View source for QGIS/C2/Geometric-Properties-of-Vectors/English-timed


{|border=1
||'''Time'''
||'''Narration'''
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|| 00:01
|| Welcome to this tutorial on '''Geometric Properties of Vectors'''.

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|| 00:06
|| In this tutorial, we will learn to

Show selected features from the '''attribute table''' on the map.

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|| 00:14
|| Add columns to the '''attribute table'''.

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|| 00:18
|| Calculate statistics for the '''attributes'''.

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|| 00:22
|| To record this tutorial, I am using,

'''Ubuntu Linux '''OS version 16.04, '''QGIS''' version 2.18

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|| 00:34
||To follow this tutorial you should be familiar with '''QGIS''' interface.

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||00:41
||If not for relevant tutorials please visit our website.

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||00:46
||To practise this tutorial, you need to download the folder given in the '''Code files''' link located below the player.

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||00:56
||Extract the contents of the downloaded zip file.

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||01:00
||Locate''' IND_rails.shp '''file in the extracted folder.

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|| 01:08
|| I have already downloaded the '''code file''', extracted and saved in a folder on the '''Desktop'''.

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|| 01:15
|| Double-click on '''Code-file''' folder on the '''Desktop''' to open it.

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||01:21
||Right-click on '''IND_rails.shp '''file.

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||01:27
||From the context menu, select '''Open with QGIS Desktop'''.

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||01:35
|| '''QGIS''' interface opens.

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||01:38
||Click on '''OK''' button to close the '''QGIS tips''' dialog-box.

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|| 01:44
|| Map of India with lines representing railroads opens on '''canvas'''.

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|| 01:51
|| We will calculate line lengths for rail roads which are operational.

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||01:57
||To view this information we need to open the '''attribute table'''.

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|| 02:02
|| Right-click on '''IND_rails layer''' in the '''Layers Panel'''.

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|| 02:09
|| From the '''context menu''' select '''Open Attribute Table '''option.

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|| 02:14
|| '''Attribute table '''opens.

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|| 02:17
|| The '''table''' has an '''attribute''' called '''EXS_DESCRI'''. 

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||02:25
||We can use the value of this '''attribute''' to select features which are operational.

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||02:31
|| This column shows the status of a particular railway line.

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||02:36
||These are categorized as '''Operational, Unexamined''' or '''Unsurveyed '''and '''not Usable'''.

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|| 02:48
|| We need to select the lines which are operational. 

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|| 02:52
|| In the '''attribute''' table window, 

Click '''Select features using an expression '''tool in the tool bar. 

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|| 03:00
|| A new dialog-box''' Select By Expression '''opens.

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|| 03:05
|| In the '''Function Editor''' panel, click on the black triangle next to '''Fields and Values''' option.

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|| 03:13
|| From the list select '''EXS_DESCRI attribute'''.

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||03:21
||Double-click on it and add it to the '''Expression''' text area. 

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|| 03:26
|| Complete the expression by typing 

'''"EXS_DESCRI" equal to''', in '''single quotes''' '''Operational'''

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|| 03:37
|| Please note, the syntax here is case-sensitive. 

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||03:42
||Type the words as they appear in the '''attribute table.'''

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||03:47
||Here “O” in '''Operational''' is capital letter.

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||03:52
|| At the bottom of the table, click on '''Select''' button followed by '''Close '''button.

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|| 03:59
|| The '''Operational category''' are selected in the '''attribute table'''.

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|| 04:04
||Close the '''attribute table'''.

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|| 04:07
|| On the map, you will see that all lines that fall in '''Operational category''' are selected.

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|| 04:14
||These lines appear in yellow.

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|| 04:17
|| Now let’s save our selection to a new '''shapefile. '''

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||04:22
||Right-click on '''IND_rail layer''' and select '''Save As....'''option.

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|| 04:31
|| '''Save Vector Layer as ….. '''dialog-box opens.

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|| 04:35
|| Click on '''Browse''' button, next to '''File name '''field.

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|| 04:40
|| '''Save Layer As...''' dialog-box opens.

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|| 04:44
||Name the output file as '''railway.shp.'''

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||04:49
|| Select a location, I will choose '''Desktop.'''

Click on '''Save''' button.

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|| 04:57
|| Now let us select '''CRS''' for this '''layer'''.

Click on '''Select CRS '''button.

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|| 05:04
|| '''Coordinate Reference System Selector '''dialog-box opens.

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|| 05:09
|| Since we are interested in calculating length, let us select an''' equidistance projection'''.

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||05:16
||Type '''Indian 1975''' in the '''Filter''' search box. 

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|| 05:22
|| Under '''Coordinate Reference Systems''' of the world, in '''Geographic Coordinate Systems''' section select '''Indian 1975 EPSG:4240.'''

