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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
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<title>ArcGIS for Economics: Lecture 8</title>
<meta name="description" content="ArcGIS for economics researchers">
<meta name="author" content="Masayuki Kudamatsu">
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</head>
<body>
<!-- To activate the CSS framework of Reveal.js -->
<div class="reveal">
<!-- To reconfigure an HTML5 <section> element as a slide using CSS. -->
<div class="slides">
<!-- Slide contents start here -->
<!-- My own slide templates -->
<section id="title">
<h3 style="color:slategrey">
Causal Inference with Spatial Data
</h3>
<p style="color:slategrey">
(ArcGIS 10 for Economics Research)
</p>
<h3>
Lecture 8
</h3>
<h2>
Map Algebra
</h2>
<br>
<p>Masayuki Kudamatsu</p>
<p>7 December, 2018</p>
<br>
<p>
<small>Press <span style="color:red;">SPACE</span> to proceed. </small>
<br>
<small>To go back to the previous slide, press <span style="color:red;">SHIFT+SPACE</span>. </small>
</p>
</section>
<!-- Standard text slide -->
<section id="map_algebra" data-transition="slide-in fade-out" style="text-align: left;">
<h3>
What is Map Algebra?
</h3>
<p>
Cell-by-cell calculation across multiple raster datasets
</p>
<p>
1. Arithmetic operations
</p>
<ul>
<li>
with numbers (e.g., <code>raster*2</code>)
</li>
<li>
across several raster datasets (e.g., <code>ras1 + ras2</code>)
</li>
</ul>
<p class="fragment" data-fragment-index="1">
2. Functions
</p>
<ul class="fragment" data-fragment-index="1">
<li>
<a href="http://desktop.arcgis.com/en/arcmap/latest/tools/spatial-analyst-toolbox/an-overview-of-the-math-tools.htm" target="_blank">Math toolset</a> (square root, logarithm, sine/cosine, etc.)
</li>
<li>
<a href="http://desktop.arcgis.com/en/arcmap/latest/tools/spatial-analyst-toolbox/cell-statistics.htm" target="_blank">Cell Statistics</a>
</li>
<li>
<a href="http://desktop.arcgis.com/en/arcmap/latest/tools/spatial-analyst-toolbox/focal-statistics.htm" target="_blank">Focal Statistics</a>
</li>
</ul>
</section>
<!-- ROAD MAP -->
<section id="road_map" style="text-align: left;">
<h3>
Today's road map
</h3>
<p>
1. Mayshar, Moav, Neeman, & Pascali (2015)
</p>
<p>
2. Arithmetics in Map Algebra
</p>
<ul>
<li>
Dictionary in Python
</li>
</ul>
<p>
3. Cell Statistics
</p>
<p>
4. Focal Statistics
</p>
<ul>
<li>
Replicate ruggedness index in Nunn & Puga (2012)
</li>
</ul>
<p>
5. Other geo-processing tools used in economics
</p>
</section>
<!-- SECTION for paper -->
<section>
<section id="section1">
<h1>
1. <a href="http://cepr.org/active/publications/discussion_papers/dp.php?dpno=10742" target="_blank">Mayshar, Moav, Neeman, & Pascali (2015)</a>
</h1>
<p>
This study was featured in <a href="https://www.washingtonpost.com/news/wonk/wp/2016/04/25/the-secret-ancient-history-of-the-potato-that-could-change-the-story-of-civilization/">Washington Post</a>
</p>
</section>
<section id="rq1" data-transition="slide-in slide-out" style="text-align: left;">
<h3>Research Question</h3>
<p>
What caused the formation of the state?
</p>
<p>
Important?
</p>
<ul class="fragment" data-fragment-index="1">
<li>
Early state formation: associated with higher living standards today (<a href="http://dx.doi.org/10.1023/A:1020827801137" target="_blank">Bockstette et al. 2002</a>, <a href="http://dx.doi.org/10.1007/s10887-007-9017-z" target="_blank">Gennaioli & Rainer 2007</a>, <a href="http://dx.doi.org/10.3982/ECTA9613" target="_blank">Michalopoulos & Papaioannou 2013</a>)
</li>
</ul>
<p>
Original?
