Q: I have defined my spectrum (u_def) using the "&data curves p_spectrum" command in MOSES.  Then I used the &env command to define my environment for use in the analysis.  With the &env command I also use an Hs, andTp value.  When I plot the u_def (user-defined) spectrum and MOSES defined spectrum (obtained from &status sea_spectrum command) in the same graph in excel. I find the peak values to be different.

A: The reason for different peak values is because the spectrum curve that you have defined (u_def) peaks at the period of 3 seconds. However, you have instructed MOSES to define the environment with the u_def curve but with a different peak period of 3.1 seconds.

When you first input the P_SPECTRUM curve MOSES will automatically find the peak period of the curve data (3 seconds in this case). If this peak value differs from the defined peak period on &ENV -SEA then MOSES will move the peak of the spectrum to the right (when plotted against period) and therefore you have different peaks in your excel graph.

To ensure the input curve matches the spectrum produced by MOSES it is best to enter the curve P_SPECTRUM at the same periods as defined in &DEFAULT -PERIOD. In addition to that, the peak period of &ENV command should be the same as the peak period of the user-defined (u_def) curve.

There is a companion document that shows commands and graphs that better illustrate the above question.  Please ask for the "&data Curves P_Spectrum" located in the Files section.