Several years of cadastre, and this question is very common always. Which method is better to do cadastre?
We admit that this is not a recipe, because there are different conditions that must be taken into account and each method can have opposite variables in different territories. So to give light to the post, chew some aspects that may be useful for decisions, in passing the productive of a discussion forum brought a couple of days ago.
Why municipal cadastre. I clarify this, because the post is applicable to an environment, in which a municipality wants to do its cadastre, either by its own means or with the support of a cooperation project. It does not apply to a large modernization project at the regional or national level, led by a centralized entity, which will have better conditions to execute the project and more money to spend ... although also more surplus indicators to fulfill.
The scenario is, therefore, a municipality, of regular size, that will have at most 5,000 urban properties in its municipal head, some 4 large communities, but with less than 1,000 real estate and the rest rural area or as they call it of the Other side, rustic.
Cadastre for what. This is important to define, because the criteria of precision for cadastre with legal approach will not be the same if the focus is only fiscal or control in the use of land. Also because if there is a Appraisal methodology, The measurement of buildings or valuation of the permanent crop requires other criteria to make the process more efficient.
The priority for a municipality that does not have a cadastre is not in its precision, it is in having it as a place to use it. So we must consider methods that are sustainable, that help end the complete uprising of the municipality, to devote to use, update and improve its accuracy.
Some methods I've tried. The last four years we have tried different alternatives, depending on the conditions of the municipality, here I summarize some:
- Photogrammetry. Little by little, in urban areas this method is falling into disuse, mainly because it is not economical in relation to its accuracy. No company will make a flight to 10,000 meters high for a very small urban area, doing so for the entire municipality may be unattainable with own funds. Then if photointerpretation is used in urban areas it is always necessary to measure the fronts and finally the precision will not be very good in areas where people take out a knife for 10 centimeters. However, in the case of rural areas, it is very practical because greater coverage is achieved without the need to travel each boundary and the precision is relatively sufficient since the plots have large areas.
- Photointerpretation + GPS. If you have the orthophoto you can apply perfectly to the rural area with very good results. Clarifying, we are talking about orthophoto coming from an aerial photograph, because the orthorectified satellite image that now has a pixel less than one meter has too many distortions in areas of irregular topography, for that grace better to use Google's ortho. In practice I have observed that combining the use of printed (orthophoto) enlargement and low accuracy GPS (Garmin 3 to 5 meters) brings more practical results than clicking the aerial photographs and stereoscope and then passing it to the enlargement.
I do not say that they are discardable but questionable for projects of small municipalities, in their validity before other facilities that now allow the GPS with raster deployment or because it is not always possible to have available the pairs or human resource capable of mastering the technique. The pricking of photographs does not bring advantages in matters of precision, because only the width of a rapidógrafo on a printed orthophoto 1: 10,000 will be 10 meters plus the error that already accumulated the orthorectification software. Also the question whether the edge of a hill that the stereoscope shows very well but is not seen in an impression, is discarded because practice shows that this is possible for a technician who comes from the conventional method, a novice will not see it by any of the two methods and it will be better to take a couple of points with the GPS to get your bearings. And then with the interpretation of other details such as land use, current remote sensing techniques do better and cheaper work with supervised classification.
- GPS + Compass. This method is very practical if you have little silver. I've used it in urban areas, taking advantage of a pair of submetric precision GPS to process the grid of streets, and using a compass to tie the ends. As tape is used to measure the fronts, the error is transferred to the street, leaving a relative precision of boundaries in less than 10 centimeters and absolute respect to the moorings with GPS near the subway. You have to measure the funds and take the courses with triangulation. It is not appropriate if the survey has legal implications, if property titles or cadastral certifications with legal value will be given; for that, it will occupy field inspection at the moment of the request.
- GPS + total station. This method is functional, because it allows very good precision and information in 3 dimensions that sooner or later will be useful. It requires a pair of GPS to georeference the first two starting points, and to take a few -sufficient- control points that prevent an error in the wrong view back shot. It is not necessary to have a total station, as it can be rented, as well as GPS points that can be hired individually. The funds will always have to be measured, which can be supported by the orthophoto, compass or triangulation by berths, which is the most appropriate.
Which I recommend.
If it were my turn to decide, for urban I would go by total station. Take some kids out of the baccalaureate in computing, train them and release them to work out. Also for a municipality or commonwealth or consortium of municipalities, acquiring a total station that runs through the $ 7,000 is not a bad investment, since the use beyond cadastre in survey, reformulation or execution of engineering projects is a good investment. You just have to look for human resource training.
And in this I speak of a conventional total station, Robotics Does not apply to environments that labor is relatively cheap and where if you close half an eye they steal the station, the cell ... And if you still have the honor.
In conclusion, whoever decides the method must understand that the cadastral map will always be an imprecise reflection of reality. And however accurate our current measurement is, in a few years Will be questioned By its relative imprecision with respect to a mountain of Mars.
The best method is the one that is sustainable with own funds, whose investment recovers in the short term and with which we will have the whole municipality raised in only two periods of government.