In this article are presented the very first maps realised in a systematic way in order to determine the boundaries and register the properties of the Kingdom of Romania, at a scale of 1:20,000 in the equal-area pseudoconical Bonne projection. This projection was not applied in a uniform way for the whole territory of the country, but differently for the areas situated east and west of the Zimnicea meridian, respectively. For each of these two zones I have shown the peculiarities of applying the Bonne projection. furthermore, I explained in detail the means of establishing the nomenclatures of the maps at a scale of 1:20,000, but also at smaller scales, 1:50,000, 1:100,000 and 1:200,000. Since the analyzed map projection preserves area measurements, I verified this property on five maps at the scale of 1:20,000, chosen from both zones. Results obtained for the zone east of the Zimnicea meridian showed a 0.0005% difference, falling within the tolerated error, but for the maps situated west of the Zimnicea meridian, the area differs by about 3%. I have proved that this large difference in the area of the analyzed maps, identified at the eastern extremity of the mapped zone, is due to the fact that they are not actually bordered by the Zimnicea meridian, but by a line related to the Cartesian system used. The Bonne projection maps made for the entire Romanian Kingdom by specialists of the Romanian Army nowadays possess significant historical importance, both in terms of mapping and cadastre.