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Student Surveyor

Good day everyone!,

Monuments are all disturbed hence, a relocation survey should be done. Can I ask if what would be the procedure that should be done?

Thanks,

Rose

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  • Land Surveyor

    Currently, there are 4 coordinate systems used in the Philippines. Local Plane Coordinate System, Luzon 1911 (old PTM), PRS-92, & ITRF. Based on personal experiences and several cases of study most common problem with land property in the Philippines is the disturbance of monuments resulting to relocation surveys or verification surveys. Go back to the question above, relocation survey in old school preferably uses traditional method using total station, transit, etc. and try to adopt the localize system where the lot/land originally surveyed by checking first all the existing monuments within the mother lot and adjoining lots especially if it is already a titled property. After calculating and checking, the surveyor needs to choose or pick the common reference point and distribute or carry the allowable errors to his orientation, to begin with, his setting out of points for the lost/disturbed monuments but in such cases that there were no good/correct monuments observed that’s the time to run a traverse and looking for a good local reference nearby the property. Perhaps, old school but effective! Now, the advent of GPS/GNSS RTK Surveying techniques is more comfortable to use by surveying professionals because it will speed up the work in less time. Mostly, the surveyor will not undergo any more with checking of the existing visible monuments within the property but rather use the equivalent/identical coordinates of the property using 7 transformation parameters while some are only 4 transformation parameters and uses the nearest PRS-92 Ground Control Point (GCP) as GPS base, to begin with. While the rotational parameter does not affect technical descriptions significantly, it should be observed that the scale parameter affects side distances and areas of parcels at a certain level. Thorough analysis is advised before the transformation of cadastral data. For me, no doubt it’s more economical & reliable, but there’s still something that needs to be considered regardless of the accuracy level you’ve got during survey execution. One of the case studies found that gaps & variances are visible to a minimum of 1-4 meters after the transformation, consequential to property disputes between adjacent owners.

  • Land Surveyor

    Dear Ms. Septimo,

    If this project is important to someone and involves datum transformations, make sure to engage someone with experience in that exercise. There are several places where a subtle error can have dire consequences.

    Software can be a very valuable tool. But, it cannot make intelligent decisions or analysis.

    JAC

    • Student Surveyor

      Thank you sir JAC. I have a companion in the field though yet I want to have knowledge as well so that I can relate and perhaps share ideas to the chief surveyor. It is my first job in surveying so I'm trying to learn as many things as I can.

  • Investigation shall be initially carried out to determine the coodinate system employed in the network the monuments are tied to. Perform coordinate transformation in order to achieve homogenious coordinate system. The original coordinate system (be it local or grid) shall be transformed into PRS92 (WGS84 variant). It is only then you can effectively carry out relocation survey using the RTK technique.

    A very common practice in Philippine RTK surveying is the localization technique. In my opinion, it is an admission of guilt. It is a way of accepting others fault and reusing it. It simply propagates error. As project in charge, you should be on top of your game. You should always be able to identify and/or verify point positions relative to what the government prescribes. It is never the other way around.

    Finally, as what J. Anthony Cavell suggests, always observe redundancy. This simply promotes precision.

    My two cents.

    • Student Surveyor

      Sir, what is a localization technique in RTK surveying in the Phil. It's vague in my understanding. I presume it is when, you discovered something wrong about the control points or your reference points  or overlapping of adjacent lots and you just go along with it?

      If we're gonna use both RTK and TS, how will it affect the survey? I learned from school that coordinates from RTK has reference datum WGS 84 while TS is in local coor?

      • Land Surveyor

        Localization is not caused by finding an error. In the most usual cases, it is about teaching one's GNSS to speak the language of a local plane survey.

        The GNSS determines one's position relative to the center of the earth (ECEF) as x, y, z and usually expresses it as Lat, Lon, Ellip. Hgt.

        Most projects are designed or measured in a plane coordinate system often expressed as North, East, Elevation.

        If the plane system is a common well-defined projection,the definition is likely relative to the ECEF.

        However, the plane system may be quite arbitrary, as in a large plant or even a city grid. In such cases we must teach our GNSS how to speak in the local dialect; to translate from ECEF to N,E,Elev. It is like a dictionary for your GNSS.

        A localization looks a lot like an adjustment. It is NOT. In an adjustment the most probably correct values are assigned to the local control points. In a localization the GNSS is taught how to present a value that is the closest possible to what one would get using the local control with conventional means (tape, transit, &c.).

        Since no measurement is exact, there will always be some disagreement between two significant surveys. One may find an older survey reported on a well defined projection, disagrees with current measurements to a degree that is uncomfortable. The project may already be underway and it is imperative to agree with control already in place. One would use a localization for this so as to agree with the control already in place.

        One last note: in the process of establishing the localization parameters, one or more control points may be impossible to fit with the others to an acceptable degree. Here the expert will have to decide what control remains "good" and what is "bad" (perhaps disturbed by equipment &c.) and eliminate the bad ones from the localization.

        JAC

      • I developed my own utility to automate local to grid transformation.

        Localization technique is a process of determining transformation parameter values in which WGS84 coordinates are transformed into a plane linear system. This simply means that your GPS/GNSS data controller/collector coordinate readouts are in local coordinate system. Localization only works if residual values are within acceptable range. This also means that the process provides the ability to determine relative accuracy of lot corner monument position.

    • Student Surveyor

      Good day sir, we're gathering the lot data computation of the sub-lots and adjacent lots. And other control points that are undisturbed so, we can have a reference.

      • Transform the coordinate system employed in your lot parcels to conform to PRS92. Please note that this involves 7-parameter transformation and not just a 3D affine solution. This ensures non-linear best fit solution thus provides a very reliable outcome. Effecting common difference would never provide you meaningful outcome.

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