</p>
<ul class="fragment" data-fragment-index="2">
<li>
Previous studies focused on agriculture in general
</li>
<li>
This study: focus on crop type (cereal vs tuber/root)
</li>
<li>
Convincing causal evidence
</li>
</ul>
<p>
Feasible?
</p>
</section>
<section id="" data-transition="slide-in fade-out" style="text-align: left;">
<h4>
Theory (1 of 3)
</h4>
<h3>Roots and Tubers</h3>
<p>
Potatoes, cassava, yam, taro, bananas...
</p>
<p class="fragment" data-fragment-index="1">
Perishable upon harvest
</p>
<p class="fragment" data-fragment-index="1">
Harvesting is non-seasonal, not visible (underground)
</p>
<p class="fragment" data-fragment-index="2">
$\Rightarrow$ Costly to steal / confiscate
</p>
</section>
<section id="" data-transition="fade-in fade-out" style="text-align: left;">
<h4>
Theory (2 of 3)
</h4>
<h3>Cereals</h3>
<p>
Wheat, rice, maize...
</p>
<p class="fragment" data-fragment-index="1">
Storable
</p>
<p class="fragment" data-fragment-index="1">
Harvest within a short period of time, clearly visible
</p>
<p class="fragment" data-fragment-index="2">
$\Rightarrow$ Cheap to steal / confiscate
</p>
</section>
<section id="" data-transition="fade-in slide-out" style="text-align: left;">
<h4>
Theory (3 of 3)
</h4>
<h3>State formation</h3>
<p>
Building a state incurs a fixed cost
</p>
<p>
With roots & tubers, no incentive to build a state
</p>
<p class="fragment" data-fragment-index="1">
Historical examples
</p>
<ul class="fragment" data-fragment-index="1">
<li>
Ancient Egypt: wheat with state
</li>
<li>
New Guinea: yam/taro without state
</li>
</ul>
<p class="fragment" data-fragment-index="2">
cf. Coltan vs gold in DR Congo (<a href="http://dx.doi.org/10.2139/ssrn.2358701" target="_blank">Sanchez de la Sierra (2017)</a>)
</p>
</section>
<section id="data1" data-transition="slide-in fade-out" style="text-align: left;">
<h3>Data</h3>
<p>
State formation
</p>
<ul>
<li>
Ethnographic Atlas for cross-sectional regression
</li>
<li>
<a href="http://dx.doi.org/10.2139/ssrn.2464285" target="_blank">Borcan et al. (2014)</a> for panel cross-country regression
</li>
</ul>
</section>
<section id="data2" data-transition="fade-out fade-in">
<h4>
Jurisdictional Hierarchy
</h4>
<img src="gis_lecture8/MMNP2015F4.png" style="border: none; background: none; box-shadow:none;">
<p>
<small>(Figure 4 of <a href="http://cepr.org/active/publications/discussion_papers/dp.php?dpno=10742" target="_blank">Mayshar, Moav, Neeman, & Pascali (2015)</a>)</small>
</p>
</section>
<section id="data3" data-transition="slide-in fade-out" style="text-align: left;">
<h4>Data (cont.)</h4>
<p>
Relative productivity of cereals vs. tuber/roots
</p>
<ul>
<li>
Potential caloric yields by crops, obtained from Global Agro-Ecological Zones (GAEZ)
</li>
</ul>
</section>
<section id="data4" data-transition="fade-in fade-out" style="text-align: left;">
<h4>
Data (cont.)
