The issue of tolerances is extremely complex, when we try to apply it to cadastral surveying processes. The problem is simple, and one day he talked about it Nancy, if one only wants to know the criterion of precision of a team; however it becomes complex when it is integrated into a regularization process of land tenure, and you must apply tolerance formulas for surveys that led to different survey methods.
It becomes almost unsustainable if the regularization involves integrating the real estate registry, where you find documents that were measured with old practices whose veracity is questionable. Such is the case of properties that were measured by saying:
... from the summit of the mountain Las Botijas (what summit?) ... to the place of La Majada (which point of all that place?) ... following the route upstream (which, if the river has changed in time ?) ... I took the path from the quebracho tree (such a tree no longer exists), and I smoked three cigars to the ridge of vicente ...
In this sense, a difference must be made between the precision of the measurement and the tolerance of the survey method. The most difficult thing about this is that many times the survey metadata does not contain the content of the methods used and it is even more difficult if the information extracted from the registration documents was not classified in such a way that it can be tabulated or parameterized for large amounts of data. data. Here I share with you how one day we worked with a case like this, perhaps at some point it will be useful to those who come to Google asking for "cadastral information" and slipping on the "search" button brings them to this page... although finally Realize that it is not that simple and that there is a lot of frustration ahead.
The problem was deciding how to enter the regularization and titling process, if the least we had was time. There were different survey methods and a work flow had to be defined towards the massive regularization of properties so that a tendency was required to follow and automate some calculations that the system could do so that the classification by the legal technicians was more fast and the prioritization of rectification in the field or analysis of the cabinet by the cabinet technicians had clear criteria.
On the Tolerances in the differences of areas.
- Accuracy of measurement.
The accuracy of the measurement is the degree of uncertainty that may exist between physical reality and the graphical model, and this is related to the survey method.
In this case, different survey methodologies had been used, so it was necessary to mark a parameter of acceptable precisions. Although I must admit it, it was an obligatory exit because the law said that the National Land Registry should create a technical standard where it would make these aspects official ... that was almost four years ago and they still have not done so.
- For the survey method by photo identification, the representation of boundaries and buildings, the graphic precision is that which allows the length of the semi-major axis of the relative standard ellipse between two points on the cadastral map as a consequence of the precision of the points to be smaller than or equivalent to root square of two times the pixel, in this sense the square root of 2 × 20 cm was considered for the built-up and urban areas, for the rural area the square root of 2 × 40 cm. (This corresponded to +/- 28 cm in built-up / urban areas and +/- 57 cm in rural areas). This was an output that was used in work done via orthophoto photo interpretation that had a 20-centimeter pixel, flight at 10,000 feet, and estimated absolute accuracies of 1: 2,000.
- For the method of submetric GPS survey was considered 0.36 mts; this was applied to works performed with dual frequency equipment and whose accuracy was assumed to be sub-metric.
- For the method of millimeter GPS survey 0.08 was considered mts; it was applied to work done with total station and georeferenced with gps points of subcentimeter precision.
- For other methods of surveying direct measurement it was considered twice the factory precision of the respective instruments; here included surveys with conventional theodolites and georeferenced with subcentimeter precision gps points.
- For survey methods in which combined measurements direct and indirect was considered the least accurate.
On tolerances between the calculated area and the document area.
This tolerance is defined to adopt a measurement made with a less accurate procedure as acceptable.
Regarding this, the real property law of this country had been prepared "as is" and there was no way to make changes unless the National Cadastre made the aforementioned technical standard official. However, in the law there were at least three articles related to tolerance.
Article 33… referred to the priority that the cadastral area has over the documentary area, when the boundaries have not changed. This article says that when there is a difference between the cadastral area and the documentary area, and the boundaries have not changed, the cadastral area will have priority.
Article 104… a tolerance of no greater than 20% of the area was mentioned, this refers specifically to remediation titles. This article mentioned that re-measurement documents that reflect area differences above 20% of the originally registered area would not be accepted.
Article 49… referred to the tolerance allowed in the Cadastral Measurement Regulations, where the margin must be established. At this point is where the law said that the National Cadastre should create a normative document where it would establish tolerance and precision ranges for different methods of cadastral surveying.
So for the computer system to solve the problem, or at least warn about it, we resorted to a formula that could calculate a tolerance range and raise a flag saying: "warning, the measurement area of this property is out of range." the margin of tolerance regarding the documentary area”
Tolerance was expressed in formula T = q √ (a + pa), taken from a study of a document that at this moment I could not find on the web ... one of these days I will find it.
"T" is indicated in square meters, which would be the tolerable area between the measurement and the documentary area.
"Q" is a uncertainty factor which expresses the desired accuracy. This factor is used to define certain parameters as the area grows and is obtained based on sampling tests, it can be used from 2 to 6, and has the objective of weighting the relationship of areas in small, urban, or urban areas. rural.
"to" is expressed in square meters and corresponds to calculated area, this came from the field measurement and calculated on the final map.
"√" refers to the square root
"P" is an adjustment factor that goes from 0 to 1, and has to do with the acceptance requirements that can be given to the techniques of measurement or documentary references, if you have as a field the cadastral record the method of survey and it is known the level of progress that had the registry system between changes of books or milestones in the reforms of notarial registry , this can also be parameterized, the closer you get to 1, the more reliability can exist in the documentation.
For urban or rural parcels with an area equal to or less than 10,000 m2 q = 2
For plots with area larger than 10,000 m2, q = 6
P = 0.1
The programmers were able to make a script that they ran in 11 minutes to a system of more than 150,000 properties. The results at the graphical level were interesting, since it was possible to know the trends of the areas where tolerance was more acceptable and at least the titration process could be prioritized. After this, a classification process and regularization opinions were carried out where both professionals from the cadastral and legal areas were included, we will talk about that another day.
Although the smoke lasted a couple of days to reach that decision, we must recognize that the institutions that regulate the regularization processes in land tenure must take firm steps in formalizing the technical rules of product acceptance ... to date, I believe which do not do that document regrettably.