Calculate unknown value between the known values

Mick

Good day, everybody.

Could you help me to develop a simple script?

I have two columns x and y (around 15 pairs).

I would like my script uses the known values to calculate the unknown values if I have only y values between the known values.

 For example, if y = 0.25, x = 2.5.

I think to provide .csv file with two columns (x and y) and another .csv with only one column y, so the script can calculate the known values applying the first .csv file.

I guess the output in python will be okay.

 

JohannesLindner

try:
    from tkinter import filedialog
except ImportError:
    import tkFileDialog as filedialog
import csv

   
def interpolate(xy, x):
    """Linear interpolation

    xy: list of given data points as [(x, y)]
    x: x-value of unknown data point

    Returns the y-value as a linear interpolation between the given data points

    """
    xy = sorted(xy, key=lambda p: p[0])  # sort list by x
    x0, y0 = [p for p in xy if p[0] <= x][-1] # get data point with next smaller x
    x1, y1 = [p for p in xy if p[0] >= x][0]  # get data point with next larger x
    y = y0 + (y1 - y0) / (x1 - x0) * (x - x0)  # linear interpolation
    return y
    

def interpolate_empty_values(xy):
    """Fills empty x or y values by linear interpolation using the known data points.

    xy: list of data points as [(x, y)], x OR y can be null

    Returns a list with interpolated values

    """
    known_xy = [(x, y) for x, y in xy if not (x is None or y is None)]
    known_yx = [(y, x) for x, y in known_xy]
    filled_xy = []
    for x, y in xy:
        if not (x is None or y is None) or (x is None and y is None):
            filled_xy.append((x, y))        
        elif x is None:
            # Be very careful when interpolating x!
            # Interpolating x for things like relief plots, time series,
            # quadratic functions, etc. will give unintended results.
            x_ = interpolate(known_yx, y)
            filled_xy.append((x_, y))
        elif y is None:
            y_ = interpolate(known_xy, x)
            filled_xy.append((x, y_))
    return filled_xy



# load your csv file
csv_file = filedialog.askopenfile(title="Select csv file")
# generate [(x, y)]
csv_data = [line for line in csv.reader(csv_file)]
csv_file.close()
header = csv_data[0]
csv_data = csv_data[1:]
xy = []
for d in csv_data:
    x = None if d[0] in ['', ' '] else float(d[0])
    y = None if d[1] in ['', ' '] else float(d[1])
    xy.append((x, y))
# run interpolate_empty_values
new_xy = interpolate_empty_values(xy)
# write the result into a new csv file
new_csv_file = filedialog.asksaveasfile(title="Save csv file")
writer = csv.writer(new_csv_file)
writer.writerow(header)
for xy in new_xy:
    writer.writerow(xy)
new_csv_file.close()

 

Paste that code into a script and run the script. You don't need arcpy or ArcGIS at all. In fact, expect errors or crashes when using the tkinter methods (filedialog.*) in the ArcGIS python window.