16.2. 栅格分析

16.2.1. Raster Calculator

The Raster Calculator in the Raster menu allows you to perform calculations on the basis of existing raster pixel values (see 图 16.20). The results are written to a new raster layer in a GDAL-supported format.

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图 16.20 Raster Calculator

The Raster bands list contains all loaded raster layers that can be used. To add a raster to the raster calculator expression field, double click its name in the Fields list. You can then use the operators to construct calculation expressions, or you can just type them into the box.

In the Result layer section, you will need to define an output layer. You can:

  • checkbox Create on-the-fly raster instead of writing layer to disk:

    • If unchecked, the output is stored on the disk as a new plain file. An Output layer path and an Output format are required.

    • If checked, a virtual raster layer, i.e. a raster layer defined by its URI and whose pixels are calculated on-the-fly, is created. It's not a new file on disk; the virtual layer is still connected to the rasters used in the calculation meaning that deleting or moving these rasters would break it. A Layer name can be provided, otherwise the calculation expression is used as such. Removing the virtual layer from the project deletes it, and it can be made persistent in file using the layer Export ► Save as... contextual menu.

  • Define the Spatial extent of the calculation based on an input raster layer extent, or on custom X,Y coordinates

  • Set the Resolution of the layer using columns and rows number. If the input layer has a different resolution, the values will be resampled with the nearest neighbor algorithm.

  • With the checkbox Add result to project checkbox, the result layer will automatically be added to the legend area and can be visualized. Checked by default for virtual rasters.

The Operators section contains all available operators. To add an operator to the raster calculator expression box, click the appropriate button. Mathematical calculations (+, -, *, ... ) and trigonometric functions (sin, cos, tan, ... ) are available. Conditional expressions (=, !=, <, >=, ... ) return either 0 for false or 1 for true, and therefore can be used with other operators and functions.

提示

See also the Raster calculator algorithm.

16.2.1.1. Examples

Convert elevation values from meters to feet

Creating an elevation raster in feet from a raster in meters, you need to use the conversion factor for meters to feet: 3.28. The expression is:

"elevation@1" * 3.28

Using a mask

If you want to mask out parts of a raster -- say, for instance, because you are only interested in elevations above 0 meters -- you can use the following expression to create a mask and apply the result to a raster in one step.

("elevation@1" >= 0) * "elevation@1"

In other words, for every cell greater than or equal to 0 the conditional expression evaluates to 1, which keeps the original value by multiplying it by 1. Otherwise the conditional expression evaluates to 0, which sets the raster value to 0. This creates the mask on the fly.

Classify a Raster

If you want to classify a raster -- say, for instance into two elevation classes, you can use the following expression to create a raster with two values 1 and 2 in one step.

("elevation@1" < 50) * 1 + ("elevation@1" >= 50) * 2

In other words, for every cell less than 50 set its value to 1. For every cell greater than or equal 50 set its value to 2.

Or you can use the IF operator.

if ( elevation@1 < 50 , 1 , 2 )

16.2.2. Raster Alignment

This tool is able to take several rasters as input and to align them perfectly, that means:

  • reproject to the same CRS,

  • resample to the same cell size and offset in the grid,

  • clip to a region of interest,

  • rescale values when required.

All rasters will be saved in another files.

First, open the tools from Raster ► Align Raster... and click on the symbologyAdd Add new raster button to choose one existing raster in QGIS. Select an output file to save the raster after the alignment, the resampling method and if the tools need to Rescale values according to the cell size. The resampling method can be (see 图 16.21):

  • Nearest Neighbor

  • Bilinear (2x2 kernel)

  • Cubic (4x4 kernel): Cubic Convolution Approximation

  • Cubic B-Spline (4x4 kernel): Cubic B-Spline Approximation

  • Lanczos (6x6 kernel): Lanczos windowed sinc interpolation

  • Average: computes the average of all non-NODATA contributing pixels

  • Mode: selects the value which appears most often of all the sampled points

  • Maximum, Minimum, Mediane, First Quartile (Q1) or Third Quartile (Q3) of all non-NODATA contributing pixels

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图 16.21 Select Raster Resampling Options

In the main Align raster dialog, you can still symbologyEdit Edit file settings or symbologyRemove Remove an existing file from the list of raster layers. You can also choose one or more other options (see 图 16.22):

  • Select the Reference Layer,

  • Transform into a new CRS,

  • Setup a different Cell size,

  • Setup a different Grid Offset,

  • Clip to Extent: it can be user-defined, bound to a layer or to the map canvas

  • Output Size,

  • Add aligned raster to the map canvas.

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图 16.22 Raster Alignment