Streets laid out in many different directions. There are historical and geographic reasons for this. One was Elliot bay long before its present shoreline was as it is today, and the location of a particular Saw Mill owned by one town Father who's skid road for transporting logs to the mill, (yes, the namesake for the term skid road) was pointed in the direction where the mill owner owned the logs he was milling, and so formed the prime meridian for street direction in that part of Seattle. Later, downtown Seattle's grid direction was changed to north when a different town Father came to power, much later in time. The big point here is that we need to know this history in order to know why. If you leave Seattle, none of this really matters.

 

Cadastral survey descriptions take some of these things into account. State plane Coordinates work just as well to describe a location but they do not take any of it into account. They rely on the grid.

 

Ok That said, Let's exercise some caution about discussing specific legal surveys here.

 

Rob

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I appreciate all your responses, and am certainly learning something, not to mention learning what I need to research. A few more details, though I don't want to beat this much more, as sooner or later I'll get an answer from the surveyor. The diagram he gave my friend was made from other survey documents, and was intended for my friend to set his GPS up for either UTC or user grid (not sure which), and walk his property line from stake to stake. The starting point was given false coordinates to prevent any negative numbers (5000 and 10000), then each corner point simply has an X and Y value listed as the easting and northing values from the first stake. There are no absolute coordinates to put this map at any particular place on the globe. I did a very primitive conversion to GPS coordinates simply by taking a reading at one stake, and figuring how many feet per degree for our latitude. Not terribly accurate, but that's not a concern here. Then I used USAphotomaps to plop those coordinates on the aerial photo where he lives. That's where it became obvious that I had a rotation. I started by drawing the property lines in a CAD program, so it was easy to rotate and convert the numbers, making the property line up properly with roads, hedgerows, and other features on the photo. Thus my conclusion that whatever frame of reference the drawing used, was rotated, compared to the lat/long used for the aerial photo. Whew.

 

On Q2, again, really interesting stuff, but I guess what I'm really asking is, when you're out there surveying, and there's a nice vertical benchmark up the road, say, half a mile, what do you do with it? I'd think with the good GPS equipment used today, benchmarks would be of limited use. Please understand that, though I have a reasonable understanding of trig and geometry, I don't have a clue about the physical process of surveying, and the various references I've found on the web weren't very enlightening. I actually far more interested in this question, than the mapping stuff above.

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