</h4>
<h3>
<a href="http://gaez.fao.org/" target="_blank">GAEZ</a>
</h3>
<p>
Resolution: 5 x 5 arc-minutes (about 10 X 10 km)
</p>
<p>
Data: potential yields by crop, based on climate & soil
</p>
<p>
$\Rightarrow$ Exogeneous to human activities
</p>
<p>
$\Rightarrow$ Widely used by economists
</p>
<ul>
<li>
<a href="http://dx.doi.org/10.1093/qje/qjr009" target="_blank">Nunn & Qian (2011)</a>, <a href="http://dx.doi.org/10.1086/684719" target="_blank">Costinot et al. (2016)</a>, etc.
</li>
</ul>
</section>
<section id="" data-transition="slide-out fade-in">
<h4>
Cereal's relative productivity to tuber/root
</h4>
<img src="gis_lecture8/MMNP2015F7.png" style="border: none; background: none; box-shadow:none;">
<p>
<small>(Figure 7 of <a href="http://cepr.org/active/publications/discussion_papers/dp.php?dpno=10742" target="_blank">Mayshar, Moav, Neeman, & Pascali (2015)</a>)</small>
</p>
</section>
<section id="equation1" data-transition="slide-out slide-in" style="text-align: left">
<h3>
Empirical specification: cross-section
</h3>
<p>
Sample: 952 pre-colonial societies
</p>
<br>
<p>
<!-- Case by Case -->
\begin{align*}
y_{ic} = \beta CAL\_DIF_i + \gamma MAX\_CAL_{i} + \mu_c + \varepsilon_{i}
\end{align*}
</p>
<br>
<table border="0" width="100%">
<tr>
<td style="text-align: center">$y_{ic}$</td>
<td># jurisdictional levels beyond villages</td>
</tr>
<tr>
<td style="text-align: center">$CAL\_DIF_i$</td>
<td>Cereal's relative caloric yield to tuber</td>
</tr>
<tr>
<td style="text-align: center">$MAX\_CAL_i$</td>
<td>Maximum caloric yield among all crops</td>
</tr>
<tr>
<td style="text-align: center">$\mu_c$</td>
<td>Continent fixed effects</td>
</tr>
</table>
<p class="fragment" data-fragment-index="1" style="color:slategrey">
We will learn how to calculate $CAL\_DIF_i$ and $MAX\_CAL_i$ from GAEZ data
</p>
</section>
<section id="result1" data-transition="slide-in slide-out">
<h4>
Main results (cross-section)
</h4>
<img src="gis_lecture8/MMNP2015T3.png" style="border: none; background: none; box-shadow:none; width: 800px;">
<p>
<small>(Table 3 of <a href="http://cepr.org/active/publications/discussion_papers/dp.php?dpno=10742" target="_blank">Mayshar, Moav, Neeman, & Pascali (2015)</a>)</small>
</p>
</section>
<section id="equation2a" data-transition="slide-in fade-out" style="text-align: left;">
<h4>
Natural experiment:
</h4>
<h3>
Colombian exchange
</h3>
<p>
Until 1500:
</p>
<table border="0" width="100%">
<tr>
<td style="text-align: center"></td>
<td>Cereals</td>
<td>Tubers</td>
</tr>
<tr>
<td>New World</td>
<td>Maize only</td>
<td>All but yams</td>
</tr>
<tr>
<td>Old World</td>
<td>All but maize</td>
<td>Yams only</td>
</tr>
</table>
<p class="fragment" data-fragment-index="1">
$\Rightarrow$ Around 1500, cereals' relative productivity to tubers changed exogenously!
