python: slope.py: Use cell size in polyfit and plot functions.

* python/slope.py (plot_curve)[PX]: New argument.

1 files changed, 10 insertions, 6 deletions

diff --git a/python/slope.py b/python/slope.py
index 0bf99b3..cd1dea7 100755
--- a/python/slope.py
+++ b/python/slope.py
@@ -122,7 +122,11 @@ def detach_display():
     fig.canvas.callbacks.disconnect(cid)
     #print(xMarker)
 
-def plot_curve():
+def plot_curve(PX):
+    """
+    Fit straight line to channel elevation data.  Pixel size PX is
+    used to convert pixel indices to co-ordinate values.
+    """
     # sort markers by y value
     #yx = list(zip(yMarker, xMarker))
     #yx.sort()
@@ -156,13 +160,13 @@ def plot_curve():
     # Add to list.  Repeat.
 
     # Fit with polyfit
-    m, c = np.polyfit([499.75 - y*0.5 for y in ych], zch, 1)
+    m, c = np.polyfit([499.75 - y*PX for y in ych], zch, 1)
     print('gradient =', m, 'intercept =', c)
 
     fig, ax = plt.subplots()
-    line1, = ax.plot([499.75 - y*0.5 for y in ych], zch)
-    line2, = ax.plot([499.75 - y*0.5 for y in ych],
-                     [c + m*499.75 - m*y*0.5 for y in ych], '--')
+    line1, = ax.plot([499.75 - y*PX for y in ych], zch)
+    line2, = ax.plot([499.75 - y*PX for y in ych],
+                     [c + m*499.75 - m*y*PX for y in ych], '--')
     ax.set_xlabel('Distance / m')
     ax.set_ylabel('Height / m')
     ax.set_title('Channel profile')
@@ -206,7 +210,7 @@ while (True):
     elif(input_str == "m"):
         detach_display()
     elif(input_str == "p"):
-        plot_curve()
+        plot_curve(DY)
     elif(input_str == "s"):
         save_xyz()