IDW
Release 9.3 Last modified September 7, 2011	Print all topics in : "Interpolation (Spatial Analyst)"

Interpolates a surface from points using an inverse distance weighted (IDW) technique.

Learn more about how IDW works

Usage tips

Command line and Scripting

• The output value for a cell using IDW is limited to the range of the values used to interpolate. Because IDW is a weighted distance average, the average cannot be greater than the highest or less than the lowest input. Therefore, it cannot create ridges or valleys if these extremes have not already been sampled (Watson and Philip, 1985).

• The influence of an input point on an interpolated value is isotropic. Since the influence of an input point on an interpolated value is distance related, IDW is not "ridge preserving" (Philip and Watson, 1982).

• The best results from IDW are obtained when sampling is sufficiently dense with regard to the local variation you are attempting to simulate. If the sampling of input points is sparse or uneven, the results may not sufficiently represent the desired surface (Watson and Philip, 1985).

• The characteristics of the interpolated surface can also be controlled by limiting the input points used in the calculation of each output cell value. The input can be limited by the number of sample points to be used (VARIABLE) or by a radius within which there are points to be used (FIXED). When using the VARIABLE search method:

  • If there are not enough points within the maximum_distance search radius to meet the number_of_points criteria, then only those points within the maximum_distance are used.
  • If no points are found within the maximum_distance, then NoData is assigned to the cell.
  When using the FIXED search method:
  • If the minimum_number_of_ points = 0 and no points are found within the specified distance, then NODATA is assigned to the cell.
  • If a minimum_number_of_points value greater than 0 is specified, and either no points are found or the required number of points are not not found, the search radius will be increased until the required nunber of points is satisified.
  • Note: When the search radius needs to be increased it is done so until the minimum_number_of_points fall within that radius, or the extent of the radius crosses the lower (southern) and/or upper (northern) extent of the output raster. NoData is assigned to all locations that do not satisfy the above condition.
  • Note: If the distance is increased, then there may suddenly be more than minimum_number_of_points within the search radius. This is because some points that are equidistant from the location at which the estimate is required are now included in the search radius.

• Some input datasets may have several points with the same X,Y coordinates. If the values of the points at the common location are the same, they are considered duplicates and have no affect on the output. If the values are different, they are considered 'coincident' points. Interpolation functions may handle this data condition differently, in some cases using the first coincident point encountered for the calculation, or sometimes the last. For IDW, the values will be averaged.

  This may cause some locations in the output raster to have different values than what you might expect. The solution is to prepare your data by removing these coincident points.

• The barriers option is used to specify the location of linear features known to interrupt the surface continuity. These features do not have z-values. Cliffs, faults, and embankments are typical examples of barriers. Barriers limit the selected set of the input sample points used to interpolate output z-values to those samples on the same side of the barrier as the current processing cell. Separation by a barrier is determined by line-of-sight analysis between each pair of points. This means that topological separation is not required for two points to be excluded from each other's region of influence. Input sample points that lie exactly on the barrier line will be included in the selected sample set for both sides of the barrier.

• Barrier features are input as polyline features. IDW only uses the x,y coordinates for the linear feature; therefore, it is not necessary to provide z-values for the left and right sides of the barrier. Any z-values provided will be ignored.

• Using barriers will significantly extend the processing time.

• This tool has a limit of approximately 45 million input points. If your input feature class contains more than 45 million points, the tool may fail to create a result. You can avoid this limit by interpolating your study area in several pieces, making sure there is some overlap in the edges, and then mosaicking the results to create a single large raster dataset. Alternatively you can use a Terrain geodatabase to store and visualize points and surfaces comprised of billions of measurement points.

• The input feature data must contain at least one valid field.

• The following environment settings affect this tool:

  • General: Current Workspace, Scratch Workspace, Output Coordinate system, Output Extent, Snap Raster.
  • Raster Analysis: Cell size, Mask.
  • Raster Storage: See the Raster Storage settings for the particular output raster format selected.

Map Algebra

• The output value for a cell using IDW is limited to the range of the values used to interpolate. Because IDW is a weighted distance average, the average cannot be greater than the highest or less than the lowest input. Therefore, it cannot create ridges or valleys if these extremes have not already been sampled (Watson and Philip, 1985).

• The best results from IDW are obtained when sampling is sufficiently dense with regard to the local variation you are attempting to simulate. If the sampling of input points is sparse or uneven, the results may not sufficiently represent the desired surface (Watson and Philip, 1985).

• The influence of an input point on an interpolated value is isotropic. Since the influence of an input point on an interpolated value is distance related, IDW is not "ridge preserving" (Philip and Watson, 1982).

• Some input datasets may have several points with the same X,Y coordinates. If the values of the points at the common location are the same, they are considered duplicates and have no affect on the output. If the values are different, they are considered 'coincident' points. Interpolation functions may handle this data condition differently, in some cases using the first coincident point encountered for the calculation, or sometimes the last. For IDW, the values will be averaged.

