This article will discuss about the creation of polygon feature using GIS.
Open QGIS by clicking on Start then go to All Programs and then clickQGIS Desktop 3.8.3.
Open the georeferenced toposheet via Layer then clickAdd Layer followed by Add Raster Layer
Following pop up window (Data source manager raster) will open. Browse and select the georeferenced toposheet for polygon features (polygon is used to demarcate area features like built-up/Urban area, forest, agriculture land, administrative boundaries,waterbody etc.) creation via Raster dataset and Add button.
Now for polygon layer creation, click on Layer then click Create layer followed by New Shapefile Layer
New Shapefile layer window will popup
Now In the above popup window define the File name.
Select Geometry type – ‘Polygon’ as here we want to create polygon layer.
Now specify Coordinate reference system (CRS) same as toposheet. For this click on Select CRS. A CRS selector popup will open, here define the co-ordinate system and Click on ‘OK’
Specify the attribute fields to the layer that we have created. If we want to create polygon layer of waterbody, then we will add ‘Name’of the road as new field and waterbody type as other fields.For New field to be added, an appropriate Name, Type(like text, whole number, decimal number and date) and Length must be selected.
Click on ‘Add to Field List’ and the attribute will be added to the field list as shown below
In the same way, you can add other fields like waterbody type etc. and once all the fields added, click on OK.
Note: If you want to remove the added field from the list, select the field you want to remove and click on ‘Remove Field’ button.
The polygon layer created by you will be visible in Layer as shown below
Now you can start digitization, but before starting digitization you have to enable the editing mode of the corresponding layer. Right click on ‘waterbody-jp.shp’ polygon layer and click on ‘Toggle Editing’ as shown below
Zoom into the toposheet where waterbody is visible, Place the cursor at feature of interest and click. But before that select the symbol of waterbody and this could be selected from Style manager. For this click on Settings and then go to Style manager as shown below
Following window will popup. Here select the symbol of your choice for area and close this Style manager window.
Now click on Add Polygon feature icon from digitizing toolbar.
Note: If Digitizing tool bar is not present in your QGIS window, then right click on main menu bar area and check theDigitizing toolbar as shown below
Now place the cursor on the waterbody and start waterbody digitization by using left mouse button to insert vertices. ‘Attributes’ window will open, fill in the required attributes for example Name is ‘Sambhar’ and Type is ‘salty lake’, then click OK.
In the same way you can digitize other waterbodies and define their attributes. Click on Save Layer Edits icon in Digitizing tool bar, to save the polygon layer along with its attributes.
Note: We will suggest that enable snapping tool to digitize the waterbody. Snapping tool is very effective to reduce the digitization errors like overshoot and undershoot. For this
Click on Settings then go to Options as shown below
A popup window will open, here select Digitizing and then in Snappingoption check the box indicating Enable snapping by default. Snapping tolerance can be set manually. After specifying the snapping tolerance, click OK
If you want to edit any attribute information, then open the attribute table of ‘waterbody-jp.shp’ by right clicking on this layer and select Open Attribute Table as shown below.
Select the attribute by double click and edit it. Now save the edits and after saving click once again on the toggle button to stop editing. Polygon layer creation and editing is over now.