</p>
</section>
<section id="equation2b" data-transition="fade-in fade-out" style="text-align: left;">
<h3>
Empirical specification: panel
</h3>
<p>
Sample: 159 countries from 1000 to 2000 (every 50 years)
</p>
<br>
<p>
<!-- Case by Case -->
\begin{align*}
y_{ct} = \beta CAL\_DIF_{it} + \gamma MAX\_CAL_{it} + \mu_c + \eta_t*x_c + \varepsilon_{i}
\end{align*}
</p>
<br>
<table border="0" width="100%">
<tr>
<td style="text-align: center">$y_{ct}$</td>
<td>Government above tribal level</td>
</tr>
<tr>
<td style="text-align: center">$\mu_c$</td>
<td>Country fixed effect</td>
</tr>
<tr>
<td style="text-align: center">$\eta_t*x_c$</td>
<td>Year fixed effect * country characteristics</td>
</tr>
</table>
</section>
<section id="result2" data-transition="fade-in slide-out">
<h4>
Main results (panel)
</h4>
<img src="gis_lecture8/MMNP2015T6.png" style="border: none; background: none; box-shadow:none;">
<p>
<small>(Table 6 of <a href="http://cepr.org/active/publications/discussion_papers/dp.php?dpno=10742" target="_blank">Mayshar, Moav, Neeman, & Pascali (2015)</a>)</small>
</p>
</section>
</section>
<!-- Preparation -->
<section>
<section id="preparation1" data-transition="slide-in fade-out" style="text-align: left;">
<h3>
Prepare for the rest of this lecture
</h3>
<p>
1. Launch ArcMap 10 (it takes time)
</p>
<p>
2. <a href="https://www.dropbox.com/s/337ah1lv05muk8v/Lecture8.zip?dl=1" target="_blank">Download the zipped dataset for Lecture 8</a>
</p>
<p>
3. Save it to <strong>Desktop</strong> (<code>C:\\Users\\<em>yourname</em>\\Desktop</code>)
</p>
<ul style="margin-left: 72px;">
<li>
Don't save in the remote server, which slows down ArcGIS
</li>
</ul>
<p>
4. Right-click it and choose <span style="background-color: Gainsboro">7-Zip > Extract to "Lecture8\"</span>
</p>
<ul style="margin-left: 72px;">
<li>
So the directory path will be: <code>C:\\Users\\<em>yourname</em>\\Desktop\\Lecture8</code>
</li>
</ul>
</section>
<section id="preparation2" data-transition="slide-out fade-in" style="text-align: left;">
<h4>
Prepare for the rest of this lecture (cont.)
</h4>
<p>
Now in ArcMap's <strong>Catalogue Window</strong>:
</p>
<p>
5. Establish connection to data folder
</p>
<ul style="margin-left: 72px;">
<li style="margin-bottom: 5px;">
Right-click <span style="background-color: Gainsboro">Folder Connections</span>
</li>
<li style="margin-bottom: 5px;">
Select <span style="background-color: Gainsboro">Connect to Folder</span>
</li>
<li>
Choose <span style="background-color: Gainsboro">Desktop > Lecture8</span>
</li>
</ul>
</section>
<section id="map_algebra_python" data-transition="slide-in slide-out" style="text-align: left;">
<h4>
Mentaly prepare for the rest of this lecture
</h4>
<h3>
No Model Builder today
</h3>
<p>
We directly write a Python script to use Map Algebra
</p>
<ul>
<li>
Raster Calculator tool does the same job in Model Builder
</li>
<li>
But it's not meant to be exported as a Python script
</li>
</ul>
</section>
</section>
<!-- SECTION for ArcMap -->
<section>
<section id="section2">
<h1>
2. Arithmetics in Map Algebra
</h1>
</section>
<section id="" data-transition="slide-in fade-out" style="text-align: left">
<h3>
Data we want to construct
</h3>
<p>
Caloric yields by crop at each 5-minute cell across the world
</p>
<ul>
<li>GAEZ provides each crop's potential yield <br> <strong>in tons per hectare</strong></li>
</ul>
<p>
</p>
<p>
$\Rightarrow$ Multiply it w/ crop-specific conversion factor
</p>
</section>
<section id="calorie_conversion_factors" data-transition="fade-in fade-out">
<h4>
Calorie conversion factors
</h4>
<img src="gis_lecture8/conversion_factors.png" style="border: none; background: none; box-shadow:none; width: 500px;">
<p style="vertical-align:text-bottom;"><small>(Table A.1 of Online Appendix for <a href="https://dx.org/10.1257/aer.20150020" target="_blank">Galor & Ozak 2016</a>)</small></p>
</section>
<section id="" data-transition="fade-in slide-out" style="text-align: left;">
<h4>
Data we want to construct (cont.)
