A fully automated, high resolution method to measure the variable line density of a spherical grating is presented. The assessment of the line density requires an accurate evaluation of the first order diffraction angle, from which the line spacing is calculated. The optic characteristics (curvature, variable line density, length) make the measurement of this angle difficult to achieve, due to the limited dynamic range of the typical long trace profiler (LTP) used as a detector. We developed a new, fully automated method to assess the line density N(w) of a 280 mm long spherical grating (R ≈ 65 m). N(w) and the curvature of the optics could be measured on the same setup without intermediate realignment of the sample. The principle: the grating position is scanned with the linear stage while a feedback loop between the LTP detector and the round table maintains the Littrow condition. The angle of the round table is recorded as function of grating position. The accuracy of the angle assessment is then improved by subtracting the recorded residual angle measured by the LTP. Our method avoids stitching and overcomes the dynamic range limit of typical LTP instruments while it still benefits from the high resolution offered by its detector. The line density of the grating under test was determined with an estimated accuracy of ∆N of 5 mm^-1 for a central line density of 1160 mm^-1 . This method can be also applied to shape measurements of long steeply curved mirrors, whose accurate curvature evaluation is limited by the dynamic range of the typical LTP.