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Which equipment to use? For topographic maps it is compulsory to work in the differential GPS mode using carrier-phase processing and preferably "real time" (RTK) methods. For supplying a GIS with data, instruments with metric accuracies are sufficient in quality. To feed a GIS, metric devices provide sufficient quality. When is GPS preferred to conventional methods? In open, non-urban spaces without high vegetation (savannah, farm land, airports, golf courses, etc.) or in residential or industrial areas (low fences or walls, sidewalks without trees, buildings with retracted façades). Advantages of GPS surveys Conventional topographic surveys require direct sight between the instrument (theodolite or total station) and the target. When surveying in urban areas, there is no getting around to setting up the instrument at every intersection, causing unnecessary loss of time. Moreover, when working alone with a motorized instrument, it is not always safe to leave the instrument unguarded. Knolls or mounds (even low ones), and slopes or hedges away from urban areas can also limit the range of the instrument. With a GPS receiver, there is no need for reciprocal sights between the base and rover stations. The only stipulation is that it must be within transmission range (usually about 3 to 4 km). It is thus easy to leave the base station in a secured location, even if distant, and survey continuously. And with a bit of luck, the working area is serviced by a real-time GPS network, so that there is no need to set up one's own base station. Direct tie to a geodetic network With the proper training, it is usually possible to tie a survey to a national geodetic or leveling network by using post-processing to calculate accurate coordinates for the reference point based on a permanent GPS network. In emerging countries where the geodetic infrastructure is often sparse, this task can be performed by relying on international GPS networks. This is especially the case for obtaining homogeneous levelings over distant areas without having to previously establish a network or a traverse.
Survey of topographic maps
Click on the picture to enlarge
Topographic survey for a new ring around Maurecourt (78)
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TopoSat - Topography by Satellite
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