1. Data Analysis using Python-I
Baburao Kamble and Ayse Kilic
University of Nebraska-Lincoln
University of Nebraska-Lincoln
GIS in Water Resources Lecture, 2014

2. Objectives To be able to understand and write Python scripts for
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Objectives
To be able to understand and write Python scripts for
Numerical, Text (Weather) & Geospatial (Environmental) data manipulation

3. Agenda Part I: Basic Python Programming
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Agenda
Part I: Basic Python Programming
Python Syntax, Strings, Array, Conditional and Control flow, Functions
File Input/Output and Text Data Processing
Part II: Geospatial Data Analysis
Installation of GDAL bindings on windows operating system
Reading Raster (Landsat and DEM) data
Using GDAL function from command line (interpolation, translation)
Raster Processing.
NDVI calculation
DN2Rad2Ref

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What is Python?

5. Why Python? Natural Language ToolKit Ease of use; interpreter
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Why Python?
Natural Language ToolKit
Ease of use; interpreter
Processing: Symbolic
Python’s built-in data-types for strings, lists, and more.
Java or C++ require the use of special classes for this.
Processing: Statistical
Python has strong numeric processing capabilities: matrix operations, etc.
Suitable for probability and machine learning code.

6. Writing a Program in Python
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Writing a Program in Python
A program is a collection of Python statements, functions, etc.
A script is a list of commands which are run by a program written in a specific language such as Python
Scripts can be edited by text editors or preferably by a specialized editor that also allows the script to run
We can use Python command line in Windows
However, the use of the command line is limited (it does not support testing scripts)
So, instead we use the Python script editor (an IDE) that has a better interface than the command line mode

7. Installing Python Python is installed on Lab Computers.
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Installing Python
Python is installed on Lab Computers.
Python for Win/Mac/Unix/Linux is available from
Generally an easy install.
On Mac, already part of OS X.
For GDAL you need Python 2.6 or 2.7.
Python 3.X is little-bit different
GUI development environment: IDLE (Integrated DeveLopment Environment).

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IDLE
We will also continue using the IDLE environment for Python scripting
IDLE is available with Python download
Using the interactive Python interpreter is great for writing small code
However, it is not meant to be saved!
For larger, more complex multi-line code, it is better to write it as a script (rather than using the interactive Python interpreter, e.g., IDLE) or configure ECLIPSE for Python

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Elements of Programs
Simpler expressions can be combined using operators.
+, -, *, /, **
Spaces are irrelevant within an expression.
The normal mathematical precedence applies.
2/4
2*3

10. Variable “Variables” in python are really object references.
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Variable
“Variables” in python are really object references.
Can contain letters, numbers, and underscores
Must begin with a letter
Cannot be one of the reserved Python
pi = HW = "Hello, world" i = 2+7 print type(pi) print type(HW) print type(i) Output: <type 'float'> <type 'str'> <type 'int'>

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Operators

12. Simple Sample of Python code
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Simple Sample of Python code
Sample.py

13. Array & Lists How to Define array How to define list
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Array & Lists
How to Define array
How to define list
myArray=numpy.array([2,3,4,5,6,7])
myArray
myArray[0]
myArray[4]
myList=[2,3,4,5,6,7]
myList
myList[1]
myList[4]=99

14. Modules (Library) & Packages in Python
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Modules (Library) & Packages in Python
A module is a file containing Python definitions and statements. 
A module is a single file (or files) that are imported under one import and used. e.g.
A package is a collection of modules in directories that give a package hierarchy.
import my_module
import numpy
from my_package.timing.danger.internets import my_module

15. Using the numpy Library
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Using the numpy Library
Besides (+, -, *, /, //, **, %, abs), we have lots of other math functions available in a math library.
A library is a module with some useful definitions/functions.
To use a library, we need to make sure this line is in our program:
Offers Matlab-ish capabilities within Python
import numpy

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Functions
A function is like a subprogram, a small program inside of a program.
The basic idea – we write a sequence of statements and then give that sequence a name. We can then execute this sequence at any time by referring to the name.
The part of the program that creates a function is called a function definition.
When the function is used in a program, we say the definition is called or invoked.

17. Functions Define them in the file above the point they're used
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Functions
Define them in the file above the point they're used
Body of the function should be indented consistently (4 spaces is typical in Python)‏
Example: square.py
def square(n): return n*n print "The square of 10 is ", print square(10) Output: The square of 3 is 9

18. Functions That Return Values
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Functions That Return Values
Sometimes a function needs to return one or more values.

19. Let’s Consider Celsius to Fahrenheit temperature conversion program
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Let’s Consider Celsius to Fahrenheit temperature conversion program
def main():
celsius = input("Input Celsius temperature:")
fahrenheit = 9/5 * celsius + 32
print "The temperature is", fahrenheit, "degrees Fahrenheit."
main()
ConvertTemp.py

20. Python Program: Quadratic equation/Numerical Calculations
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Python Program: Quadratic equation/Numerical Calculations
Let’s write a program to compute the roots of a quadratic equation!
The only part of this we don’t know how to do is find a square root… but it’s in the mathematical library!
Lets get mathematical library inside python.
quadratic.py

21. Let’s Consider Celsius to Fahrenheit temperature conversion program
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Let’s Consider Celsius to Fahrenheit temperature conversion program
Let’s say we want to modify that program to print a warning when the weather is extreme.
Any temperature over 90 degrees Fahrenheit and lower than 30 degrees Fahrenheit will cause a hot and cold weather warning, respectively.

22. Three major control constructs of programming
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Three major control constructs of programming
Sequential: Simply do steps one after the other in order they are listed.
Conditional: Decide which statement to do next based on some true/false test.
Iterative: A set of statements is repeated over and over until some condition is met.

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Multi-Way Decisions
if <condition1>: <case1 statements> elif <condition2>: <case2 statements> elif <condition3>: <case3 statements> … else: <default statements>

24. Forming Simple Conditions
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Forming Simple Conditions
Python
Mathematics
Meaning
<
Less than
<= ≤
Less than or equal to == =
Equal to
>= ≥
Greater than or equal to
>
Greater than != ≠
Not equal to
ControlStruct_Demo1-3.py

25. Text Data Processing file = open("test.txt")
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Text Data Processing
file = open("test.txt")
# Read everything into single string:
content = file.read()
# file.read(20) reads (at most) 20 bytes
print (len(content))
print content
# close file
file.close()
#CSV file:
#import csv library
import csv
#open csv file
myfile = open('weather.csv', "r")
#read content of file
reader = csv.reader(myfile)
# iterate over the rows
for row in reader:
print row,
myfile.close()
File_IO.py

26. CSV file read/write/manipulation
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CSV file read/write/manipulation
Estimating daily evapotranspiration (ET) using the California Irrigation Management Information System (CIMIS) Weather Data
CIMIS_ETr.py
Please refer ASCE manual for Evapotranspiration equations
(Allen et al, 2005)