  This may cause some locations in the output raster to have different values than what you might expect. The solution is to prepare your data by removing these coincident points.

• GENERATE input files in the POINT format provide an easy method to create surfaces and two-dimensional coverages from ASCII files containing x,y coordinates and z-values. The x,y coordinates and z-values can be expressed as integers or single- or double-precision floating-point values. The x,y coordinates for the output raster are stored in DOUBLE precision. The z-values are stored in SINGLE precision. GENERATE files in the POINT format contain the feature ID, x,y coordinates, and z-values for point features. The ID values are not used by IDW. The format for GENERATE files containing points is:

       ID,X,Y,Z
       ...
       END

  The end of the file is indicated with the END keyword.

• A typical GENERATE file containing points is shown below. The file can be comma or space delimited.

       1,234.065,837.123,125.845
       2,735.774,729.345,93.739
       37,840.934,385.700,157.037
       5,385.888,286.602,25.764
       END

• The barriers option is used to specify the location of linear features known to interrupt the surface continuity. These features do not have z-values. Cliffs, faults, and embankments are typical examples of barriers. Barriers limit the selected set of the input sample points used to interpolate output z-values to those samples on the same side of the barrier as the current processing cell. Separation by a barrier is determined by line-of-sight analysis between each pair of points. This means that topological separation is not required for two points to be excluded from each other's region of influence. Input sample points that lie exactly on the barrier line will be included in the selected sample set for both sides of the barrier.

• The barriers option will significantly extend the processing time.

• Barrier features can be input as either a coverage or a GENERATE file in the LINE format. IDW will look for the {barriers} coverage first, and if not found, it will look for the {barrier} file. IDW only uses the x,y coordinates for the linear feature; therefore, it is not necessary to provide z-values for the left and right sides of the barrier. Any z-values provided will be ignored.

  LINE input files contain the feature ID, followed by the x,y coordinates for each vertex in the file. Each line feature must have at least two, and no more than 20,000, vertices. The general LINE input file format is:

       ID X,Y,(Z) ... X,Y,(Z) END END

  The end of each line is indicated with the END keyword, and the end of the file is indicated with a second END keyword.

  The following is an example of a 3D GENERATE file in the LINE format. In this case, only the x,y coordinates are used by IDW. The feature IDs and z-values are ignored.

       2
       345321.5  678543.1  123
       345879.0  689231.2  145
       345999.9  690579.3  157
       END
       2
       367809.0  712004.0  209 
       368098.9  710776.8  214
       369890.6  709768.2  206
       370023.4  716098.3  198
       END
       2
       390879.0  602090.0  133
       391000.6  606668.2  169
       397547.3  609608.4  125
       END
       END

• IDW does not respect the Mask setting.

• Learn more about how to specify the input feature dataset in the Map Algebra expression of Raster Calculator.

ArcObjects

• The output value for a cell using IDW is limited to the range of the values used to interpolate. Because IDW is a weighted distance average, the average cannot be greater than the highest or less than the lowest input. Therefore, it cannot create ridges or valleys if these extremes have not already been sampled (Watson and Philip, 1985).

• The best results from IDW are obtained when sampling is sufficiently dense with regard to the local variation you are attempting to simulate. If the sampling of input points is sparse or uneven, the results may not sufficiently represent the desired surface (Watson and Philip, 1985).

• The influence of an input point on an interpolated value is isotropic. Since the influence of an input point on an interpolated value is distance related, IDW is not "ridge preserving" (Philip and Watson, 1982).

• Some input datasets may have several points with the same X,Y coordinates. If the values of the points at the common location are the same, they are considered duplicates and have no affect on the output. If the values are different, they are considered 'coincident' points. Interpolation functions may handle this data condition differently, in some cases using the first coincident point encountered for the calculation, or sometimes the last. For IDW, the values will be averaged.

  This may cause some locations in the output raster to have different values than what you might expect. The solution is to prepare your data by removing these coincident points.

• The barrier option is used to specify the location of linear features known to interrupt the surface continuity. These features do not have z-values. Cliffs, faults, and embankments are typical examples of barriers. Barriers limit the selected set of the input sample points used to interpolate output z-values to those samples on the same side of the barrier as the current processing cell. Separation by a barrier is determined by line-of-sight analysis between each pair of points. This means that topological separation is not required for two points to be excluded from each other's region of influence. Input sample points that lie exactly on the barrier line will be included in the selected sample set for both sides of the barrier.

• Barrier features are represented by a shapefile containing arcs. IDW only uses the x,y coordinates for the linear feature; therefore, it is not necessary to provide z-values for the left and right sides of the barrier.

• Using barriers will significantly extend the processing time.

