GEOG 378

Introduction to Geocomputing

Well-known Text (WKT)

a text markup language for representing vector geometry objects on a map, spatial reference systems of spatial objects and transformations between spatial reference system

Well-known Binary (WKB)

used to transfer and store the same information on databases

Points/Lines/Polygons

an area consists of lines, which consist of points, which consist of coordinates

.shp

shapefiles are storing vector-based geometric informtion of geographic features

.prj

a file containing coordinate system information to provide spatial referencing of data in ArcGIS

.shx

the index file that stores the index feature geometry

.dbf

the dBASE table that stores the attribute information of features

Topology Matters

allows many GIS operations to be done without accessing the point files (efficiency)

Incorrect calculations (perimeter, area, attributes)

Avoid data analysis nightmares

Raster data is best for:

Continuous data- things that change smoothly across space

Examples: elevation, temperature, rainfall, satellite images or aerial photos

Vector data is best for:

Discrete data- things that have clear boundaries or locations

Examples: raods, rivers, city boundaries, land parcels, utility lines

Interpreter

a program that executes computer code (source code →interpreter→ output)

Compiler

translates the program into low-level machine instructions (source code→compiler→object code→executor→output)

Syntax Errors

refer to the structure of a program and the rules about syntax

Runtime Errors

error does not appear until you run the program; also called exceptions

Semantic/Logic Errors

codes can run succesfully but it will give out the wrong result

long (data type in python)

used for huge integers that are outside the range of int

complex (python data type)

representing numbers with a fractional part

type()

displays the python type for any value

type(3) → int

type(True) → bool

Logical Operators

<,>,==,!=,>=,<=, and, or, not

Membership Operators

in, not in

float(), str(), int()

converts the value inside to whatever type it will be (type conversion)

id() function

takes a value or a variable and returns an integer that acts as a unique identifer which is guaranteed to be unique and constant for this object during its lifetime

dot notation

specify the module and then the function followed by a dot (e.g. math.log(x))

Modularity

breaking a system or program into smaller, independart parts that can be developed, tested, and reused separtely

if__name==”__main__”:

to make sure certain code only runs when the file is executed directly, not when it is imported as a module into another program

for loop

picks each element out of the sequence and sets the loop variable equal to that item (perform a block of code for a number of times)

a sequence is:

1. a string of characters: “Python”

2. a list ex: [1,2,3], [‘a’,’b’,’c’]

3. a tuple (“geog”, 378, “intro to geocomputing”)

range(start,stop,step)

range(5) → [0,1,2,3,4]

range(7,3,-1) → [7,6,5,4]

break

end (or break out of) a loop

continue

skip the rest of the loop body statements, but do not end the loop

while loops

causes a block statement to be repeated as long as iteration condition is true

ex:

count = 10

while count > 0

     print count

     count -=1

print(“Blastoff”)

try/except statements

let you “try” a piece of code that might cause an error, and then “handle” that error if it happens instead of stopping your whole program

example:

try:

     #code that might cause an error

except:

     #code that runs if there is an error

5 Types of Containers

lists, strings, typles, dictionaires, and sets

Lists

values that make up lists are elements [0,1,2,3]

list()

use this when converting another iterable/copying

Slicing Lists

a[1:3] → 1,2

a[2:] → 2, 3,4,5…

[:] → the whole list

Lists are…

mutable (we can change elements)

del

removes an element from a list

ex: del a[1]

len()

returns the length of a list

list.append(x)

append an item x to the end of a list

list.sort()

order items in ascending orders

list.sort(reverse = True)

order items in descending order

Give a sequence of numbers, how to find the largest? (8,15,45,23,88,68,75)

numbers = [8,15,45,23,88,68,75]

largests = numbers[0]

for n in numbers:

if n > largest:

largest = n

print(largest)

dictionary {}

key value pairs

• keys must be immutable (cannot be change)

• keys must be unique - no dulpicates

  • repeated values are okay

Access to Dict

dictName[Key] = value

Strings

• can slice strings

• strings are immutable, can’t change an existing strings

• to get the last letter do:

  • last[length-1]

String Transversal

processing a string one character at a time

How to count the frequency of a character in a string? Define a function in Python

Find the frequency of ‘a’ in “banana”

fruit = “banana”

count = 0

for char in fruit:

      if char == ‘a’:

         count = count + 1

print count

Tuples

like list, elements can be any type and variable from element to element

also immutable (cannot be change)

Set

an unordered collection of unique elements

used for checking duplicates or generating unquie collections

open a file

f = open(filename, mode, buffering)

• mode/buffering optional

Read and/or Write a file

f.write(…)

Close a File

f.closed

Open File Modes

rt", "rb", "wt"
read text, read binary, write text

Methods for Reading a File

• use a for loop to read every line

• use a while loop to read every line until the end of the line

Good practice to use the with keyword when dealing with files

• PRO: file is closed properly close after it is suite finishes, even if exception is raised on the way

with open (filename, ‘r’) as f:

read_data = f.readlines()

print(f.closed) #TRUE

How to list all files in a directory, and then print all files names with the .csv extensions?

import os

files = os.list.dir(‘.’)

for f in files:

     if f.endwith(‘csv’):

print(f)

Class

describes what something is and what it can do

(concept of lake)

Object

a thing (something you can imagine or describe)

• attribute: detailes or characteristics (what it has)

• behaviors: actions (what it can do)

  • Ex: a paticular lake

Syntax for generating an instance of a class

classInstance = ClassName()

Data Attribute Assignment

data attributes, like methods, are accessed by using dot notation

ex: pt1 = Point()

pt1.latitude = 34.05

pt1. longitude = -118.24

Protected Attributes

Prefixed with a single underscore
Can still be accessed outside of the class

Private Attributes

Prefixing with a double underscore
Can be access but cannot be changed

Class Methods

like functions but are defined within the class and are therefore part of the class

class ClassName:

     def method1(self, parm1, parm2,…):

            statement 1 

            statement2

dir() function

returns a list of strings containing all the names of valid attributes and methods for the object

init_:

initialize object attributes automatically when created

str:

defines how the object prints as text

dict or vars(obj)

shows all attributes of an object

Inheritance

lets a new class (child) reuse and exten code from another parent

the child inherits all parent method but can override them

How to represent cities in GIS using the classes and objects in Python?

create a class with several attributes, such as name, latitude, longitude and so one

Class Composition

classes can be nested like functions