</h4>
<p>
So we need to edit each 5-minute cell of GAEZ
</p>
<p>
$\Rightarrow$ Use Map Algebra
</p>
</section>
<section id="" data-transition="slide-in fade-out" style="text-align: left;">
<h4>
Map Algebra basics #1
</h4>
<h3>
Import Spatial Analyst Module
</h3>
<p>
To use Map Algebra, start the script by:
</p>
<pre style="font-size:1em;">
<code data-trim data-noescape>
import arcpy
from arcpy.sa import *
arcpy.CheckOutExtension("spatial")
</code>
</pre>
<ul>
<li>
I provide these lines in <code>.../Lecture8/code/template4L8ex*.py</code>
</li>
</ul>
</section>
<section id="" data-transition="fade-in fade-out" style="text-align: left;">
<h4>
Map Algebra basics #1
</h4>
<h3>
Import Spatial Analyst Module
</h3>
<p>
To use Map Algebra, start the script by:
</p>
<pre style="font-size:1em;">
<code data-trim data-noescape>
import arcpy
<mark>from arcpy.sa import *</mark>
arcpy.CheckOutExtension("spatial")
</code>
</pre>
<p>
Without this line, every method for Map Algebra has to have the prefix <code>arcpy.sa.</code>
</p>
</section>
<section id="" data-transition="fade-in fade-out" style="text-align: left;">
<h4>
Map Algebra basics #1
</h4>
<h3>
Import Spatial Analyst Module
</h3>
<p>
To use Map Algebra, start the script by:
</p>
<pre style="font-size:1em;">
<code data-trim data-noescape>
import arcpy
from arcpy.sa import *
<mark>arcpy.CheckOutExtension("spatial")</mark>
</code>
</pre>
<p>
Map Algebra is part of Spatial Analyst extension.
</p>
<p>
Without this, you cannot use it
</p>
</section>
<section id="" data-transition="slide-in fade-out" style="text-align: left;">
<h4>
Map Algebra basics #2
</h4>
<h3>
Raster Object
</h3>
<p>
To use Map Algebra, we need to understand the concept of <strong>Raster Object</strong>
</p>
<p>
cf. <a href="http://desktop.arcgis.com/en/arcmap/latest/extensions/spatial-analyst/map-algebra/working-with-raster-objects.htm" target="_blank">ArgGIS Help on Raster Object</a>
</p>
</section>
<section id="" data-transition="fade-in fade-out" style="text-align: left;">
<h4>
Raster Object (cont.)
</h4>
<p>
To use arithmetic operators, first convert each raster data into a Raster object by <a href="http://desktop.arcgis.com/en/arcmap/latest/analyze/arcpy-classes/raster-object.htm" target="_blank">Raster method</a>:
</p>
<pre style="font-size:1em;">
<code data-trim data-noescape>
maize_yield = <mark>Raster("maize.tif")</mark>
</code>
</pre>
<p style="font-size:0.7em;" class="fragment" data-fragment-index="1">
cf. If the script didn't start with
</p>
<p style="font-size:0.7em; text-align: center;" class="fragment" data-fragment-index="1">
<code>from arcpy.sa import *</code>
</p>
<p style="font-size:0.7em;" class="fragment" data-fragment-index="1">
we have to write:
</p>
<pre style="font-size:0.7em;" class="fragment" data-fragment-index="1">
<code data-trim data-noescape>
maize_yield = <mark>arcpy.sa.</mark>Raster("maize.tif")
</code>
</pre>
</section>
<section id="" data-transition="fade-in fade-out" style="text-align: left;">
<h4>
Raster Object (cont.)
