There are a number of position-fixing methods. Feature matching compares features
at the current location, such as landmarks, waypoints, or terrain height, with a
map to determine the current position. This is generally easier for a human than
for a machine. Position fixes may also be obtained by measuring the ranges and/or
bearing to known objects. This is illustrated for the two-dimensional case by Figure
1.1, whereby X marks the unknown user position, and A and B the known positions
of two reference objects.