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Home » BAS Research » Instruments and Techniques » Instruments » Global Positioning System (GPS) »
Global Positioning System (GPS) data gives us accurate positions for ground features, which are required for compiling our maps. Photogrammetry, (the technique for making maps from aerial photographs), and tying satellite imagery to its true ground position require coordinates and elevation information for features that can be clearly identified on the images or photos.
We use a highly accurate GPS so that error is kept to a minimum from the start of the project. To do this we use high precision geodetic GPS (dual-frequency Trimble 5700 receivers) and a technique called Differential GPS Surveying to measure these 'ground control points'. This method involves having one GPS receiver on an already accurately known survey point, the 'base-station' collecting GPS data simultaneously with other 'roving' receivers measuring new, previously unsurveyed points. It requires the base station position to be measured for over an hour but means the observation time at each point collected by the remote receiver needs to be only 15 minutes. The data is recorded on a compact flashcard inside the receiver and it can then be post-processed using Trimble software. Precise positions are calculated for the base stations (using NASA's JPL AutoGIPSY processing service) and the baseline between the two receivers is used to calculate the position of the unknown point to sub-centimetre level accuracy.
GPS Survey example
The area surveyed by GPS
At the cairn on Stork Ridge
On South Stork
Measuring the tripod height on Leonie Island
Later this data was post-processed and used for orientating the aerial photographs in space for producing a large-scale new map of the area with contours and other topographic information.
Identification of survey points on an aerial photograph
An additional method of acquiring this information is by recording GPS on the aircraft as we take the aerial photographs of the area . The precise positions of each of the photograph frame centres reduce the number of positions we need to collect on the ground. For example, with frame-centre GPS positions, the minimum number of ground control points for a block adjustment is only 4 well-placed points.
Other recent GPS survey projects have included taking positions in an area at the southern end of the Antarctic Peninsula in order to georeference satellite images in this previously un-surveyed region. This involved setting up a base camp for a few days and being flown on a Twin Otter aircraft close to each nunatak being surveyed, walking across the ice and climbing up to the summit to set up the roving receiver ( see 2002/3 Diary entry ).
Strategy for GPS survey fieldwork in Ellsworthland
If there are areas that are inaccessible by aeroplane or boat from our stations, then sometimes the only option is to reach the points by helicopter. This was the case in 2005 when we carried out a survey on Pourquoi Pas Island and were assisted by the Royal Navy ship, the HMS Endurance , Lynx helicopters ( see 2004/5 Diary entry ). It is certainly an incredible experience to be able to survey areas that have been visited rarely, if ever, before.
Surveying operation assisted by Lynx helicopters on Pourquoi Pas Island
Surveying operation assisted by Lynx helicopters on Pourquoi Pas Island
© NERC-BAS 2012