</h4>
<p>
Raster object can be used just like X in high-school algebra
</p>
<pre style="font-size:1em;">
<code data-trim data-noescape>
maize_yield = Raster("maize.tif")
maize_calorie <mark>= maize_yield * 36.5</mark>
</code>
</pre>
<ul>
<li>
<a href="#calorie_conversion_factors" target="_blank">Calorie from Maize: 3.65 kilo per 100g</a>
</li>
<li>
Unit in GAEZ data: kg
</li>
</ul>
</section>
<section id="" data-transition="fade-out fade-in">
<h4>
Arithmetic operators for Map Algebra / Python
</h4>
<img src="gis_lecture8/
map_algebra_operators.png" style="border: none; background: none; box-shadow:none; width: 300px;">
<p>
<small>(taken from <a href="http://desktop.arcgis.com/en/arcmap/latest/extensions/spatial-analyst/map-algebra/working-with-operators.htm" target="_blank">ArcGIS Help</a>)</small>
</p>
</section>
<section id="" data-transition="fade-in fade-out" style="text-align: left;">
<h4>
Raster Object (cont.)
</h4>
<p>
Output from calculation is also a Raster object
</p>
<pre style="font-size:1em;">
<code data-trim data-noescape>
maize_yield = Raster("maize.tif")
<mark>maize_calorie</mark> = maize_yield * 36.5
</code>
</pre>
<p>
It's not saved in the disk. Just in memory.
</p>
</section>
<section id="" data-transition="fade-in fade-out" style="text-align: left;">
<h4>
Raster Object (cont.)
</h4>
<p>
To save a raster object in the disk, use the save method
</p>
<pre style="font-size:1em;">
<code data-trim data-noescape>
maize_yield = Raster("maize.tif")
maize_calorie = maize_yield * 36.5
<mark>maize_calorie.save("maize_calorie.tif")</mark>
</code>
</pre>
</section>
<section id="" data-transition="fade-in fade-out" style="text-align: left;">
<h4>
Raster Object (cont.)
</h4>
<p>
To save a raster object in the disk, use the save method
</p>
<pre style="font-size:1em;">
<code data-trim data-noescape>
maize_yield = Raster("maize.tif")
maize_calorie = maize_yield * 36.5
maize_calorie<mark>.save(</mark>"maize_calorie.tif"<mark>)</mark>
</code>
</pre>
<p>
Add <code>.save()</code> to raster object name
</p>
</section>
<section id="" data-transition="fade-in fade-out" style="text-align: left;">
<h4>
Raster Object (cont.)
</h4>
<p>
To save a raster object in the disk, use the save method
</p>
<pre style="font-size:1em;">
<code data-trim data-noescape>
maize_yield = Raster("maize.tif")
maize_calorie = maize_yield * 36.5
maize_calorie.save(<mark>"maize_calorie.tif"</mark>)
</code>
</pre>
<p>
Within ( ), specify the output raster file name
</p>
</section>
<section id="" data-transition="slide-in fade-out" style="text-align: left;">
<h4>
Map Algebra basics #3
</h4>
<h3>
Functions
</h3>
<p>
Various mathematical functions available
</p>
<ul>
<li>
Ln, SquareRoot, Cos, Sin, Tan, etc.
</li>
<li>
See <a href="http://desktop.arcgis.com/en/arcmap/latest/tools/spatial-analyst-toolbox/an-overview-of-the-math-tools.htm" target="_blank">ArcGIS Help</a> for the complete list
</li>
</ul>
</section>
<section id="" data-transition="fade-in slide-out" style="text-align: left;">
<h4>
Functions (cont.)
</h4>
<p>
To use functions, the input raster <strong>doesn't have to be</strong> a Raster object
</p>
<p>
Output of a function is a Raster object, though
</p>
<p class="fragment" data-fragment-index="1">
$\Rightarrow$ To save the output, use save method
</p>
</section>
<section id="file_path1" data-transition="slide-in fade-out" style="text-align: left;">
<h4>
Map Algebra basics #4
</h4>
<h3>
File path
</h3>
<p>
For some reason, the following way of specifying relative file paths (cf. <a href="../gis_lecture3.html#relative-file-path" target="_blank">Lecture 3</a>) does not work with Map Algebra
</p>
<pre>
<code style="font-size: 1.4rem; line-height: 1.4;" data-trim data-noescape>
import os
work_dir = os.path.dirname(os.path.realpath(__file__))
os.chdir(work_dir)
arcpy.env.workspace = "../"
input_raster = "input/GAEZ/maize.tif"
output_raster = "output/maize_calorie.tif"
</code>
</pre>
</section>
<section id="file_path2" data-transition="fade-in slide-out" style="text-align: left;">
<h4>
File path (cont.)
