PostGIS is a powerful spatial database extension for PostgreSQL that enables the storage, retrieval, and manipulation of geospatial data. One of the fundamental operations in geospatial analysis is measuring the length and area of features, which can be achieved in PostGIS using various functions. In this article, we will discuss the measurement of area and length in PostGIS, the differences between using geometry and geography types, and how to perform these measurements in QGIS.

Geometry vs. Geography Types

PostGIS supports two types of spatial data: geometry and geography. Geometry represents data on a flat (Euclidean) coordinate system, while geography represents data on a round (spherical) coordinate system. The primary difference between these two types is that geometry is suitable for small-scale analysis (within a single country or region), while geography is more appropriate for large-scale analysis that spans multiple countries or continents.

Measurement of Length in PostGIS

To measure the length of a line feature in PostGIS, we use the ST_Length function. This function calculates the Euclidean distance between the start and end points of a line. For example, the following query returns the length of a line feature with ID 1 in a table called roads:

   
	SELECT ST_Length(roads.geom) as length FROM roads WHERE roads.id = 1;
  

Measurement of Area in PostGIS

To measure the area of a polygon feature in PostGIS, we use the ST_Area function. This function calculates the area of a polygon using the coordinate system of the data. For example, the following query returns the area of a polygon feature with ID 1 in a table called land_use:

   
	SELECT ST_Area(land_use.geom) as area FROM land_use WHERE land_use.id = 1;
  

Geometry vs. Geography Types for Length and Area Measurements

As mentioned earlier, PostGIS supports two types of spatial data: geometry and geography. When measuring length and area using geometry, the results are in the unit of the coordinate system used by the data. For example, if the data is in UTM (meters), the length and area measurements will be in meters. On the other hand, when measuring length and area using geography, the results are in meters or kilometers, depending on the coordinate system used by the data.

Performing Measurements in QGIS

QGIS is a popular open-source geographic information system that supports PostGIS. To perform length and area measurements in QGIS, we can use the Measure tool. To use this tool, we need to activate it by clicking on the Measure button in the toolbar or by pressing Ctrl + Shift + M on the keyboard. Once the tool is active, we can select a line or polygon feature on the map and click on the feature to measure its length or area.

Conclusion

Measuring length and area is an essential operation in geospatial analysis. PostGIS provides various functions to calculate length and area measurements of spatial features. By understanding the differences between geometry and geography types, we can choose the appropriate type for our analysis. Furthermore, using QGIS, we can easily perform length and area measurements on our spatial data.