QGIS Sample Raster Values (assign raster cell values to points)Ī variation on this theme would be to create and assign an average value around each point at a given distance, such as the average temperature within five miles. If you needed to interpolate, you can go to the Plugins menu, enable the SAGA plugin, and in the Processing toolbox try the SAGA tool Raster Values to Points instead. Unlike ArcGIS, there isn’t an option for interpolation from surrounding cells you simply get the value for the cell that the point falls within. Input the points, choose a raster layer, and write the output to a new vector point file. In QGIS the name of this tool is Sample raster values, which you can find in the Processing toolbox. ArcGIS Pro Extract Values to Points (assign raster cell values to points) I created some phony point data and used it to generate the output below. The output point feature contains a new column called RASTERVALU. The default is to take the value for the cell that the point falls within, but there is an Interpolate option that will calculate the value from adjacent cells. You select your input points and raster, and a new point feature that will include the raster values. But what if you had point features, and wanted to assign each point the value of the raster cell that it falls within? In ArcGIS Pro, search the toolbox for the Extract Values to Points tool. Zonal stats allows you to summarize raster data within a polygon. QGIS Zonal Statistics Extract Raster Values for Point Features Unlike ArcGIS, QGIS allows you to write output as a table or a new shapefile / geopackage, which carries along the feature geometry from the Input zones and adds the summaries, allowing you to skip the step of having to do a table join (if you opted to create a table, you could join it to the zones using the Joins tab under the Properties menu for the vector features). By default the summary stats are the count, sum, and mean, but you can check the Statistics to calculate box to select others. The vector with the zones is the Input layer, and the Raster layer is the grid with the values. In QGIS, this tool is simply called Zonal Statistics search for it in the Processing toolbox. ArcGIS Pro Zonal Statistics Table Output and Join to Show Average Temperature per County This table can be joined back to the original vector feature (select the county feature in the Contents, right click, Joins and Relates – Join) to thematically map the average temp. The output table consists of one record for each zone / county, with the count of the cells used to create the average, and the mean temperature (in degrees Celsius). ArcGIS Pro Zonal Statistics as Table Temperature Grid and Southern New England Counties I’m calculating the mean temperature for the counties on that day. In the example below, I’m using counties from the census TIGER files for southern New England as my Input Feature Zone, the AFFGEOID (Census ANSI / FIPS code) to identify the Zone Field, and a temperature grid for Janufrom PRISM as the Input Value Raster. You can join this resulting table back to the vector file using their common unique identifier in a table join. The output is a table containing the unique identifiers of the raster and vector, the summary stats you’ve generated (average, sum, min, max, etc), and a count of the number of cells used to generate the summary. That’s not desirable for the use case I’m presenting here the better choice is the Zonal Statistics as Table tool. The ArcGIS Pro toolbox has a Zonal Statistics tool where the output is a new raster file with cells that are summarized by the input zones. The term zonal statistics is used to define any operation that calculates statistics on cell values of a raster within an area or zone defined by another dataset, either a raster or a vector. The goal is to have a new attribute column in the vector layer that contains the summarized raster value, perhaps because you want to make thematic maps of that value, or you want to use it in conjunction with other variables to run spatial statistics, or you just want a plain and simple summary for given places. Imagine that you have quantitative values such as temperature or a vegetation index in a raster grid, and you want to use this data to calculate an average for counties or metro areas. Zonal Statistics: Summarize Rasters by Area I’ll summarize some approaches in this post using ArcGIS Pro and QGIS, to summarize raster values for polygons (zonal statistics) and to assign raster values to points (aka raster sampling). It’s been a busy few months, but I have a few days to catch my breath now that it’s spring break and most people (except me) have gone away! One question that’s come up quite a bit this semester is how to associate raster data with coinciding vector data.
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