[http://www.bayo.com/]
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Aerial photograph and orthophoto map Compared to a topographic map, an aerial photograph has the advantage of showing all of a given territory at the moment the picture is taken with the following reservations: - The minimum size of visible objects depends on both the scale and the resolution of the photograph. - Elements covered by vegetation are not visible (unless the image is taken when trees have no foliage). - The scale is not homogenous, which means that the raw aerial photograph cannot be used as a background map except in very limited cases. Orthophoto maps are aerial photographs which were rectified by photogrammetric methods to eliminate deformations due to differences in height, perspective or the optics of the camera. The resulting photographs have geometric qualities similar to those of topographic maps and can be assembled (image mosaicking) for showing an overview of a project area. This process requires the survey of ground control points. Survey of ground control points These surveys mostly use post-processing GPS techniques (dual frequency) involving base data from one or (usually) several permanent GPS station(s) or base data from a hinge point set up nearby, whose coordinates are determined by data from several distant permanent GPS stations. Whenever possible, points which seem permanent were chosen. Supplied items Each surveyed point is supplied with an identification sheet including: - a situation map, - a survey sketch, - 3 digital photographs For points surveyed in France, TopoSat provides RGF93 (ETRF89) geodetic coordinates with ellipsoidal heights as well as grid coordinates (Lambert 93, Lambert 2 extended or Lambert Zone) with NGF heights (or other legal systems for the French overseas territories (DOM TOM)). Abroad, we provide WGS84 (ETRF89, ITRF2000 or ITRF2005) geodetic coordinates as well as grid coordinates (UTM / EGM96) or coordinates in local systems when the required transformation parameters are available or can be calculated. All our points are calculated with an accuracy better than 10 cm (estimation) and are usually chosen in such a way that they can be used with aerial photographs (10 cm pixel size) as well as with very high-resolution satellite images. Archived points We have built a large database comprising all points surveyed since 2007 (including some surveyed in 2006). The distribution of these points and the corresponding station descriptions (with approximate coordinates) can be seen on our web page "Archived GPS ground control points".
Rectifying aerial photographs with GPS
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Archived points
[http://www.toposat.fr/gcp/ALCR08/ALCR08-01.pdf]
PDF report sample (535 ko)
Click on the picture to enlarge
Survey of ground control points
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TopoSat - Topography by Satellite
[http://www.toposat.fr/pages/imag_aeropag.html]
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