1. A method for estimating at least one control parameter of an injection system in an internal combustion engine for a target injection quantity, the method comprising the following steps: determining, by a processor, at least one injection control grid with a plurality of grid points, each described by at least one grid parameter and one grid injection quantity, wherein the injection control grid describes an operating range of the injection system, performing at least one isolated test injection by injecting fuel into the internal combustion engine according to at least one test parameter, determining, by the processor, at least one test point based on results of the at least one isolated test injection, each test point being described by at least one test parameter and one test injection quantity, generating, by the processor, a first linear equation based on a restricted linear regression of a first grid point and one or more test points, the first linear equation defining a first line segment extending from a first grid point and approaching an interval around the target injection quantity from a first direction, generating, by the processor, a second linear equation based on a restricted linear regression of a second grid point and one or more test points, the second linear equation defining a second line segment extending from a second grid point and approaching an interval around the target injection quantity from a second direction generally opposite the first direction, wherein the first and second line segments have different slopes, determining, by the processor, an intersection point of the first and second line segments having different slopes, the intersection point defining an estimated value for the control parameter of the target injection quantity, and determining, by the processor, a new value for the control parameter of the target injection quantity based on the determined estimated value for the control parameter of the target injection quantity.