55 research outputs found
Transformação de coordenadas cartográficas: o exemplo da região de Granja (NW do Ceará, Brasil)
Este estudo tem como principal objetivo mostrar a importância de se efetuar a análise metódica da cartografia de
base que suporta a produção de Cartografia Geológica, antes de iniciar o trabalho de campo. Em certas regiões, as cartas
base publicadas estão ainda associadas a redes geodésicas clássicas, o que obriga a proceder à transformação de coordenadas
para assegurar que todos os dados geo-espaciais são definidos no mesmo sistema de referência espacial. Neste
estudo, selecionaram-se 8 pontos localizados sobre as cartas publicadas na região de Granja (NW do Ceará, Brasil),
determinaram-se as coordenadas no Datum topocêntrico das cartas (CA 70-72) e procedeu-se à sua transformação para
os Data geocêntricos WGS84 e SIRGAS2000. As diferenças entre as coordenadas obtidas nos três sistemas são residuais,
indicando que erros introduzidos quando se desconsideram parâmetros essenciais como o Datum Geodésico são, neste
caso, mínimos para efeitos de Cartografia Geológica.This study draws attention to the importance of conducting a detailed analysis of cartographic products that sustain the Geological Mapping before
starting the field work. In some regions, the published base maps are still related to classical geodetic networks. Therefore, it is fundamental to perform the
adequate coordinate transformation to ensure that all spatial data are defined relatively to the same spatial reference system. This is particularly relevant
when GPS and GIS are used to obtain and plot spatial data. The present study involved the selection of 8 points located on the published maps for the
Granja region (NW Ceará, Brazil), the determination of their coordinates in the CA 70-72 topocentric map Datum and their subsequent transformation
to the geocentric Data WGS84 and SIRGAS2000. The differences between coordinates are residual, indicating that the error introduced when
essential parameters such as the Geodetic Datum are disregarded are, in this case, minimum
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Gaia Early Data Release 3: The celestial reference frame (Gaia-CRF3)
Context. Gaia-CRF3 is the celestial reference frame for positions and proper motions in the third release of data from the Gaia mission, Gaia DR3 (and for the early third release, Gaia EDR3, which contains identical astrometric results). The reference frame is defined by the positions and proper motions at epoch 2016.0 for a specific set of extragalactic sources in the (E)DR3 catalogue. Aims. We describe the construction of Gaia-CRF3 and its properties in terms of the distributions in magnitude, colour, and astrometric quality. Methods. Compact extragalactic sources in Gaia DR3 were identified by positional cross-matching with 17 external catalogues of quasi-stellar objects (QSO) and active galactic nuclei (AGN), followed by astrometric filtering designed to remove stellar contaminants. Selecting a clean sample was favoured over including a higher number of extragalactic sources. For the final sample, the random and systematic errors in the proper motions are analysed, as well as the radio-optical offsets in position for sources in the third realisation of the International Celestial Reference Frame (ICRF3). Results. Gaia-CRF3 comprises about 1.6 million QSO-like sources, of which 1.2 million have five-parameter astrometric solutions in Gaia DR3 and 0.4 million have six-parameter solutions. The sources span the magnitude range G = 13-21 with a peak density at 20.6 mag, at which the typical positional uncertainty is about 1 mas. The proper motions show systematic errors on the level of 12 μas yr-1 on angular scales greater than 15 deg. For the 3142 optical counterparts of ICRF3 sources in the S/X frequency bands, the median offset from the radio positions is about 0.5 mas, but it exceeds 4 mas in either coordinate for 127 sources. We outline the future of Gaia-CRF in the next Gaia data releases. Appendices give further details on the external catalogues used, how to extract information about the Gaia-CRF3 sources, potential (Galactic) confusion sources, and the estimation of the spin and orientation of an astrometric solution
Gaia Early Data Release 3: acceleration of the solar system from Gaia astrometry
Stars and planetary system
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