In this paper we explore the maximum precision attainable in the location of a point source imaged by a pixel array detector in the presence of a background, as a function of the detector properties. For this we use a well-known result from parametric estimation theory, the so-called Cramér-Rao lower bound. We develop the expressions in the 1-dimensional case of a linear array detector in which the only unknown parameter is the source position. If the object is oversampled by the detector, analytical expressions can be obtained for the Cramér-Rao limit that can be readily used to estimate the limiting precision of an imaging system, and which are very useful for experimental (detector) design, observational planning, or performance estimation of data analysis software: In particular, we demonstrate that for background-dominated sources, the maximum astrometric precision goes as B/F 2 , where B is the background in one pixel, and F is the total flux of the source, while when the background is negligible, this precision goes as F −1 . We also explore the dependency of the astrometric precision on: (1) the size of the source (as imaged by the detector), (2) the pixel detector size, and (3) the effect of source de-centering. Putting these results into context, the theoretical Cramér-Rao lower bound is compared to both groundas well as spaced-based astrometric results, indicating that current techniques approach this limit very closely. It is furthermore demonstrated that practical astrometric estimators like maximum likelihood or least-squares techniques can not formally reach the Cramér-Rao bound, but that they approach this limit in the 1-dimensional case very tightly, for a wide range of S/N of the source. Our results indicate that we have found in the Cramér-Rao lower variance bound a very powerful astrometric "benchmark" estimator concerning the maximum expected positional precision for a point source, given a prescription for the source, the background, the detector characteristics, and the detection process.