A summary of the main concepts on Global Ionospheric Map(s) (hereinafter GIM(s)) of Vertical Electron Content (VTEC), with special emphasis on their assessment, is presented in this paper. It is based on the experience accumulated during almost two decades of collaborative work in the context of the International GNSS Service (IGS) ionospheric working group. A representative comparison of both main asessments (VTEC-altimeter and dSTEC-GNSS) is performed around a chain of GNSS receivers mostly placed in islands distributed at different latitudes over the Atlantic Ocean. The consistency between dSTEC-GNSS and VTEC-altimeter assessments is shown (with typical errors of 20% when using one the most accurate IGS GIMs), and explained by means of a simple random model. The authors expect that this manuscript can be useful for new analysis contributor centers and in general for the scientific and technical community interested on simple and truly external ways of validating electron content models of the ionosphere.