Definability over Linear Constraints

We settle a number of questions concerning definability in first order logics with an extra predicate symbol ranging over semilinear sets. These questions are motivated by the constraint database model for representing spatial data. We give new results both on the positive and negative side: we show that in first-order logic one cannot query a semi-linear set as to whether or not it contains a line, or whether or not it contains the line segment between two given points. However, we show that some ofthese queries become definable if one makes small restrictions on the semi-linear sets considered.