I often see the same questions in the keywords by which users arrive at Geofumadas from the Google search engine.
Can I register using Google Earth?
How accurate are the images in Google Earth?
Why is my survey displaced with respect to Google Earth?
Before I get penalized for what you will read in this article, let me put you in context of the experience I had when carrying out a cadastral survey in a project where the breaking of paradigms in search of results was more valuable than adherence to methodologies and protocols. of traditionalists.
When I dimensioned the variables of what it meant to perform orthophoto for the 25 municipalities that needed to do the survey, I realized that there were things that did not converge:
-The time to make flights had already passed, because the country is tropical and there is an optimum time before which the cloudiness, smoke and weather conditions,
-They were years in which the satellite image bought by scraps was not an option with the precisions that are now offered,
-The public institution that granted the permits to fly was half plated the old way, wanted a million of money (above the table, of course), by the dispersion of each municipality. Apart from that the plane charged me an extra for uploading a chubby that the institution said was the only one able to monitor those flights.
-The money that was available had barely reached to make good orthophoto, but little uprising.
-While I had the money, doing the orthophoto entailed times that prevented obtaining results before the dates of approval of the cadastral values for the change of five years.
In reviewing the results sought by the project, I realized that the disruptive methodology was more important through a joint cadastre model than precision. It was more important to demonstrate the multifunctional model than the optimal quality of the simple legal approach. So, I preferred to submit to derision and rush for short-term results.
The article is based on that experience, more as an approach of technical application and common sense than a magic recipe; although arbitrarily I used examples from a municipality, where I must admit that the «choros soup»It brings back memories that go beyond the merely geomatic context.
Google Earth images have good precision (Relative).
Let's see the sample image. At the level of continuous coverage, we can see that the survey we did with the total station and georeferenced with geodetic gps, fits very well with the image that was before 2013. Of course, for this, it was necessary to lower the image to make it hunt with the control points that we had raised. In this case, a displacement (of the image, not of the survey) of about 11 meters to the north-west had to be made.
The image of Google Earth needs to be displaced with respect to our precision survey. Once this is done, the image shows consistency.
Google Earth images do not have consistency in absolute precision.
Continuing with the same example, we see that the image that Google Earth updated in 2013 has a different displacement than the previous one. The overlap between the images has a gradient so well done that little is seen discontinuity; in the case of the road, see that as it is in the direction of the cut it is not seen as if it were transversal, but to the right of the survey you can see how the stream no longer coincides with the survey; Although it could have changed course, who knows it knows that there is a retaining wall from the bridge that has not changed in years.
So, using that image that is more recent implies the same work as with the previous one; determine identifiable points and associate them with control points in the field, to then move the image a vector, which we are clear is different from that of the other image. The practice led to interesting solutions, in which the image of Google Earth was a reference to errors made with the total station, such as losing the view back, the detection that a team needed calibrated, the validation of the cadastral quadrants that obey a partition according to exact degrees, minutes and seconds, and what not to say as the evidence of a three-dimensional map showing the differences in cadastral value paid for land and buildings. These things, without a reference image or the simple use of a CAD / GIS program are almost impossible.
The base of images of Google Earth is a set of fragments of shots of different years, with different sources and with inconsistency of absolute position between these pieces.
The positional accuracy of Google Earth is accurate.
Putting aside the problem of the images, the characteristics of the spheroid that Google Earth uses, are precise with respect to any survey projected on it. In the image above, when showing the UTM coordinates in Google Earth, on a file of my survey that I have uploaded as kml, the precision of the coordinate has no discussion about the Datum WGS84, because that is a mathematical fact.
In the middle of the Project, the gurus of the state institution passed with high-precision super-teams. We told them that they could use our survey as support, because they were georeferencing properties for a titling project on demand. It was difficult for me to remove a lump from my throat when they disparaged one of the cadastre kids, telling him that his uprising did not work.
The utility of Google Earth for cadastre is your benefit as support
The fact is that taking the previous decisions allowed us to give Google Earth the use and merit that it deserves. Like all the other tools used in the project, Google Earth is one more.
Google Earth is irreplaceable for the use of images that are not available, not only for a current year but also for other historical dates. I remember a meeting in which a mayor participated, explaining the results: «Google Earth has beenan accessible solution to have images of the entire municipality, which never before offered us any company or institution of the state«. When asked by a 70s Catastro guru, that accuracy was bad, his words are recorded in my videos: «These guys solved it, if you have a better proposal, write it down and we'll consider it«.
What we should not forget, is that those in management require easy tools to demonstrate results at the central level. There are people who will never go to the field, and before them graphic demonstrations are needed for which Google Earth was irreplaceable at that time. Open a kml or a WMS service and show them that there are the urban and rural areas of the municipality, with the digital terrain model and the buildings with their elevations depending on before and after the entrance of the project ... it is an invaluable experience. They are unaware of the accuracy, they ignore how we adapt the method, but they are satisfied to see graphical results and they approve rupture of administrative bottlenecks or absurd requirements of reports of many pages.
Downloading the images from Google Earth was a carpentry thing. The value of the project was in the joint management; it was not necessary to buy a total station or millimeter GPS per municipality. With one per commonwealth it was enough, and they continue doing so since they only take turns during the year to follow up the survey or update with the resources that they committed annually as a re-investment of the collected and topographic survey in road projects or systems hydrosanitary.
Resist the option to use Google Earth as a reference, it is only to fight against a wall. Those who have gone to provide cadastral services to the municipalities in this context will tell me that now the mayors do not want to pay for a utraprecise job, but for a local capacity building service, use of low-cost resources and advice so that the decisions be correct
And then ...
After all those years, I must admit that there were mistakes that I made, and of which I would write a broader article than this one. I would have liked free software to be more mature, so we would have saved another stack of money; or that the crowd mapping and cadastre fit for purpose had more diffusion, because it would have cost me less to explain practices that since that time we implemented. But to the results today I refer:
- The cadastre model based on joint management of municipalities is there, being adopted by other ventures, not because of its innovation but because of the lessons learned.
- Instead of doing cadastre in 25 municipalities, the experience led to doing it in 89. Just to take advantage of the economy of scale of joint management, and of course, use Google Earth images as support.
- The innovation in local human resource and economic counterparts from where others would not look, led to demonstrate that every dollar invested by the municipality, recovered it in less than two years and multiplied it six times in revenue in a period of 10 years.
- The municipalities that achieved full coverage of their territories, are currently updating using the income resource they receive from the entire territory, and they are changing the metadata of accuracy and quality and geometry.
Near the end of the project, I met again the gurus of the state institution, who were validating some problems that had their uprisings, which had already given titles and entered the national system. First, with an authoritarian tone, they told us that we should give them the maps they had uploaded. When we gave them an address for download via WFS we were seen as aliens, then one of the guys they despised showed them using gvSIG; He told them that from there they could be downloaded whenever they wanted. His arrogance changed his face, and we exchanged our irony for something of pity, before his next comment:
Excuse me, Don Golgi, what we really want is that you provide us with the images that you downloaded from Google Earth.