Wordwide analysis of minimum number of GPS satellites available and Maximum PDOP during 24 hours.
If an area has only 4 satellites then there is at least one moment during that in that area only 4 satellites are available. If PDOP is colored purple with number 14 than there is a moment in the day at that location that the PDOP is 14.
Rule of thumb: 3D Position accuracy = PDOP x Range accuracy (Range accuracy: GPS in equatorial areas = 5 meter, GPS in mid-latitude areas = 2 meter, VBS = 0.1 meter (1 decimeter), HP/XP worldwide = 0.02 (2 cm).
To investigate at which time of day the number of satellites is too low or the PDOP is too high see Geosky OmniSTAR worldwide reference stations and GPS Prediction Software.