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api:dk11:delphi:gisinterpolation.tgis_interpolationidw

TGIS_InterpolationIDW class

DK11 for Delphi | GisInterpolation.TGIS_InterpolationIDW | Classes | Constructors | Methods | Properties

Implementation of the Inverse Distance Weighting (IDW) interpolation method.

Available also on: .NET | Java | ActiveX.

Syntax

// Delphi
type
  TGIS_InterpolationIDW = class( TGIS_VectorToGridAbstract )
  end;
// C++ Builder
class PASCALIMPLEMENTATION TGIS_InterpolationIDW : public TGIS_VectorToGridAbstract
{
};

Inheritance

Constructors

Inherited Overrides Protected
Name Visibility Description
Create public

Methods

Inherited Overrides Protected
Name Visibility Description
Generate public Populates the pixel (grid) layer with the values resulting from computations based on the set of sample points from the vector layer.
(Overrides TGIS_VectorToGridAbstract.Generate)
raiseBusyEvent protected Prepares and raises the BusyEvent.
(Inherited from TGIS_VectorToGridAbstract)

Properties

Inherited Protected
Name Visibility Description
Coordinate public Defines which coordinate is taken as interpolation value if the interpolation is not based on an attribute field; default is Z.
(Inherited from TGIS_VectorToGridAbstract)
DefaultValue public If UseDefaultValue is true then this value will be set for each grid cell for which the interpolated value cannot be computed.
(Inherited from TGIS_VectorToGridAbstract)
Exponent public Exponent in the formula for weight calculation; default value is 2.
Radius public Applies only to the windowed method (Windowed = True).
UseDefaultValue public If true then each grid cell for which the interpolated value cannot be computed will be set to DefaultValue.
(Inherited from TGIS_VectorToGridAbstract)
Windowed public If True then the windowed version of the algorithm is used; true by default.

Remarks

Method

The Inverse Distance Squared (IDW) interpolation method assumes that the influence of the data points on the interpolated point diminish with distance i.e. close points have greater weight than the points further away.

Usage

There are two different scenarios for generating an IDW interpolated grid:

  1. based on a Z, M or Z+M coordinate value:
    • set the Coordinate property to the desired option
    • pass an empty string as the field name to the Generate method
  2. based on a attribute field value:
    • pass the field name to the Generate method
    • with above the Coordinate property is neglected

The Exponent property pays the role of a smoothening factor. For most cases, reasonable results are obtained for the exponent value between 2.0 (default) and 3.0. For big exponent values, the algorithm converges to the Voronoi diagram.

Use a windowed version of the algorithm (Windowed=True) for large datasets (thousands of points and more). For up to few hundred points the non-windowed (full sample) algorithm is efficient enough and gives best possible results. The Radius property is the size of the window in map units.

Pros

  • good for interpolating data with extreme value changes at short distances
  • gives intuitive results for clusters of points with small value variation

Cons

  • interpolated values don't exceed the range of minimum to maximum values of the input data
  • not good for mountain-like areas, flattens peaks

Example

// Delphi
<code delphi>
var
  src : TGIS_LayerVector ;
  dst : TGIS_LayerPixel ;
  vtg : TGIS_InterpolationIDW ;  
  ext : TGIS_Extent ; 
begin
  // get the source layer (GIS is TGIS_ViewerWnd)
  src := TGIS_LayerVector( GIS.Get( 'points' ) ) ;
  ext := src.Extent ;
 
  // create the destination layer (grid, 200x200 pixels)
  dst := TGIS_LayerPixel.Create ;
  dst.Build( True, lsrc.CS, ext, 200, 200 ) ;
  dst.Name := 'Heatmap' ;
  dst.Params.Pixel.GridShadow := False ;
 
  // create an instance of the IDW interpolation engine
  vtg := TGIS_InterpolationIDW.Create ;
  try
    // attach event to track the progress
    vtg.BusyEvent := doBusyEvent ;   
 
    // set the exponent for weight calculation
    vtg.Exponent := 3.0 ;
 
    // initiate the grid generation process for the
    // POPULATION attribute field
    vtg.Import( lsrc, ext, 'POPULATION', ldst, ext ) ;
  finally
    vtg.Free ;
  end ;  
 
  // add the grid to the viewer
  GIS.Add( dst ) ;
 
  // update the viewer
  GIS.FullExtent ;
end ;

References

2019/01/25 16:26