added initial changelog

docs/changes/2403.bugfix.rst

@@ -0,0 +1,14 @@
+Fixed the definition of ``h_max``, which was both inconsistent between
+`ctapipe.reco.HillasReconstructor` and `ctapipe.reco.HillasIntersection`
+implementations, and was also incorrect since it was measured from the
+observatory elevation rather than from sea level.
+
+The value of ``h_max`` is now defined as the height above sea level of the
+shower-max point (in meters), not the distance to that point. Therefore it is
+not corrected for the zenith angle of the shower. This is consistent with the
+options currently used for *CORSIKA*, where the *SLANT* option is set to false,
+meaning heights are actual heights not distances from the impact point, and
+``x_max`` is a *depth*, not a *slant depth*. Note that this definition may be
+inconsistent with other observatories where slant-depths are used, and also note
+that the slant depth or distance to shower max are the more useful quantities
+for shower physics.