forked from Automating-GIS-processes/2016
-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy pathLesson7-read-raster-array.html
372 lines (258 loc) · 17.2 KB
/
Lesson7-read-raster-array.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
<!DOCTYPE html>
<!--[if IE 8]><html class="no-js lt-ie9" lang="en" > <![endif]-->
<!--[if gt IE 8]><!--> <html class="no-js" lang="en" > <!--<![endif]-->
<head>
<meta charset="utf-8">
<script type="text/javascript">
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-88382509-1']);
_gaq.push(['_trackPageview']);
(function() {
var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
})();
</script>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Reading raster as a numerical array — GeoPython - AutoGIS 1 documentation</title>
<link rel="stylesheet" href="_static/css/theme.css" type="text/css" />
<link rel="index" title="Index"
href="genindex.html"/>
<link rel="search" title="Search" href="search.html"/>
<link rel="top" title="GeoPython - AutoGIS 1 documentation" href="index.html"/>
<link rel="next" title="GDAL command line tools" href="Lesson7-gdal-utilities.html"/>
<link rel="prev" title="Reading raster files with GDAL" href="Lesson7-read-raster.html"/>
<script src="_static/js/modernizr.min.js"></script>
</head>
<body class="wy-body-for-nav" role="document">
<div class="wy-grid-for-nav">
<nav data-toggle="wy-nav-shift" class="wy-nav-side">
<div class="wy-side-scroll">
<div class="wy-side-nav-search">
<a href="index.html" class="icon icon-home"> GeoPython - AutoGIS
<img src="_static/logo_hy_geo_135.png" class="logo" />
</a>
<div class="version">
2016 Autumn
</div>
<div role="search">
<form id="rtd-search-form" class="wy-form" action="search.html" method="get">
<input type="text" name="q" placeholder="Search docs" />
<input type="hidden" name="check_keywords" value="yes" />
<input type="hidden" name="area" value="default" />
</form>
</div>
</div>
<div class="wy-menu wy-menu-vertical" data-spy="affix" role="navigation" aria-label="main navigation">
<p class="caption"><span class="caption-text">Course information</span></p>
<ul>
<li class="toctree-l1"><a class="reference internal" href="course-info.html">General info</a></li>
<li class="toctree-l1"><a class="reference internal" href="Installing_Anacondas_GIS.html">Installing Python + GIS</a></li>
<li class="toctree-l1"><a class="reference internal" href="License-terms.html">License and terms of usage</a></li>
</ul>
<p class="caption"><span class="caption-text">Lesson 1</span></p>
<ul>
<li class="toctree-l1"><a class="reference internal" href="Lesson1-Intro-Python-GIS.html">Introduction to Python GIS</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson1-Geometric-Objects.html">Geometric Objects - Spatial Data Model</a></li>
<li class="toctree-l1"><a class="reference internal" href="Exercise-1.html">Exercise 1</a></li>
</ul>
<p class="caption"><span class="caption-text">Lesson 2</span></p>
<ul>
<li class="toctree-l1"><a class="reference internal" href="Lesson2-overview.html">Lesson 2 Overview</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson2-download-data.html">Download datasets</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson2-overview-pandas-geopandas.html">Pandas and Geopandas -modules</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson2-pandas-intro.html">Introduction to Pandas</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson2-geopandas-basics.html">Introduction to Geopandas</a></li>
<li class="toctree-l1"><a class="reference internal" href="Exercise-2.html">Exercise 2</a></li>
</ul>
<p class="caption"><span class="caption-text">Lesson 3</span></p>
<ul>
<li class="toctree-l1"><a class="reference internal" href="Lesson3-overview.html">Lesson 3 Overview</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson3-geocoding.html">Geocoding</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson3-table-join.html">Table join</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson3-projections.html">Re-projecting data</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson3-point-in-polygon.html">Point in Polygon & Intersect</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson3-spatial-join.html">Spatial join</a></li>
<li class="toctree-l1"><a class="reference internal" href="Exercise-3.html">Exercise 3</a></li>
</ul>
<p class="caption"><span class="caption-text">Lesson 4</span></p>
<ul>
<li class="toctree-l1"><a class="reference internal" href="Lesson4-overview.html">Lesson 4 Overview</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson4-download-data.html">Download datasets</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson4-geometric-operations.html">Geometric operations</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson4-reclassify.html">Data reclassification</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson4-nearest-neighbour.html">Nearest Neighbour Analysis</a></li>
<li class="toctree-l1"><a class="reference internal" href="Exercise-4.html">Exercise 4</a></li>
</ul>
<p class="caption"><span class="caption-text">Lesson 5</span></p>
<ul>
