Mohammed Eltahir's blog

  Coordinates system are baaed on assumption that the vertical axes coincide with the rotation axe of the earth, that is derived from another assumption that says the rotation axe of the earth is fixed. Although many scientists proved that this assumption is invalid, it still acceptable for some Geodetic application with resolution of 1 cm or more.   The rotation axe actually moves slightly resulting in very small changes on measurements, which taking them into account when high accuracy is need specially in applications that require accuracy of milometer or less such as detecting and measuring curst movement.   Also the over all shape of Earth change continuously so when the earth’s curst is measured it most be referred . Photo source   For that the International Terrestrial Reference Frame (ITRF) was developed. And an international organization called: International Earth Rotation and Reference Systems service (IERS), which was created in 1988, measure the rotation axe of the

  it defines the relationship between a local and a global three dimensional coordinate system of the same type, and it is defined as a re-position of a reference ellipsoid to be more accurate in representing a specific area . it also called local datum or national datum or just a datum.   Because an ellipsoid may be a best fit globally and the difference between the geoid and ellipsoid (n) be very minimum (globally) but not necessary for a specific area (country for example) so that a datum is created.   As every country wish to have an ellipsoid that is much accurate and close to the real surface of its land, some modifications may be applied to the previously best fitting global ellipsoid (correction for the ellipsoid parameters: a, b or e and f) or global reference system. Photo source   And that by taking a point and consider it as a datum centre point (datum origin point, it’s fixed and all other points are calculated from it), by assuming that this specific point i