Reverse chainages so elevations are monotonically increasing

extract_contours.py

@@ -40,14 +40,17 @@ for beach in beaches:
                     survey = df.loc[(beach, block, profile, date), :]
                     survey = survey.set_index('Chainage')
 
-                    # Find last point in profile above ref contour elevation
-                    last_idx = survey.where(
-                        survey['Elevation'] > contour_z).last_valid_index()
-                    elevation = survey.loc[last_idx:, 'Elevation']
-                    eastings = survey.loc[last_idx:, 'Easting']
-                    northings = survey.loc[last_idx:, 'Northing']
-                    x.append(np.interp(contour_z, elevation, eastings))
-                    y.append(np.interp(contour_z, elevation, northings))
+                    # Reverse survey chainage for interpolation
+                    survey = survey[::-1]
+
+                    # Find largest chainage in profile above contour elevation
+                    idx = survey.where(
+                        survey['Elevation'] > contour_z).first_valid_index()
+                    elevation = survey.loc[:idx, 'Elevation']
+                    eastings = survey.loc[:idx, 'Easting']
+                    northings = survey.loc[:idx, 'Northing']
+                    x = np.interp(contour_z, elevation, eastings)
+                    y = np.interp(contour_z, elevation, northings)
 
                 except KeyError:
                     pass