</h4>
<p>
The problem seems to be <code>arcpy.env.workspace</code>
</p>
<p>
So we directly specify the file paths:
</p>
<pre>
<code style="font-size: 1.4rem; line-height: 1.4;" data-trim data-noescape>
import os
work_dir = os.path.dirname(os.path.realpath(__file__))
os.chdir(work_dir)
input_raster = "<mark>../</mark>input/GAEZ/maize.tif"
output_raster = "<mark>../</mark>output/maize_calorie.tif"
</code>
</pre>
<p>
We will later use <code>arcpy.env.workspace</code> <br>for another purpose
</p>
</section>
<section id="" data-transition="slide-in fade-out" style="text-align: left;">
<h4>
Exercise #1
</h4>
<h3>
Calorie conversion for maize
</h3>
<p>
Start with <code>code/template4L8ex1.py</code>
</p>
<p>
Inputs
</p>
<ul>
<li>
GAEZ data: <code>../input/GAEZ/maize.tif</code>
</li>
<li>
Conversion factor: 36.5
</li>
</ul>
<p>
Output
</p>
<ul>
<li>
<code>../output/maize_calorie.tif</code>
</li>
</ul>
</section>
<section id="" data-transition="fade-in slide-out" style="text-align: left;">
<h4>
Exercise #1 (cont.)
</h4>
<pre style="font-size:0.7em;">
<code data-trim data-noescape>
print "Inputs being set..."
input_raster = "../input/GAEZ/maize.tif"
conversion_factor = 36.5
print "Outputs being set..."
output_raster = "../output/calorie_maize.tif"
try:
maize_yield = Raster(input_raster)
maize_calorie = maize_yield * conversion_factor
maize_calorie.save(output_raster)
except arcpy.ExecuteError:
...
</code>
</pre>
</section>
<section id="" data-transition="slide-in fade-out" style="text-align: left;">
<h3>
How to repeat for 15 crops
</h3>
<p>
How do we repeat this for other 14 crops?
</p>
<p class="fragment" data-fragment-index="1">
Naive approach by 1st-year PhD student:
</p>
<ul class="fragment" data-fragment-index="1">
<li>
Copy and paste
</li>
<li>
Replace file names and conversion factor for each crop
</li>
</ul>
<p class="fragment" data-fragment-index="2">
We should avoid copy-and-paste
</p>
<ul class="fragment" data-fragment-index="2">
<li>
Not readable for other people and your future self
</li>
<li>
Difficult to revise
</li>
<ul>
<li>
You have to change <strong>15 lines</strong> for each revision
</li>
<li>
You may forget to change a few of them
</li>
</ul>
</ul>
<p class="fragment" data-fragment-index="3">
Use a loop over 15 crops
</p>
</section>
<section id="" data-transition="fade-in slide-in" style="text-align: left">
<h3>
detour: defensive programming
</h3>
<p>
If tempted to copy-and-paste during programming
</p>
<p>
$\Rightarrow$ Youl should better use a loop
</p>
<p>
One example of what's known as defensive programming
</p>
</section>
<section id="" data-transition="fade-in fade-out" style="text-align: left">
<h4>
detour: defensive programming (cont.)
</h4>
<p>
"Computer scientists <em>assume</em> programmers will make mistakes, and instead of chiding people to “just be careful,” they have developed a battery of practices to address the problem." (<a href="https://github.com/nickeubank/DefensiveProgramming/blob/master/Eubank_DefensiveProgramming.pdf" target="_blank">Nick Eubank</a>)