<li class="toctree-l1"><a class="reference internal" href="Lesson5-overview.html">Lesson 5 Overview</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson5-download-data.html">Download datasets</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson5-static-maps.html">Static maps</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson5-interactive-map-bokeh.html">Interactive maps with Bokeh</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson5-share-on-github.html">Sharing interactive plots on GitHub</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson5-interactive-map-folium.html">Interactive maps on Leaflet</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson5-World-3D.html">Inspiration: World 3D</a></li>
<li class="toctree-l1"><a class="reference internal" href="Exercise-5.html">Exercise 5</a></li>
</ul>
<p class="caption"><span class="caption-text">Lesson 6</span></p>
<ul>
<li class="toctree-l1"><a class="reference internal" href="Lesson6-overview.html">Lesson 6 Overview</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson6-arcpy.html">Python in ArcGIS</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson6-toolbox.html">ArcGIS Toolbox</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson6-arcpy-script.html">Writing arcpy scripts</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson6-run-the-tool.html">Running the Python script from ArcGIS</a></li>
</ul>
<p class="caption"><span class="caption-text">Lesson 7</span></p>
<ul class="current">
<li class="toctree-l1"><a class="reference internal" href="Lesson7-overview.html">Lesson 7 Overview</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson7-download.html">Download data</a></li>
<li class="toctree-l1"><a class="reference internal" href="Lesson7-read-raster.html">Reading raster files with GDAL</a></li>
<li class="toctree-l1 current"><a class="current reference internal" href="#">Reading raster as a numerical array</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#excluding-nodata-values">Excluding NoData values</a></li>
<li class="toctree-l2"><a class="reference internal" href="#closing-raster-dataset">Closing raster dataset</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="Lesson7-gdal-utilities.html">GDAL command line tools</a></li>
<li class="toctree-l1"><a class="reference internal" href="Exercise-7.html">Exercise 7</a></li>
</ul>
<p class="caption"><span class="caption-text">Lesson 8</span></p>
<ul>
<li class="toctree-l1"><a class="reference internal" href="Lesson8-network-analysis.html">Network analysis in Python</a></li>
</ul>
<p class="caption"><span class="caption-text">Final Assignment</span></p>
<ul>
<li class="toctree-l1"><a class="reference internal" href="Final-assignment.html">Final assignment</a></li>
</ul>
<p class="caption"><span class="caption-text">Map Challenge 2016</span></p>
<ul>
<li class="toctree-l1"><a class="reference internal" href="map-challenge.html">Map Challenge 2016</a></li>
</ul>
</div>
</div>
</nav>
<section data-toggle="wy-nav-shift" class="wy-nav-content-wrap">
<nav class="wy-nav-top" role="navigation" aria-label="top navigation">
<i data-toggle="wy-nav-top" class="fa fa-bars"></i>
<a href="index.html">GeoPython - AutoGIS</a>
</nav>
<div class="wy-nav-content">
<div class="rst-content">
<div role="navigation" aria-label="breadcrumbs navigation">
<ul class="wy-breadcrumbs">
<li><a href="index.html">Docs</a> »</li>
<li>Reading raster as a numerical array</li>
<li class="wy-breadcrumbs-aside">
<a href="https://github.com/Automating-GIS-processes/2016/blob/master/source/Lesson7-read-raster-array.rst" class="fa fa-github"> Edit on GitHub</a>
</li>
</ul>
<hr/>
</div>
<div role="main" class="document" itemscope="itemscope" itemtype="http://schema.org/Article">
<div itemprop="articleBody">
<div class="section" id="reading-raster-as-a-numerical-array">
<h1>Reading raster as a numerical array<a class="headerlink" href="#reading-raster-as-a-numerical-array" title="Permalink to this headline">¶</a></h1>
<p>GDAL is a powerful library when it comes to accessing geospatial raster data, but it does not provide many functionalities for doing calculations.
For more advanced computing, we will read in the raster data as a numerical array in order to use the capabilities in the NumPy-library.</p>
<p>In case you want to convert and existing Gdal Dataset or a Band into a numpy array you can convert it with <code class="docutils literal"><span class="pre">ReadAsArray()</span></code> -function:</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="c1"># Read raster data as numeric array from GDAL Dataset</span>
<span class="n">rasterArray</span> <span class="o">=</span> <span class="n">raster</span><span class="o">.</span><span class="n">ReadAsArray</span><span class="p">()</span>
</pre></div>
</div>
<p>So what is the difference?</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="c1">#Check the datatype of variables</span>
<span class="nb">type</span><span class="p">(</span><span class="n">rasterArray</span><span class="p">)</span>
</pre></div>
</div>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="nb">type</span><span class="p">(</span><span class="n">raster</span><span class="p">)</span>
</pre></div>
</div>
<p>As you can see, we have now stored the same raster data into two different types of variables:</p>
<blockquote>
<div><ul class="simple">
<li><strong>raster</strong> is a <code class="docutils literal"><span class="pre">Gdal</span> <span class="pre">Dataset</span></code></li>
<li><strong>rasterArray</strong> is a <code class="docutils literal"><span class="pre">numpy</span> <span class="pre">array</span></code></li>
</ul>
</div></blockquote>
<p>The GDAL library also comes with a module gdal_array that works as an interface between NumPy and GDAL.
