Dear Colleagues, I would like to know, that how are geo coordinates obtained from the actual version of google maps accurate? Are they 'quasi absolutely' (I know there is no absolut location determination)? In other words, if I record the geocoordinates of a field object (e.g. a smaller tree or schrub in a meadow) using a two-frequency geodetic GPS receiver and I also obtain from the geocoordinates of the same object from the the Google maps layer, are these lat and lon coordinates quasi the same or there is a deviation berween the coordinates recorded by high precision receiver and the coordinates derived from the software? I mean question in the same coordinate system, of course. Sorry for my question, but I am not a GIS expert, just a biologist, so I am a little bit confused regarding that. And I must to know it for field measurement purpose. I am waiting for you helpful answers, Thanks, Imre
Depends largely on the size of the object, where it is located (a shrub in a park in downtown NY is better georeferenced than one somewhere in Siberia), the imagery creation date, the sensor used, the quality of the GPS signal on site, etc.
Estimado Imre, tuve el mismo problema al tratar de ubicar con coordenadas geográficas sitios donde estudiaba migraciones de aves en Argentina. Encontré que los mapas de Google no eran tan precisos como los de mi GPS, creo que los mapas de google toman solo localidades bien conocidas. En Argentina muchas localidades en zonas montañosas aisladas que es casi la mitad del país no están bien indicadas en los mapas de Google
Dear Imre Majláth, the positions/coordinates obtained with high-precision GPS receivers are absolute while those of the Google maps are quasi. The coordinates determined using high-precision GPS receivers are absolute in the sense that during the field observation, the errors in the GPS signal are modelled/computed and applied as a correction to the computed coordinates. When using the relative/differential method of GPS observation, two or more receivers (rover and base/reference receivers) are employed. The base receiver is usually set at a control station (a station whose coordinates are known) while the rover receiver(s) is/are set at the new points i.e., points whose coordinates are to be determined. During the observation, both receivers (rover and base receivers) receive signal from the same set of satellites. Since the base receiver knows its position accurately in a specified coordinate system, it computes its position/coordinates using the received satellites signal and compares it with the known coordinates to determine the error in the computed position. As the rover and the base receivers receive signal from the same set of satellites, it is assumed that the error in the signal received by the base receiver is the same as the one in the signal received by the rover receivers. The error determined by the base receiver is used to adjust the rover receivers coordinates to obtain the final/adjusted coordinates of the new points. The GPS observations can also be processed with respect to COR station data.
On the other hand, the coordinates obtained from the Google maps are quasi as their reliability depends on the following:
1. the accuracy with which the images used for the production of the maps are merged and georeferenced.
2. the accuracy with which the actual points is snap on the maps. Using the geographic coordinate system, latitude and longitude, one has to be very careful, because 1'' (one second in arc) is approximately 30m. Therefore, if the point is not precisely snap on the maps, coordinates different from the actual position of the point might be obtained.
Best regards
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