QGIS Polygon Analysis: Shape Properties

by ADMIN 40 views

Hey guys! Ever found yourself swimming in a sea of polygons in QGIS and thinking, "How in the world do I sift through all this and find the ones that actually matter?" Well, you're in luck! This guide is all about helping you identify polygons with specific shape properties. Whether you're dealing with a dataset of farm boundaries, building footprints, or any other polygon-based data, understanding how to filter and analyze these shapes is super important. We'll cover how to deal with those pesky irregular polygons, their varying sizes, and how to zero in on the ones that meet your specific criteria. Let's dive in and get those polygons under control!

Understanding Your Polygon Data in QGIS

Alright, before we jump into the nitty-gritty, let's get acquainted with the data. Imagine you've got a dataset of WKT (Well-Known Text) polygons in QGIS, like those farm boundaries you mentioned. These polygons can be a real mixed bag. You've got some with a ton of vertices (making them irregular and detailed), and others that are simpler. Size can vary wildly too – some farms are massive, while others are tiny little plots. This diversity is completely normal, but it can make it challenging to identify specific shapes without the right tools. Think about it: you might be interested in farms that are roughly square, or maybe ones that are above a certain size threshold. Without a clear strategy, you could spend hours manually inspecting each polygon! This is where QGIS's powerful tools come into play. We'll explore how to use them to automatically analyze and filter your polygons based on their geometric properties.

Specifically, we will examine some of the key attributes you can leverage to find your ideal polygons. These include area, perimeter, aspect ratio, and even more complex measurements such as the ratio of a polygon's area to that of its bounding box (also called the rectangularity or form factor). These attributes can unlock a whole new level of understanding. Consider the area, for example; it's the simplest measurement, yet it can immediately reveal size disparities. Perimeter, the total length of the boundary, is also straightforward, giving you an understanding of the length of the boundary. Aspect ratio gives you an indication of how stretched out or elongated the shape is. By using these attributes, you'll be able to quickly filter out the irrelevant polygons and concentrate on the shapes that fulfill your exact criteria. For instance, you could single out farms with an area greater than 10 hectares or those with an aspect ratio that resembles a square (close to 1). Knowing these measures is essential for making sense of your data and setting up effective analysis workflows.

Key Polygon Properties to Consider

Let's take a closer look at some of the essential shape properties you can use for filtering:

  • Area: The space enclosed by the polygon. Useful for filtering by size (e.g., find all polygons larger than a certain threshold).
  • Perimeter: The total length of the polygon's boundary. Can be useful for identifying long, thin shapes.
  • Aspect Ratio: The ratio of a polygon's width to its height. Helps identify elongated or square-like shapes.
  • Rectangularity/Form Factor: The ratio of a polygon's area to the area of its bounding rectangle. A value closer to 1 indicates a shape that is more rectangle-like.
  • Compactness: Metrics that describe how close a shape is to a circle. These measure the roundness or the irregularity of the polygon shape.

Analyzing Polygon Shapes in QGIS: Step-by-Step Guide

Okay, now let's get into the actual steps of analyzing your polygon shapes in QGIS. I'll walk you through the process, from calculating the properties to filtering your data. It's all about using the right tools and understanding how to interpret the results.

1. Loading Your Polygon Data

First things first, make sure your polygon data is loaded into QGIS. Go to "Layer" -> "Add Layer" -> "Add Vector Layer." Browse to your WKT file (or whatever format your data is in) and add it to your project. Make sure your data is correctly projected in a suitable coordinate reference system (CRS). This is critical because all the geometric calculations will be affected by your CRS. Check the CRS of your layer in the layer properties and change it if necessary. Choosing the right projection is essential for accurate area and distance measurements.

2. Calculating Shape Properties Using Field Calculator

This is where the magic happens! QGIS's Field Calculator is your best friend. Open the attribute table of your polygon layer (right-click the layer and select "Open Attribute Table"). Then, click the "Open field calculator" button (looks like an abacus). Now, here's how you can calculate each property:

  • Area: In the Field Calculator, create a new field called "area_ha" (or whatever you like). Use the expression $area / 10000 (this converts square meters to hectares; adjust the divisor if your units are different). Select the field type (e.g., Decimal number) and set the output field length and precision as appropriate.
  • Perimeter: Create a new field named "perimeter_m". Use the expression $perimeter. Choose an appropriate field type (Decimal number) and set field length and precision. These values will be measured in the same units as the data's CRS.
  • Aspect Ratio: This one is a little more involved, since QGIS does not directly calculate aspect ratio. We will calculate a new field as the ratio of a polygon's bounding box dimensions. Create a new field called "aspect_ratio". Use the expression CASE WHEN $width = 0 THEN 0 ELSE $height / $width END. Here, $width and $height are the width and height of the bounding box. You'll need to use the bounds_width() and bounds_height() functions, respectively. Choose an appropriate field type (Decimal number) and set field length and precision. This will give you the aspect ratio.
  • Rectangularity/Form Factor: Create a new field called "rectangularity". Use the expression $area / (bounds_width($geometry) * bounds_height($geometry)). This will give you an understanding of how much the polygon resembles a rectangle, with values closer to 1 being more rectangular. Again, choose an appropriate field type (Decimal number) and set field length and precision.
  • Compactness: Compactness is a bit more complex, as there isn't a single universally accepted measure. You might calculate the ratio of the area to the area of a circle with the same perimeter. Create a new field called "compactness". Use the expression (4 * pi() * $area) / ($perimeter * $perimeter). It returns values between 0 and 1, where 1 represents a perfect circle. Choose an appropriate field type (Decimal number) and set field length and precision.

After entering each expression, click "OK" to calculate the values for your new fields. You should now have a series of new columns in your attribute table, each containing a calculated shape property.

3. Filtering Polygons Based on Shape Properties

Now that you've calculated the shape properties, it's time to filter your data. Right-click your layer in the Layers panel and select "Filter." Here, you can enter an expression to filter the features based on the field values. For example:

  • To filter for farms larger than 5 hectares, you would use the expression: `