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Cadastral Disks

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As we know, some cadastral disks have been included for convenience in the NGS database. But what are they really? What little I 'know' is that they are grid (township?) partitioning markers and that they are established using the 'cadastral method of surveying', whatever that is. I tried looking a few months ago on the internet for the difference between cadastral surveying and geodetic surveying and couldn't find much and haven't tried since.

 

What is cadastral surveying anyway? How is it different from the the surveying to establish the horizontal control marks in the NGS database?

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Cadastral surveying deals with the measurement of land boundaries and most generally applies to surveys of the Public Land Survey System (PLSS), but can also include parcel and tract boundaries, and limits of ownership or land use. Cadastral surveys are horizontal only in nature as there is no vertical component. They are a two-dimensional type of land measurement.

 

Geodetic surveying is three-dimensional and used to help define the shape of a particular area of land, land mass, or the earth. Geodetic surveying uses both triangulation stations (horizontal) and true benchmarks (vertical). These are the marks that are primarily in the NGS database. While the triangulation stations used may not have a vertical component applied to them, the vertical angles between the stations are measured along with the horizontal angles in order to define the shape of said area of land. The triangles are then linked together to model that shape. From this type of measurement, and the resulting mathmatical calculations, map datums (ie. NAD 27, NAD 83, & WGS 84) can be defined, and refined. Geodetic surveys are generally used as survey control points and/or positions used to define other smaller control networks and surveys, done on a more local level.

 

- Kewaneh

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OK, so how are they surveyed differently, or are they?

 

In the case of a slope downhill from one station to the next, both a geodetic survey and a cadastral survey will have to measure and remove the vertical component of the distance between the two in order to get the 2D distance. Whether or not the end result of the survey is to get the horizontal distance or horizontal coordinates, this separation of the vertical and horizontal components must be done for both types of surveying. Either that or the cadastral type measures slope distance, which I doubt.

 

I still don't understand the difference in how these are surveyed.

 

Also, what is the difference in 'end result'? I know that the 'end result' of geodetic surveys are the latitude and longitude coordinates, in the case of triangulation stations and other types of horizontal control, but what is the 'end result' of a cadastral survey; do the disks have latitude and longitude coordinates, or just PLSS coordinates?

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I will add my 2 cents worth since I am a cadastral surveyor and have also been a geodesist.

 

Cadastral Surveying is just a term for a land surveying specializing in locating and sometimes establishing boundaries of land, i.e. ownership boundaries. It is sort of 2d in that we think of the boundaries of lands being defined or shown on a map which represents measurements of directions and distances on the ground. Of course the ground is not flat, but legal descriptions of parcels act like it is. Large scale cadastral surveys must also take into account the size and shape of the earth, but are not usually concerned directly with the elevation of the land.

 

Geodetic Surveying has more to do with precise absolute location of things, usually involving large areas and particular care in order to maintain high precision. Such surveys ares usually related to established control. The 'things' being located are sometimes boundary related but don't have to be. In order to perform geodetic surveying usually requires paying attention to the size and shape of the earth in a fairly scientific way.

 

Actually not sure if that helps or not. These days with a lot of surveyors using GPS for surveying even some ordinary land surveying tasks are being done with 'geodetic' tools.

 

jerry wahl

 

Simple explanation.

 

Cadastral=Grid system flat plane. 2D.

 

Geodetic=Grid system Curved plane.3D.

Edited by jwahl

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Also, what is the difference in 'end result'? I know that the 'end result' of geodetic surveys are the latitude and longitude coordinates, in the case of triangulation stations and other types of horizontal control, but what is the 'end result' of a cadastral survey; do the disks have latitude and longitude coordinates, or just PLSS coordinates?

 

The end result of a cadastral survey is (re)establishment of legal ownership and tax boundaries. There is usually no tie to geographic coordinates like latitude and longitude. The "tie" of a cadastral survey is to the history of the deed to the land, and that means that the controlling point might be the location of the old oak at the bend in the stream behind John Brown's barn in 1750.

