Interpolate Continuous Data From Non-Continuous Data. Examples. Dialog Box

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1 Interpolate Continuous Data From Non-Continuous Data Examples Dialog Box Interpolating Continuous Elevation Data From Contours Interpolating Continuous Bathymetric Data Using a Mask Map Layer Using the operation dialog box interface Using the dialog box interface to create or edit scripts Sparse Data Map This drop-down list allows you to specify a map layer of non-continuous data to be interpolated into a map layer of continuous data. Non-continuous data can include contour data, incomplete thematic information, and remote sensing imagery with data gaps. For sparse point data such as spot heights, geological surveys, and random points, use the Krige operation to interpolate continuous data. Output Precision Use the Output Precision field and drop-down list to specify the precision of the interpolation. Specify 5m if you want the interpolated values to be calculated only to the nearest 5m; specify 0.001m or 1cm if you want the OP-INT-1

2 interpolated values to be calculated to the nearest centimetre. If your data values are not measured in mm, cm, m, or km, specify None. Mask Map Use the Mask Map drop-down list to specify a map layer that restricts the interpolation to the cells in the mask map layer that have a non-void value. All cells that have the value ÒVOIDÓ in the mask map will not be included in the interpolation. Search Radius The Search Radius field and drop-down list specify a distance from each cell for which the Interpolate operation will search for values to use in the first pass of the interpolation. The Interpolate operation uses a two pass process to calculate values (see First Pass Grid Spacing below). If no value is specified in this field, the Interpolate operation will search in all directions until it locates a value. First Pass Grid Spacing The Interpolate operation uses a two pass process to interpolate values. The First Pass Grid Spacing option field is used to specify the coarseness of the first pass of the interpolation. A spacing of 1 is equivalent to interpolating all cells on the first pass. A spacing of 3 is the default: it interpolates values for every third cell moving from left to right and top to bottom on the first pass. The higher the spacing number the faster the operation; however, increasing the spacing value may reduce accuracy. The maximum value is 10. For tightly spaced contours, use a grid spacing of Ò1Ó. Weighting Method There are two weighting methods that the Interpolate operation can use to calculate unknown values based on known values: Inverse and Inverse Square. These are based on a gravity model (see Details below). The further away a cell is from the target cell, the less influence it has on the resulting cell value. This influence is based either on the inverse of the distance, or the inverse square of the distance. With Inverse Square the distance factor is squared, greatly reducing the influence of cells that are far away. Syntax Syntax and type conventions Using the Script window interface Using the dialog box interface to create or edit scripts OP-INT-2

3 Interpolate map [In area map] [To vertical distance] [Within horizontal distance] [Step increment value] [InverseSquare]; Interpolate map The Interpolate statement specifies a map layer of non-continuous data to be interpolated into a continuous data set. In area map The In modifier is used to specify a mask map layer that restricts the interpolation to specific cells. For example, you may wish to restrict the interpolation to non-urban areas only or, if creating a Digital Bathymetry Model, to those areas below sealevel only. Non-VOID cells in the mask map identify those cells that are to be interpolated. VOID cells in the mask map will result in corresponding VOID cells in the resulting map layer. To vertical distance The To modifier is used to determine the data type and precision of the resulting map layer. If the To parameter contains a decimal point (e.g., 1.0cm), the resulting map layer will be of floating point type, otherwise (e.g., 1cm) the resulting map layer will be of fixed point type. For example, for floating point results to be within 1 cm, specify To 0.01m or To 1.0cm. The units of measure specified will be the data units of the resulting map layer. Within horizontal distance The Interpolate operation uses a two pass process to calculate the unknown values. The Within modifier can be used to limit the radius of the search area of the first pass. If no distance units are given, horizontal distance is assumed to represent number of cells. The default is unlimited distance. Note: the use of this modifier may produce VOID areas in the resulting map layer if the input map layer data is more disbursed than the radius specified. Step increment value The Interpolate operation uses a two pass algorithm. The Step modifier identifies the coarseness of the first pass of the interpolation. Step 1 is equivalent to interpolating all cells on the first pass. Step 3 is the default; it only interpolates values for every third cell moving from left to right and top to bottom on the first pass. The higher the Step number, the faster the operation; however, increasing the step value may reduce accuracy. The OP-INT-3

4 step value must be an integer between 1 and 10. To interpolate values from tightly spaced contours, use Step 1. InverseSquare By default, an inverse weighting process is used unless the InverseSquare modifier is specified. Inverse weighting works like a gravity model. The further away a cell is from the centre of the window, the less influence it has on the value of a cell in the resulting map layer. Influence is based on the inverse or inverse square of the distance between cells. More distant values have a lesser effect if the InverseSquare modifier is used. Details The Interpolate operation creates a continuous data set from sparse data by estimating unknown values based on known values using a two pass approach. The non-data cells in the non-continuous data map layer should be assigned the value ÒVOIDÓ. The Interpolate operation is commonly used with contour data and/or ridge-and-channel data to generate a DEM (Digital Elevation Model). The Interpolate operation uses a two pass approach. The first pass looks for non-void cells within the maximum specified distance surrounding a VOID cell (i.e., a cell with an unknown value). A coarse grid of cells (every 2nd or 3rd or nth cell, whatever is specified by the Step modifier in the Script window or the First Pass Grid Spacing in the Interpolate dialog box) is interpolated on the first pass. The second pass takes the resulting coarse grid and interpolates the remaining cells. Either way, the interpolated value of a cell is based on the following formula: e å i i d i target value = å 1 i di e is the elevation of a neighbouring cell d is the distance between the neighbouring cell and the target cell. i is the number of cells Each neighbouring cell contributes to the value of the target cell based on its distance from the target cell. Closer cells contribute more than cells further away. If the InverseSquare modifier is specified, the weighting is based on the square of the distance from the target, otherwise the weighting is based on the distance. What Do I Need? Use the Interpolate operation on non-continuous data to fill in gaps and voids. Cells with no data should be assigned the value ÒVOIDÓ. The Interpolate operation works best on semi-continuous data such as contours and remote sensing imagery with gaps. If you have a map layer with sparse data points, use the Krige operation to interpolate unknown values. OP-INT-4

5 Troubleshooting Error Messages Here are some of the most common error messages for the Interpolate operation with suggestions on what to do if you see them: Error, the original map data units are not compatible with the units specified in the ÒToÓ modifier. Either change the map layer units in the Information window to match the units of the To modifier or change the units of the To modifier to match the map layer units. Error, negative or zero distance/resolution specified. The distance modifier must be a value greater than zero. Error, the cell resolution units of the original map are not compatible with the units specified in the ÒWithinÓ modifier. Either change the map layer units in the Information window to match the units of the Within modifier or change the units of the Within modifier to match the map layer units. Error, the specified ÒStepÓ value is outside the valid range (1Ñ10). Specify the Step value in the operation with an integer between 1 and 10. Error, the specified ÒStepÓ value is too large for the original map. Must be less than the number of rows/columns. Consult the Information window to determine the number of rows/columns. Sorry, the map has exceeded the maximum number of zones allowed (65Ê000). The map layer is too large. Try subscening the operand map layer and perform the operation on the subscenes then use Cover (Mosaic) to reassemble the maps. OP-INT-5