Validation of satellite data is performed via a comparison of derived altimetric time series and ground-based stage measurements. Gauging stations closest to the satellite overpass are selected and data compared on a daily basis.
The accuracy of the altimeter data is dependent on a number of factors, including the size and nature of the target. To date, elevation variations deduced from the TOPEX/POSEIDON mission have been observed to be accurate to 3-4cm rms for the largest lakes, (e.g. Lake Ontario). This will decline to ~5cm-10cm rms for the smaller lakes (100km2) under consideration. The accuracy of T/P elevation time series for rivers, wetlands and floodplains can range from ~10cm to tens of centimetres depending on the target size. See the Reference section (Birkett, 1995a and Birkett, 1998) for full time series accuracy and validation details.
Example 1 - Lake Ontario - TOPEX/POSEIDONWater level variation for 1 pass across Lake Ontario in comparison with traditional gauge (stage) data. Lake results are denoted by triangles (TOPEX) and squares (POSEIDON). The altimeter results have been shifted vertically, by an arbitrary constant amount for gauge comparisons. Dates are marked at intervals of 4 months. Lake stage data from the Oswego gauge courtesy of NOAA.Click on the image below for the full-scale version.
Example 2 - Amazon River - TOPEX/POSEIDONWater level changes for 3 passes across the main stem of the Amazon River in comparison with river stage data. River results are denoted by triangles, floodplains by squares. The altimeter results have been shifted vertically, by an arbitrary constant amount for gauge comparisons. Dates are marked at intervals of 2 months. River stage data from Tabatinga, Manacapuru and Parintins, courtesy of ANEEL/Brazil.Click on the image below for the full-scale version.
Example 3 - Paraguay River - TOPEX/POSEIDONHere, the river itself is too narrow for the altimeter to detect, particularly at low water levels. However, time series of water height variations for its floodplain can be derived and compared with river gauge data. River stage measurements courtesy of the Brazilian Navy at Ladario.Click on the image below for the full-scale version.
Time series of Jason-1 IGDR lake height variations (stage) compared to averaged daily gauge data for the 2002-2003 time period. The rms comparison is ~7.5cm. This value will be representative of similar sized lakes. The lower than expected stage value in April, 2002 is due to poor surface tracking on this overpass.
