Inverse distance weighted interpolation

Introduction

Added in version 8.0.

The inverse distance weighted interpolation is a method to rasterize vector features with a weight that is the inverse of a power of the distance between the target pixel and the source feature.

Configuration

First, you must set the LAYER CONNECTIONTYPE parameter to IDW. The vector-to-raster interpolation takes the following parameters:

• CONNECTION “layername” : reference to the NAME or GROUP of a LAYER to use as an input vector datasource. NAME takes precedence, followed by the first layer from GROUP who’s minscaledenom/maxscaledenom matches the current map scale. The referenced layer should be a TYPE POINT layer. Other layer types will result in one sample being added for each vertex of the input features.

• PROCESSING “IDW_RADIUS=10” : Radius in pixels of the search radius of features to take into account. Higher values result in increased cpu time needed to compute the filtered data. The default is the dimension of the image to render.

• PROCESSING “IDW_COMPUTE_BORDERS=ON|OFF” : A kernel of radius “r” cannot be applied to “r” pixels along the borders of the image. The default is to extend the search rectangle of the input datasource to include features “r” pixels outside of the current map extent so that the computed result extends to the full extent of the resulting image. This can be deactivated when tiling if the tiling software applies a metabuffer of “r” pixels to its requests, to avoid the performance overhead of computing this extra information.

• PROCESSING “IDW_POWER=numeric” :

  Value such that 1 / distance^power is the weight applied when computing pixel values. The default is 1.0.

Advanced sample weighting and filtering

See corresponding section of Kernel Density Estimation (Dynamic Heatmap)

Raster Color Ramping

See corresponding section of Kernel Density Estimation (Dynamic Heatmap)

Example mapfiles

MAP
  SIZE 1000 500
  EXTENT -180 -90 180 90
  NAME "test heat"
  IMAGETYPE "png"

  WEB
    METADATA
      "ows_srs" "epsg:4326  epsg:3857 epsg:900913"
      "ows_enable_request" "*"
    END # METADATA
  END # WEB

  PROJECTION
    "+init=epsg:4326"
  END # PROJECTION

  LAYER
    NAME "idw"
    TYPE raster
    CONNECTIONTYPE idw
    CONNECTION "points"
    STATUS on
    PROCESSING "IDW_RADIUS=20"
    PROCESSING "IDW_COMPUTE_BORDERS=OFF"
    PROCESSING "IDW_WEIGHT=1"
    OFFSITE 0 0 0
    CLASS
      STYLE
        COLORRANGE  "#0000ff00"  "#0000ffff"
        DATARANGE 0 32
      END # STYLE
      STYLE
        COLORRANGE  "#0000ffff"  "#ff0000ff"
        DATARANGE 32 255
      END # STYLE
    END # CLASS
  END # LAYER

  LAYER
      NAME "points"
      STATUS on
      TYPE POINT
      DATA "pnts.shp"
   END # LAYER
END # MAPFILE