The Unofficial XCSoar Manual
Faq:Terrain
From XCSoar
Why do the terrain heights seem to show an error?
Most of the terrain data files provided use GTOPO30 data as the source. This data uses a 30 arc second grid, approximately 900m square. The heights on this grid are probably an average height of the terrain within the cell.
As a result of this, the presence of small hills and gradual slopes will result in terrain heights reported by XCSoar being in error.
Work is planned to process higher resolution terrain data in order to produce terrain files where the heights on the grid are the maximum heights of the terrain in the cell. This will result in more conservative AGL heights.
Secondly, GPS altitude is not particularly accurate, so AGL heights may show an additional error due to the GPS altitude error. The configuration option "Nav by Baro Altitude" uses barometric altitude for all navigation functions, and if the barometric altimeter has been properly QNH compensated, then this error is substantially reduced; however, changing QNH pressure during the day may result in pressure altitude errors being reflected in the AGL heights.
Where XCSoar is used with the Vega variometer, QNH compensation is performed automatically by setting the QNH such that the barometric altitude equals the terrain altitude when on the ground. Due to the limited resolution of the terrain data as explained above, this itself can result in a small offset error.