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||05:36
||Click on '''OK''' button.

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||05:39
||In the '''Save vector layer as... '''dialog-box, by default, '''Add saved file to map''' is already checked. 

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|| 05:48
|| Check '''Save only selected features '''check box.

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|| 05:53
|| Click on '''OK''' button.

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|| 05:56
|| Once the export process finishes, you will see a new layer '''railway''' loaded in '''Layers Panel'''.

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|| 06:04
|| In the '''Layers Panel''', you can uncheck the box next to''' IND_rail '''layer, to turn it off as we don’t need it any more.

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|| 06:15
|| On the canvas you will see the map of India with only operational railway lines.

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|| 06:22
|| Right-click on the '''railway''' layer and select '''Open Attribute Table'''.

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|| 06:29
|| Now we will add a column with length of each feature. 

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|| 06:34
|| Put the '''layer''' in editing mode by clicking on the '''Toggle editing''' tool on the '''tool bar'''.

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|| 06:41
|| Then click the '''Open field calculator''' button, available on the right-hand corner of the '''tool bar'''.

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|| 06:49
|| In the '''Field Calculator '''dialog-box, check '''Create a new field check box'''.

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|| 06:55
|| Type '''length hyphen km''' in the '''Output field name '''text box.

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|| 07:02
|| Choose '''Decimal number (real)''' as the '''Output field type'''.

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|| 07:07
|| Change the output '''Precision''' to 2. 

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|| 07:10
|| In the '''Function editor panel''', Click the triangle next to '''Geometry '''and select''' $length'''. 

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|| 07:20
|| Double-click on '''$length''' to add it to the '''Expression''' text box.

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|| 07:26
|| Complete the expression as '''$length '''divided by 1000.

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||07:32
||Click on '''Division Operator''' button on top of the text window.

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||07:37
||Type 1000 on the keyboard.

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|| 07:40
|| We need to divide the output length by 1000, 

because '''railway''' layer '''CRS''' is in meters unit and we want the output in kilo meters. 

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||07:52
||Click on '''OK''' button.

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|| 07:55
|| Click on '''Toggle editing''' tool to stop editing.

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|| 08:00
|| In the '''Stop editing''' dialog-box, click on '''Save''' button, to save the changes to the '''attribute table'''.

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|| 08:07
|| Back in '''attribute table''', a new column '''Length hyphen km '''has been added.

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|| 08:15
|| Now we have length of each individual line in '''Railway layer'''. 

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||08:20
||We can add it all up and find the total length. 

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||08:25
|| Close the '''attribute table.'''

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|| 08:28
|| Click on''' Vector''' menu on the menu bar.

Select '''Analysis Tools''' option.

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||08:36
||From the sub-menu, click on '''Basic Statistics for numeric tools'''.

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|| 08:42
|| A dialog-box opens.

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|| 08:45
|| Select '''Input vector layer''' as '''railway'''. 

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||08:50
||Choose '''Field to calculate statistics on''' as '''length hyphen km'''.

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|| 08:57
|| Click on '''Run '''button at the bottom right corner of the dialog-box.

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|| 09:03
|| '''Results''' window opens.

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||09:06
||Here you will see various statistics results. 

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||09:11
||The '''Sum value''' shown here is the total length of the railroads.

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|| 09:16
|| Note, the answer may vary slightly if a different '''projection''' is chosen. 

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||09:23
|| In practice, line lengths for roads and other linear features are measured on the ground.

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||09:30
||These values are provided as '''attributes''' to the '''dataset'''. 

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|| 09:35
|| The method demonstrated above works in the absence of such '''attribute'''

And as an approximation of actual line lengths.

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||09:46
|| Let us summarize,

In this tutorial we have learnt to,

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||09:51
||Show selected features from the '''attribute table''' on the map.

Add columns to the '''attribute table''' and Calculate statistics for the '''attributes'''.

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|| 10:04
|| As an Assignment

Use the code file '''world_1.shp''', downloaded from the '''Code files''' link.

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||10:14
||Find the area in square-kilometers for different countries.

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||10:20
|| Your completed assignment should look as shown here.

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|| 10:26
||The video at the following link summarises the spoken tutorial project.

Please download and watch it.

|-
|| 10:34
||  The spoken tutorial project team conducts workshops and gives certificates on passing online tests

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||10:41
||For more details please write to us.

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|| 10:45
|| Please post your timed queries on this forum.

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|| 10:49
|| The Spoken Tutorial Project is funded by '''NMEICT, MHRD''' Government of India.

More information on this mission is available at the following link.

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|| 11:01
|| This tutorial is contributed by Prajwal.M from NIT Surathkal and Snehalatha from IIT Bombay.

Thank you for joining.

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