 

The end result of a geodetic survey is a measurement of the earth's figure, either geographic location or elevation. Geodetic surveys are ultimately tied to a datum which is a mathematical ideal based on some model of the earth's figure.

 

The way I think of it is that cadastral surveys are meant to establish who has the right to use or tax the land, and geodetic surveys are meant to facilitate science and engineering.

 

PLSS is only one kind of cadastral survey.

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CADASTRE

 

Cadastral is derived from cadastre.

 

Initial Points are the key's to the maps.

But here in lies the mysteries.

If most of you study it as I have you come to these same conclusions.

 

John Brown's corner is an adverse posession.

It does not meet the requirement of the TOWNSHIP,SECTION AND RANGE.

From the initial points we find these errors from combined and double porportion measurements.

And also from the more precise geodetic survey's.

In our State the State Plane is the same as the Geodetic maps.

The Clarks Spheroid with 1927 datum.

What now?

 

He is being taxed for X land yet he lives on Y land.

 

I know we been here.=))

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Thank you all for the responses, I am getting it now, I think.

 

One last point I want to understand explicitly... Let's imagine a piece of land that's on a 45 degree slope. The horizontal component of the land is X acres and the slope surface area is Y acres. Y is a lot larger than X in this case. Does the cadastral type of surveying establish borders strictly on the horizontal component , or the slope component? (I realize that the curvature of the Earth brings in a component that cadastral surveying usually does not consider.)

 

I have zero interest in any particular situation, my interest is merely to understand more about surveying.

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BDT, perhaps a different analogy would be better. Suppose a person has a 'full section' (1 square mile) that has a hill 750 feet high in the center.

 

His land boundary is 1 mile in length not 1.3 miles in length. 750 X 2=1500 feet= about.3 mile. 750X2= going up the hill on one side and down on the opposite side. that is an additional 1500 feet the owner has in linear 3D over the linear 2D measurement.

 

John

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Short answer is 'horizontal component'.

 

All land surveys I am aware of measure and report distances and compute acreages using horizontal component of the distances. In the old days one way that could be done was to hold the chain level plumbed to the uneven ground by such means as a plumb bob. (with an exception described below, some surveys now report grid distances and areas).

 

In the technology of the 20th century the distance may be directly measured on the slope, but a vertical angle is measured and by using almost basic trig the horizontal component is derived and used. Most total station instruments actually spit out the horizontal distance directly if you want it, or it is dumped into a data collector and reduced to horizontal later.

 

Now with survey grade GPS this is harder to get to from the 3 dimensional vector one gets between points to the proper horizontal ground equivalent. People use a number of techniques to compute or closely approximate the equivalent horizontal ground distance to report for a land survey. One solution is now more common and that is to convert the lat long to some projection grid such as UTM but more often one of the published State Plane Coordinate Systems, and treat it like a 2d coordinate deriving a grid distance. You may see boundary descriptions with grid distances more often now as a result. Another solution is that many surveyors develop custom projections that are at the average terrain elevation of the project area and map the geographic coordinates to them, thus computing with those coordinates give you close to horizontal ground equivalent.

 

Technically it is still not a perfect process. if the land varies much in elevation the question becomes horizontal based on what elevation. Anyway it is probably best not get into the technical details.

 

- jerry

 

Thank you all for the responses, I am getting it now, I think.

 

One last point I want to understand explicitly... Let's imagine a piece of land that's on a 45 degree slope. The horizontal component of the land is X acres and the slope surface area is Y acres. Y is a lot larger than X in this case. Does the cadastral type of surveying establish borders strictly on the horizontal component , or the slope component? (I realize that the curvature of the Earth brings in a component that cadastral surveying usually does not consider.)

 

I have zero interest in any particular situation, my interest is merely to understand more about surveying.

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