update field with change in Z value from one point to the next

[IanIrmischer]

I am trying to write some python code to calculate a change in Z value from one point to the next. I have a number of point files with Z values in a field called "RASTERVALU".  I have a field called zdiff that I would like to update with " Zdiff(row1)=Z(row2)-Z(row1)". I can use the update cursor it to look at the backwards change (Zdiff(row1)=z(row1)-Z(row0)) but not the forward change (Zdiff(row1)=z(row1)-Z(row0)).

I used the following code below but it seems like the updaterow.next() is messing up my sequential flow through the rows.  Let me know if anyone has any ideas:

updateRows=arcpy.da.UpdateCursor(OutputFileName,["RASTERVALU","zDiff"])

for row in updateRows:

nextRow=updateRows.next()

zdiff=nextRow[0]-row[0]

row[1]=zdiff

updateRows.updateRow(row)

The first run through the loop works ok but then it iterates to the third row instead of the second row. I assume it is because of my use of updateRows.next() but I am not sure. I can’t find very good documentation of the updateRow.next or any of the cursor information.

Any thoughts?

Ian

[JakeSkinner]

Hi Ian,

Here is one way you can do this:

with arcpy.da.UpdateCursor(OutputFileName, ["RASTERVALU", "zDiff"]) as cursor:
    firstTime = True
    for row in cursor:
        if firstTime:
            diff = row[0]
            row[1] = 0
            firstTime = False
        else:
            row[1] = row[0] - diff
            diff = row[0]
        cursor.updateRow(row)

del cursor

[IanIrmischer]

Hi Jake, Thanks for your help.  I may be mistaken but I think your code gets me the backward calc.  I need the forward calc. I put your code in and ran it and it gave me the below output.  I need the -1.442169 in the first row, not the second row.  This is because it is associated with other attributes in that row. 

Thanks again for your help,

Ian

[JakeSkinner · Accepted solution]

Try the following:

valueList = []

with arcpy.da.SearchCursor(OutputFileName, ["RASTERVALU"]) as cursor:
    for row in cursor:
        valueList.append(row[0])
del cursor

x = 1

with arcpy.da.UpdateCursor(OutputFileName, ["RASTERVALU", "zDiff"]) as cursor:
    for row in cursor:
        try:
            value = valueList
        except:
            value = valueList[-1]

        row[1] = row[0] - value
        x += 1
        cursor.updateRow(row)

del cursor

[IanIrmischer]

Jake, Thanks so much.  That worked and it was really helpful.  The code with the searchCursor will also help me many other places in my work.

Thanks again,

Ian