Ellipsoid, geoid, proiezioni, DATUM termini sconosciuti troppo spesso a chi approaches the GIS world. The GIS sector is in full evolution, especially in the last decade, and professionalism has approached it which are increasingly distant from the world of engineering and architecture which, historically, are those that most they used GIS knowledge/skills.
Like any self-respecting discipline, GIS also has its rules, its fundamentals, essential actions that are the basis of any type of GIS-based project started by any professional and knowing the meanings of the terms mentioned above is one of the fundamentals.
Yesterday I launched a small survey on my page Facebook
Just taking inspiration from this question I will try to do a little of clarity on the ellipsoid and why it is necessary to use it.
The Earth does not have a regular shape, it is not spherical(never flat!!geodetica?
The solid that most closely approximates the shape of the Earth is l’ellipsoid; normal text-decoration: none; vertical-align: baseline white-space: pre-wrap;"> an ellipse that rotates on one of its two axes; in our specific case it rotates on the minor axis.
The ellipsoid therefore allows us to represent the Earth mathematically, albeit by approximation. There are many ellipsoids, with the advent of GPS technology at the beginning of the 70s of the last century, WGS72 and WGS84; geocentric ellipsoids, i.e. the center of the ellipsoid coincides with the center of the Earth.
Another example can be the Hayford ellipsoid, of reference for example for the reference system Roma 40 and oriented on the Monte Mario meridian. Always the same ellipsoid is used for reference system ED50, oriented on Potsdam.
The Bessel ellipsoid is known, for example, because it is fundamental part of the reference system Cassini-Soldner, reference system used for the Italian land registryThe WGS84 is used, for example, in the reference system UTM.
Will It is clear now that the ellipsoid is used to approximate the shape of our planet but on its own, the ellipsoid is not always useful. It is in fact a fundamental part of the cartographic reference systems and based on the area of interest there will be one or more reference systems that can use a specific ellipsoid, perhaps oriented on that area to further reduce positioning errors.