Introduction
In spatial data processing, it is common to have only the boundary points of a polygon rather than the polygon itself. This situation can arise due to various reasons, such as incomplete datasets, GPS-based data collection, or manual digitization errors. When only boundary points are available, it becomes necessary to reconstruct the polygon to ensure proper spatial analysis and visualization.
Failing to convert boundary points into polygons can lead to issues such as inaccurate area calculations, misalignment in spatial overlays, and difficulties in integrating data with other geospatial datasets. Converting these points into a polygon ensures that the data is structured correctly for further analysis, visualization, and land management applications.
QGIS, a powerful open-source Geographic Information System, provides tools that facilitate this conversion process. In this tutorial, you will learn how to transform boundary points into a polygon using QGIS.
This tutorial assumes that QGIS is already installed on your machine. If not, you can follow this official guide to install it.
This tutorial was created using a macOS machine, but users on other operating systems should be able to follow along without significant differences.
Check for Plot ID
Before converting points into a polygon, it is essential to ensure that the dataset has a column indicating the correct order of the points. This ordering allows QGIS to first create a polyline (a connected sequence of points) before converting it into a polygon.
To check the point order:
Right-click the point layer in the Layers panel
Select Open Attribute Table.
Look for a column (e.g.,
plot_id) that defines the order in which the points should be connected. If such a column does not exist, you may need to create it manually by assigning a unique sequential order to each point.
Ensuring this column is present is crucial for correctly linking the points to form a polyline.
Convert Points to Polyline
Once the point order is confirmed, the next step is to use the Points to Path tool in QGIS to convert the ordered points into a polyline.
Go to Processing Toolbox by clicking on the Tools button in the toolbar.
In the Processing Toolbox panel, search for Points to Path. Double-click to open its configuration window.
Select the Input layer that contains the boundary points.
Check Closed Path option. This setting is important because it ensures that the start and end points are connected, forming a closed shape instead of an open line.
Configure Order Expression: the column that ensures the points are connected in the correct sequence.
Configure Path Group Expression: the column that will be base how the points are grouped into separate polylines.
Click Run to execute the process.
Once the process is completed, a new polyline layer will be created. This layer represents the connected boundary points as a continuous path. You can verify the result by visually inspecting the map in QGIS.
Convert Polyline to Polygon
After successfully creating a polyline, the next step is to convert it into a polygon using the Lines to Polygon tool in QGIS.
Open the Processing Toolbox in QGIS if it is not already open.
Search for Lines to Polygon in the toolbox.
Double-click the Lines to Polygon tool to open its configuration window.
In the Lines to Polygon window, select the Input layer which is the polyline layer created in the previous step.
Click Run to start the process.
After the process is complete, a new polygon layer will be generated. This layer represents the reconstructed polygon created from the original boundary points.
Export the Result as a Shapefile
Once the polygon has been successfully created, it is recommended to export it as a Shapefile for further use in GIS applications.
Right-click the newly created polygon layer in the Layers panel.
Select Export
Select Save Features As….
Set the Format. In this case, it is going to be ESRI Shapefile (.SHP).
Choose the desired directory and file name for the exported shapefile.
Click OK to save the file.
You can now navigate to the specified directory to find the saved polygon shapefile, which can be used for further spatial analysis or integration with other datasets.
Conclusion
Converting boundary points into a polygon is essential for ensuring accurate spatial representation and analysis. By following this tutorial, you have successfully:
Verified the point order using the attribute table.
Converted the points into a polyline using the Points to Path tool.
Transformed the polyline into a polygon using the Lines to Polygon tool.
Exported the final polygon as a Shapefile for further use.
This process is valuable for various geospatial applications, including land management, agricultural mapping, and spatial data integration. We encourage users to practice this method with different datasets to gain proficiency in QGIS.