Gdal_array reads and writes raster files to and from NumPy arrays directly from file:</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">osgeo</span> <span class="k">import</span> <span class="n">gdal_array</span>
<span class="c1"># Read raster data as numeric array from file</span>
<span class="n">rasterArray</span> <span class="o">=</span> <span class="n">gdal_array</span><span class="o">.</span><span class="n">LoadFile</span><span class="p">(</span><span class="n">filepath</span><span class="p">)</span>
</pre></div>
</div>
<div class="section" id="excluding-nodata-values">
<h2>Excluding NoData values<a class="headerlink" href="#excluding-nodata-values" title="Permalink to this headline">¶</a></h2>
<p>What is the minimum value of our array?</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="n">rasterArray</span><span class="o">.</span><span class="n">min</span><span class="p">()</span>
</pre></div>
</div>
<p>As you can see, the numpy array still contains the original nodata values. Any calculations will be wrong if these are not taken care of.</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="c1"># Get nodata value from the GDAL band object</span>
<span class="n">nodata</span> <span class="o">=</span> <span class="n">band</span><span class="o">.</span><span class="n">GetNoDataValue</span><span class="p">()</span>
<span class="c1">#Create a masked array for making calculations without nodata values</span>
<span class="n">rasterArray</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">ma</span><span class="o">.</span><span class="n">masked_equal</span><span class="p">(</span><span class="n">rasterArray</span><span class="p">,</span> <span class="n">nodata</span><span class="p">)</span>
<span class="nb">type</span><span class="p">(</span><span class="n">rasterArray</span><span class="p">)</span>
<span class="c1"># Check again array statistics</span>
<span class="n">rasterArray</span><span class="o">.</span><span class="n">min</span><span class="p">()</span>
</pre></div>
</div>
<p>Now you have a two-dimensional array ready for further calculations.
However, for completing our exercise for this week we will use a very simple command line tool <code class="docutils literal"><span class="pre">gdal_calc.py</span></code>.</p>
</div>
<div class="section" id="closing-raster-dataset">
<h2>Closing raster dataset<a class="headerlink" href="#closing-raster-dataset" title="Permalink to this headline">¶</a></h2>
<p>It might be useful to close an existing GDAL object in the middle of the code if you want to free resources and remove unnecessary variables from memory.</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="n">raster</span> <span class="o">=</span> <span class="kc">None</span>
<span class="n">band</span> <span class="o">=</span> <span class="kc">None</span>
</pre></div>
</div>
<p>It is however not necessary to close raster datasets or bands at the end of the python script as Python will automatically take care of this.</p>
</div>
</div>
</div>
</div>
<footer>
<div class="rst-footer-buttons" role="navigation" aria-label="footer navigation">
<a href="Lesson7-gdal-utilities.html" class="btn btn-neutral float-right" title="GDAL command line tools" accesskey="n">Next <span class="fa fa-arrow-circle-right"></span></a>
<a href="Lesson7-read-raster.html" class="btn btn-neutral" title="Reading raster files with GDAL" accesskey="p"><span class="fa fa-arrow-circle-left"></span> Previous</a>
</div>
<hr/>
<div role="contentinfo">
<p>
© Copyright 2016, Henrikki Tenkanen.
Last updated on Feb 20, 2017.
</p>
</div>
Built with <a href="http://sphinx-doc.org/">Sphinx</a> using a <a href="https://github.com/snide/sphinx_rtd_theme">theme</a> provided by <a href="https://readthedocs.org">Read the Docs</a>.
</footer>
</div>
</div>
</section>
</div>
<script type="text/javascript">
var DOCUMENTATION_OPTIONS = {
URL_ROOT:'./',
VERSION:'1',
COLLAPSE_INDEX:false,
FILE_SUFFIX:'.html',
HAS_SOURCE: true
};
</script>
<script type="text/javascript" src="_static/jquery.js"></script>
<script type="text/javascript" src="_static/underscore.js"></script>
<script type="text/javascript" src="_static/doctools.js"></script>
<script type="text/javascript" src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>
<script type="text/javascript" src="_static/js/theme.js"></script>
<script type="text/javascript">
jQuery(function () {
SphinxRtdTheme.StickyNav.enable();
});
</script>
<li><a href="http://project.invalid/">Project Homepage</a> »</li>
<div class="footer">
<img src="../img/GPLv3_Logo.svg">
</div>
</body>
</html>