A method of geometrically determining Position Dilution of Precision (PDOP) is given. The method gives an insight which is lacking in the purely mathematical determination. The method applies to 3-dimensional GPS fixes. With slight modification it can be applied to 2-dimensional GPS fixes, to 2-dimensional hyperbolic fixes, or to 2-dimensional range-range fixes. Practical examples are given, and the results are shown to agree with the purely mathematical determination.