Zonal Statistics

What is Zonal Statistics?

Zonal Statistics is a spatial analysis technique used to calculate statistics of a raster dataset within the boundaries of vector geometries. This method involves summarizing raster values based on predefined zones defined by the vector geometries, such as polygons. Common statistical measures that can be derived include mean, sum, median, minimum, and maximum values for raster data that fall within each zone.

When would you use Zonal Statistics?

You would use Zonal Statistics when you need to understand the distribution and characteristics of raster data within specific geographic areas. This analysis is particularly useful in scenarios such as environmental studies where you assess vegetation indices within administrative boundaries, urban planning to evaluate land use patterns within city limits, and resource management to analyze resources like soil or water quality within different regions. Zonal statistics are also vital in agriculture for evaluating climate variables or yield data within farm plots.

FAQs

What types of vector geometries can be used in Zonal Statistics?

Polygon geometries are typically used for Zonal Statistics as they define distinct zones within which the raster statistics are calculated. However, lines and points can also be used, depending upon the specific requirements of the analysis.

Can Zonal Statistics be used with any type of raster data?

Yes, Zonal Statistics can be applied to any type of raster data, including elevation models, satellite imagery, and climate data, as long as the data is appropriately pre-processed and georeferenced to align with the vector geometries.

Is it possible to perform Zonal Statistics on multiple raster layers simultaneously?

Typically, Zonal Statistics is performed on a single raster layer at a time. However, by running the analysis iteratively or using batch processes, you can calculate statistics for multiple raster layers individually and compare results across different datasets.

How are NoData values handled in Zonal Statistics?

NoData values are usually excluded from the statistical calculations in Zonal Statistics. Therefore, the statistics are computed only for the valid data pixels that lie within the vector zones, ensuring that missing values do not skew the results.

What is a suitable cell size for raster data in Zonal Statistics?

The cell size for raster data should be suitable for the scale of the analysis and the size of the vector zones. Smaller cell sizes provide more detailed results but may increase processing time, while larger cell sizes result in faster computations but may offer less precision.