QGIS/C2/Geometric-Properties-of-Vectors/English-timed

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Time Narration
00:01 Welcome to this tutorial on Geometric Properties of Vectors.
00:06 In this tutorial, we will learn to

Show selected features from the attribute table on the map.

00:14 Add columns to the attribute table.
00:18 Calculate statistics for the attributes.
00:22 To record this tutorial, I am using,

Ubuntu Linux OS version 16.04, QGIS version 2.18

00:34 To follow this tutorial you should be familiar with QGIS interface.
00:41 If not for relevant tutorials please visit our website.
00:46 To practise this tutorial, you need to download the folder given in the Code files link located below the player.
00:56 Extract the contents of the downloaded zip file.
01:00 Locate IND_rails.shp file in the extracted folder.
01:08 I have already downloaded the code file, extracted and saved in a folder on the Desktop.
01:15 Double-click on Code-file folder on the Desktop to open it.
01:21 Right-click on IND_rails.shp file.
01:27 From the context menu, select Open with QGIS Desktop.
01:35 QGIS interface opens.
01:38 Click on OK button to close the QGIS tips dialog-box.
01:44 Map of India with lines representing railroads opens on canvas.
01:51 We will calculate line lengths for rail roads which are operational.
01:57 To view this information we need to open the attribute table.
02:02 Right-click on IND_rails layer in the Layers Panel.
02:09 From the context menu select Open Attribute Table option.
02:14 Attribute table opens.
02:17 The table has an attribute called EXS_DESCRI.
02:25 We can use the value of this attribute to select features which are operational.
02:31 This column shows the status of a particular railway line.
02:36 These are categorized as Operational, Unexamined or Unsurveyed and not Usable.
02:48 We need to select the lines which are operational.
02:52 In the attribute table window,

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

03:00 A new dialog-box Select By Expression opens.
03:05 In the Function Editor panel, click on the black triangle next to Fields and Values option.
03:13 From the list select EXS_DESCRI attribute.
03:21 Double-click on it and add it to the Expression text area.
03:26 Complete the expression by typing

"EXS_DESCRI" equal to, in single quotes Operational

03:37 Please note, the syntax here is case-sensitive.
03:42 Type the words as they appear in the attribute table.
03:47 Here “O” in Operational is capital letter.
03:52 At the bottom of the table, click on Select button followed by Close button.
03:59 The Operational category are selected in the attribute table.
04:04 Close the attribute table.
04:07 On the map, you will see that all lines that fall in Operational category are selected.
04:14 These lines appear in yellow.
04:17 Now let’s save our selection to a new shapefile.
04:22 Right-click on IND_rail layer and select Save As....option.
04:31 Save Vector Layer as ….. dialog-box opens.
04:35 Click on Browse button, next to File name field.
04:40 Save Layer As... dialog-box opens.
04:44 Name the output file as railway.shp.
04:49 Select a location, I will choose Desktop.

Click on Save button.

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

Click on Select CRS button.

05:04 Coordinate Reference System Selector dialog-box opens.
05:09 Since we are interested in calculating length, let us select an equidistance projection.
05:16 Type Indian 1975 in the Filter search box.
05:22 Under Coordinate Reference Systems of the world, in Geographic Coordinate Systems section select Indian 1975 EPSG:4240.
05:36 Click on OK button.
05:39 In the Save vector layer as... dialog-box, by default, Add saved file to map is already checked.
05:48 Check Save only selected features check box.
05:53 Click on OK button.
05:56 Once the export process finishes, you will see a new layer railway loaded in Layers Panel.
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.
06:15 On the canvas you will see the map of India with only operational railway lines.
06:22 Right-click on the railway layer and select Open Attribute Table.
06:29 Now we will add a column with length of each feature.
06:34 Put the layer in editing mode by clicking on the Toggle editing tool on the tool bar.
06:41 Then click the Open field calculator button, available on the right-hand corner of the tool bar.
06:49 In the Field Calculator dialog-box, check Create a new field check box.
06:55 Type length hyphen km in the Output field name text box.
07:02 Choose Decimal number (real) as the Output field type.
07:07 Change the output Precision to 2.
07:10 In the Function editor panel, Click the triangle next to Geometry and select $length.
07:20 Double-click on $length to add it to the Expression text box.
07:26 Complete the expression as $length divided by 1000.
07:32 Click on Division Operator button on top of the text window.
07:37 Type 1000 on the keyboard.
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.

07:52 Click on OK button.
07:55 Click on Toggle editing tool to stop editing.
08:00 In the Stop editing dialog-box, click on Save button, to save the changes to the attribute table.
08:07 Back in attribute table, a new column Length hyphen km has been added.
08:15 Now we have length of each individual line in Railway layer.
08:20 We can add it all up and find the total length.
08:25 Close the attribute table.
08:28 Click on Vector menu on the menu bar.

Select Analysis Tools option.

08:36 From the sub-menu, click on Basic Statistics for numeric tools.
08:42 A dialog-box opens.
08:45 Select Input vector layer as railway.
08:50 Choose Field to calculate statistics on as length hyphen km.
08:57 Click on Run button at the bottom right corner of the dialog-box.
09:03 Results window opens.
09:06 Here you will see various statistics results.
09:11 The Sum value shown here is the total length of the railroads.
09:16 Note, the answer may vary slightly if a different projection is chosen.
09:23 In practice, line lengths for roads and other linear features are measured on the ground.
09:30 These values are provided as attributes to the dataset.
09:35 The method demonstrated above works in the absence of such attribute

And as an approximation of actual line lengths.

09:46 Let us summarize,

In this tutorial we have learnt to,

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

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

10:04 As an Assignment

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

10:14 Find the area in square-kilometers for different countries.
10:20 Your completed assignment should look as shown here.
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
10:41 For more details please write to us.
10:45 Please post your timed queries on this forum.
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.

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

Thank you for joining.

Contributors and Content Editors

PoojaMoolya