Introduction to Programming

What is programming for GIS?

What is GIS?

“Maps are both the raw material and the product of geographic information systems (GIS).”

— Xi Liu, Penn State University

“Everything is related to everything else, but near things are more related than distant things.”

—The First Law of Geography by Waldo Tobler

“GIS is like the machinery that transforms data
into the commodity information
that is needed to solve problems or create opportunities.”

—David DiBiase, Director of Education, Industry Solutions, Esri

Welcome to the (GIS) Machine

The Purpose of Tools

Simple Task: Adding First 2 Numbers

\(0 + 1 =\) \(1\)

Simple Task: Adding First 3 Numbers

\(0 + 1 + 2 =\) \(3\)

Simple Task: Adding First 5 Numbers

\(0 + 1 + 2 + 3 + 4 =\) \(10\)

Simple Task: Adding First 10 Numbers

\(0+1+2+3+4+5+6+7+8+9 =\) \(45\)

Simple Task: Adding First 25 Numbers

\(0+1+2+3+4+5+\ldots +23+24 =\) \(300\)

Okay, adding sequences is just …

\(0+1+2+3+4+5+\ldots +23+24 =\) \(\frac{n\;\left(n+1\right)}{2}\)

Examples in GIS

Calculating Distance

Euclidean

Manhattan

Geodesic

GIS: a tool for spatial analysis

“It is difficult to dig a ditch without a shovel,
but it is easier if you have a backhoe.”

High-level language

Object-oriented programming

Third-party packages
(namely ArcGIS and QGIS)

Why Python?

First, it’s considered a high-level language, which means it’s easier—and as a result, generally faster—for us to read and write. This tends to make development quicker and simpler.
Secondly, it’s an object-oriented language, which has several advantages over structured or functional programming, by simplifying the process of creating programs through the use of specially defined classes of objects; these objects encapsulate a specific set of rules, values and functions that can easily be maintained, reused, shared and scaled.
Third, in addition to the standard library, there is a plethora of well-documented and supported third-party packages to help support a variety of projects
- namely, the industry standard GIS software company, ArcGIS, and the world’s leading open-source desktop GIS software, QGIS, both provide Python support

Language & interpreter

We need both.

Semantics & syntax

It’s a language after all.

syntax: the set of symbols and rules that define terms used collectively to form a correctly structured document

semantics: the meaning (intent or purpose) behind the syntax

we prescribe meaning to words; the same goes for statements in programming terms
if I spell out ‘h’ ‘e’ ‘l’ ‘l’ ‘o’ or if I say “hello” you get a sense that I’m greeting you; there’s meaning behind those words
we also know that the general greeting, “hello,” can be spoken in other languages: ni hao (Mandarin), gutentag (German), bonjour (French), shalom (Hebrew), hola (Spanish), ciao (Italian)
the same semantics, but different syntax
does that make sense?
in my experience, a lot of programming begins with you knowing what the intent is (e.g., I want to add all these numbers together) and it’s your job as a programmer to figure out the proper syntax to accomplish the task
just as an aside, know that there are pros and cons to using different programming languages—there’s not one perfect language for everything
some are more catered to or popular with certain fields of study
- FORTRAN for atmospheric science (speed is unparalleled)
- R for biology (strong in statistics)
- MATLAB for engineering
- JAVA and Flash for web apps
- C#, C++ and Lua for video games (I was writing a video game in Python, too)

Examples

English	Python
Write the word “Hello”	`>>> print("Hello")` `Hello`
Add `0+1+2+3+4`	`>>> sum(range(0,5))` `10`
My computer’s home directory is …	`>>> import os` `>>> print(os.path.expanduser("~"))` `C:\Users\twdavis`
What version of Python am I using?	`>>> import sys` `>>> print(sys.version_info.major)` `3`

In the first example, Python doesn’t actually speak, but it writes out the word “Hello”
- Here, we see the first example of what is called a function
- A function typically performs one process or procedure, in this case, it prints whatever is given to it
- If you are familiar with Microsoft Excel (or other spreadsheet software), you’ll know about functions—they have a name and they take values inside their parentheses
- The quotes here are just to let the function know that we want it to print the characters ‘h-e-l-l-o’
In the second example, we see two functions: sum and range
- let’s start inside with the range function: we give this function a value to start at and a value to end before, so range in this example will gives us the values 0, 1, 2, 3, and 4
- we are sending these five values into a function called sum, which like the name suggests, adds up the numbers it’s given
In the third example, we are first introduced to the concept of packages and modules (these are more commonly referred to as libraries; they define new functions and provide additional capabilities);
- the ‘import’ statement is a keyword for Python to lookup and load an “expansion pack” to the standard library
- we also see the dot notation—packages are often organized hierarchically (library --> book --> chapter)
- the output here shows the directory of the user’s home folder, which you can tell is on a Windows machine because of the backslashes
The fourth example, we see the import keyword again to load the “sys” library
- Here, we access the major number of the version_info module in the sys library
- It’s probably good to mention here that the Python community is currently split (and has been for over a decade—since late 2008) between those who stay in Python 2 land and those who moved on to Python 3 (Python 3 has some fundamental differences in syntax and semantics that many who developed for years in Python 2 more or less refused to update all their code; hence we have two major versions available)
- For GIS, QGIS works with both 2 and 3, but ArcGIS Pro works only with Python 3; if you’re new to Python, learn 3

Exercises

Shared on our AGOL Group.

Exercise 1 - Python Fundamentals
Exercise 2 - More Fundamentals