Calculating slope for points with downhill slopes rendered as negatives in ArcGIS

Question

I have a DEM and a series of points. I'd like to measure the change in slope along the series of points. Distinguishing between uphill and downhill slopes matter to me, and ArcGIS' slope function renders all slopes as absolute values.

I'm working with ArcGIS' Spatial Analyst and 3D Analyst toolkits.

This is what I've done so far.

1. Created points
2. Run the "interpolate shape" tool
3. Run the "slope" tool
4. Run the "extract multi values to points" tool to add two fields Elevation and Slope to the points feature class.

I have followed the advice here (http://blogs.esri.com/esri/supportcenter/2012/03/09/calculating-the-slope-of-a-hiking-trail/) to change the downhill slope values into negatives, but have not succeeded. When I run the code (below), nothing happens.

fc = r"C:\Points\Test.gdb\Points_3d"     
cursor = arcpy.da.UpdateCursor (fc, ["Elevation","Slope"])     

firstRun = True 

for row in cursor:
   if firstRun:
      oldValue = row[0]
      firstRun = False
   else:
      if oldValue < row[0]:
         row[1] = row[1]
      else:
         row[1] = (-1)*row[1]
      oldValue = row[0]
      cursor.updateRow(row)
del row 
del cursor

Answer 1

You don't need to be updating row[1] for every feature, so cursor.updateRow(row) should be under the final else statement. Also, I'd suggest using with statements as closing is better supported:

fc = r"C:\Points\Test.gdb\Points_3d"     
with arcpy.da.UpdateCursor (fc, ["Elevation","Slope"]) as cursor:
    firstRun = True 

    for row in cursor:
       if firstRun:
          oldValue = row[0]
          firstRun = False
       else:
        #We don't need to check less than since we aren't negating it.
          if oldValue > row[0]:
             row[1] *= -1
             cursor.updateRow(row)
          oldValue = row[0]

I checked similar code on a sample dataset (see below) and it correctly negated the features based on the if statement. If this still isn't working, I'd check to see if your if statement ever evaluates to True.