• A FeatureDescriptor is needed in ArcObjects to specify the field to be used in the interpolation. The following is an example:

       Dim pFDesc As IFeatureClassDescriptor
       Set pFDesc = New FeatureClassDescriptor
       PFDesc.Create pInputDataset, Nothing, "Value" 
       'Value is the field name used for interpolation

• When the selected field is FID in the FeatureClassDescriptor object, the SPOT field will be used. If no SPOT field is present, NONE is used.

• The SPOT item will be used from the input feature class object. If another item is desired, then that item must be specified in a FeatureClassDescriptor.

• As with the Raster Calculator, when using the IDW method in ArcObjects, the output cell size must be defined in the raster environment. The following is an example of setting it:

       Dim pEnv As IRasterAnalysisEnvironment
       Set pEnv = pInterpolationOp
       pEnv.SetCellSize esriRasterEnvValue, 50

• This tool has a limit of approximately 45 million input points. If your input feature class contains more than 45 million points, the tool may fail to create a result. You can avoid this limit by interpolating your study area in several pieces, making sure there is some overlap in the edges, and then mosaicking the results to create a single large raster dataset. Alternatively you can use a Terrain geodatabase to store and visualize points and surfaces comprised of billions of measurement points.

Command line syntax
An overview of the Command Line window
Idw_sa <in_point_features> <z_field> <out_raster> {cell_size} {power} {search_radius} {in_barrier_polyline_features}

Command line example

Idw_sa c:/data/pts.shp spot final_1 1 2 "VARIABLE 12"

Scripting syntax
About getting started with writing geoprocessing scripts
Idw_sa (in_point_features, z_field, out_raster, cell_size, power, search_radius, in_barrier_polyline_features)

Script example

# IDW_sample.py
# Description:
#   Interpolate a series of point features onto a rectangular 
#   raster using Inverse Distance Weighting (IDW).
# Requirements: None
# Author: ESRI
# Date: Sept 6, 2005
# Import system modules
import arcgisscripting

# Create the Geoprocessor object
gp = arcgisscripting.create()

try:
    # Set the output workspace name
    gp.Workspace = "c:/data"

    # Set the input feature dataset
    inputFeatureDataset = "c:/data/pts.shp"

    # Set the output raster name
    outputRaster = "final_1" 

    # Set some of the IDW parameters
    cell_size = 1
    IDW_power = 2
    IDW_neighborhood = "VARIABLE 12"
    Input_barrier_polyline_features = ""          # None

    # Set the output extent
    gp.Extent = "0 0 6 6"

    # Set the attribute field
    attributeName = "spot"

    # Check out Spatial Analyst extension license
    gp.CheckOutExtension("Spatial")

    # Process: IDW
    gp.Idw_sa(inputFeatureDataset, attributeName, outputRaster, cell_size, _ 
              IDW_power, IDW_neighborhood, Input_barrier_polyline_features)

except:
    # If an error occurred while running a tool, then print the messages
    print gp.GetMessages()

Map Algebra syntax
Idw(<point_feature | point_file>, {spot_item}, {barriers}, {power}, {SAMPLE, {num_points}, {max_radius}}, {cellsize}, {xmin, ymin, xmax, ymax})

Idw(<point_feature | point_file>, {spot_item}, {barriers}, {power}, {RADIUS, {radius}, {min_points}}, {cellsize},{xmin, ymin, xmax, ymax})

Map Algebra example

idw(pointcov)
idw(ptcover, depth, #, 1, sample, 10)
idw(pntfile, #, faultcov, #, radius, 500, 30)
idw(wellcover, flowrate, linefile, 3, sample, 20, #, 1.5, 34934, 89342, 35468, 93457)
idw(popdense, density, shores, #, radius, 30)	

ArcObjects syntax
IInterpolationOp::IDW (geoData As IGeoDataset, power As Double, radius as IRasterRadius, [barrier As Variant]) As IGeoDataset

   set pRad.SetFixed 207.1

   set pRad.SetVariable 12
 will use the args "... ,SAMPLE,12, ....", while: 




   set pRad.SetVariable 12

   set pRad.SetFixed 207.1
 will use the args "... ,RADIUS,207.1, ....".

ArcObjects example

' Create the RasterInterpolationOp object
Dim pInterpolationOp As IInterpolationOp
Set pInterpolationOp = New RasterInterpolationOp

' Create the input point object
Dim pInputDataset As IGeoDataset

' Calls function to open the point dataset from disk
Set pInputDataset = OpenFeatureDataset("D:\SpatialData", "inputpoints")

' Define the search radius
Dim pRadius As IRasterRadius
Set pRadius = New RasterRadius
pRadius.SetVariable 12

' Create the output dataset object
Dim pOutputDataset As IGeoDataset

' Calls the method
Set pOutputDataset = pInterpolationOp.IDW(pInputDataset, 3, pRadius