Image Morphometric Calculator (Grid)

From UMEP
Jump to: navigation, search

Image Morphometric Parameters (Grid)

The Image Morphometric Parameters (Grid) plugin calculates various morphometric parameters based on digital surface models. These morphometric parameters are:

  • Mean building height (zH). The average building heights from ground in meters.
  • Standard deviation of building heights (zHσ).
  • Maximum building heights (zHmax).
  • Plan area index (λP). Fraction of buildings related to total ground area.
  • Frontal area index (λF). Fraction of building walls in wind direction related to total ground area.
  • Roughness length (z0). A parameter of some vertical wind profile equations that model the horizontal mean wind speed near the ground; in the log wind profile, it is equivalent to the height at which the wind speed theoretically becomes zero.
  • Zero displacement height (zd). The height in meters above the ground at which zero wind speed is achieved as a result of flow obstacles such as trees or buildings.

The morphometric parameters above are used to describe the roughness of a surface and are included in various local and mesoscale climate models. The parameters above will vary depending on what angle (wind direction) you are interested in. Thus, this plugin is able to derive the parameters for different directions. Preferable, a ground and 3D object DSM and a DEM should be used as input data. The 3D objects are usually buildings but can also be 3D vegetation (i.e. trees and bushes). A 3D object DSM with no ground heights makes it also possible to derive the parameters. This plugin is the same as the Image Morphometric Parameters (Point) except that this plugin calculates the parameters for each polygon object in the polygon vector layer. The polygons should preferable be squares or any other regular shape. To create such a grid, built in functions in QGIS can be used (see Vector -> Research Tools -> Vector Grid...).

Location

The Image Morphometric Parameters (Point) is located at

  • UMEP
    • Pre-Processor
      • Urban Morphology
        • Image Morphometric Calculator (Grid)

Running the Plugin

When you run the plugin, you will see the dialog shown below. It consists of three sections. The top section sets the parameters for the area of interest where the morphometric parameters are calculated. You also set the search interval in degrees and from where the search should take place within each grid. The next section helps you to specify the input data regarding 3D objects and ground as well as specifying what grid that should be used. The bottom section is for specifying the output and for running the calculations.

Start Dialog for the Morphometric Calculator package

Search Throughout the Grid Extent

Tick this in if the search should be performed from one side of the grid to the opposite side. Tick the other option (Search from grid centroid if the search should start from the centroid of the grid. Setting the Search distance can then allow for the search to extent beyond the grid. See the figure below for illustration. The left one performs a search using the grid extent whereas the right illustrates a search from the centroid and extending outside of the grid.

Grid Extent Options

Wind Direction Search Interval (degrees)

This decides the interval in search directions for which the morphometric parameters will be calculated.

Vector Polygon Grid

Here the grid polygon layer should be specified.

ID Field

Choose an attribute from the selected polygon layer that will be used to separated the different polygon objects from each other. An attribute field of unique numbers or letters should be used.

Raster DSM (only 3D Objects) Exist

Tick this in if a 3D-object DSM without ground heights is available. 3D objects (e.g. buildings) should be meters over ground.

Raster DSM (3D Objects and Ground)

A raster DSM (e.g. geoTIFF) consisting of ground and e.g. building height (meters above sea level).

Raster DEM (only Ground)

A DEM (e.g. geoTIFF) consisting of pixels with ground heights (meters above sea level).

Raster DSM (only 3D Objects)

A DSM (e.g. geoTIFF) consisting of pixels with object (e.g. buildings or vegetation) heights above ground. Pixels where no objects are present should be set to zero.

Roughness calculation Method

Here, options to choose methods for roughness calculations regarding zero displacement height (zd) and roughness length (z0) are available. For more information, see the online UMEP manual.

File Prefix

A prefix that will be included in the beginning of the output files.

Output Folder

A specified folder where result will be saved.

Run

This starts the calculations.

Close

This button closes the plugin.

Output

Two different files per polygon objects are saved after a successful run. The first one includes anisotropic result where the morphometric parameter for each wind direction as specified are included. The second one includes isotropic results where all directions are integrated into one value for each parameter. If the raster data includes no data values within a polygon object, this grid will not be considered in the calculation.

Remarks

  • All DSMs need to have the same extent and pixel size.